CS 352 Compilers: Principles and Practice

1. Introduction 2

2. Lexical analysis 31

3. LL parsing 58

4. LR parsing 110

5. JavaCC and JTB 127

6. Semantic analysis 150

7. Translation and simpliﬁcation 165

8. Liveness analysis and register allocation 185

9. Activation Records 216

1 Chapter 1: Introduction

2 Things to do

¯ make sure you have a working mentor account

¯ start brushing up on Java

¯ review Java development tools

¯ ﬁnd http://www.cs.purdue.edu/homes/palsberg/cs352/F00/index.html

¯ add yourself to the course mailing list by writing (on a CS computer)

mailer add me to cs352

for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and full citation on the first page. To copy otherwise, to republish, to post on servers, or to redistribute to lists, requires prior specific permission and/or Copyright c 2000 by Antony L. Hosking. Permission to make digital or hard copies of part or all of this work fee. Request permission to publish from [email protected].

3 Compilers

What is a compiler?

¯ a program that translates an executable program in one language into an executable program in another language

¯ we expect the program produced by the compiler to be better, in some way, than the original

What is an interpreter?

¯ a program that reads an executable program and produces the results of running that program

¯ usually, this involves executing the source program in some fashion

This course deals mainly with compilers

Many of the same issues arise in interpreters

4 Motivation

Why study compiler construction?

Why build compilers?

Why attend class?

5 Interest

Compiler construction is a microcosm of computer science

artiﬁcial intelligence greedy algorithms learning algorithms algorithms graph algorithms union-ﬁnd dynamic programming theory DFAs for scanning parser generators lattice theory for analysis systems allocation and naming locality synchronization architecture pipeline management hierarchy management instruction set use

Inside a compiler, all these things come together

6 Isn’t it a solved problem?

Machines are constantly changing

Changes in architecture µ changes in compilers

¯ new features pose new problems

¯ changing costs lead to different concerns

¯ old solutions need re-engineering

Changes in compilers should prompt changes in architecture

¯ New languages and features

7 Intrinsic Merit

Compiler construction is challenging and fun

¯ interesting problems

¯ primary responsibility for performance (blame)

¯ new architectures µ new challenges

¯ real results

¯ extremely complex interactions

Compilers have an impact on how computers are used

Compiler construction poses some of the most interesting problems in computing

8 Experience

You have used several compilers

What qualities are important in a compiler?

1. Correct code 2. Output runs fast 3. Compiler runs fast 4. Compile time proportional to program size 5. Support for separate compilation 6. Good diagnostics for syntax errors 7. Works well with the debugger 8. Good diagnostics for ﬂow anomalies 9. Cross language calls 10. Consistent, predictable optimization

Each of these shapes your feelings about the correct contents of this course

9 Abstract view

source machine code compiler code

errors

Implications:

¯ recognize legal (and illegal) programs

¯ generate correct code

¯ manage storage of all variables and code

¯ agreement on format for object (or assembly) code

Big step up from assembler — higher level notations

10 Traditional two pass compiler

source front IR back machine code end end code

errors

Implications:

¯ intermediate representation (IR)

¯ front end maps legal code into IR

¯ back end maps IR onto target machine

¯ simplify retargeting

¯ allows multiple front ends

¯ multiple passes µ better code

11 A fallacy

FORTRAN front code end back C++ front end target1 code end back target2 CLU front end code end back end target3 Smalltalk front code end

Canwebuildn¢ mcompilers with n · m components?

¯ must encode all the knowledge in each front end

¯ must represent all the features in one IR

¯ must handle all the features in each back end

Limited success with low-level IRs 12 Front end

source tokens code scanner parser IR

errors

Responsibilities:

¯ recognize legal procedure

¯ report errors

¯ produce IR

¯ preliminary storage map

¯ shape the code for the back end

Much of front end construction can be automated 13 Front end

source tokens code scanner parser IR

errors

Scanner:

¯ maps characters into tokens – the basic unit of syntax

Ý; · Ü = Ü

becomes ¯

< < = id,> Ü < · id,> Ü ; id,> Ý

character string value for a token is a lexeme ¯

¯ typical tokens: number, id, · , ¹ , ¶ , » , dÓ , eÒd eliminates white space (tabs, blanks, comments )

¯ a key issue is speed

µ use specialized recognizer (as opposed to ÐeÜ )

14 Front end

source tokens code scanner parser IR

errors

Parser:

¯ recognize context-free syntax

¯ guide context-sensitive analysis

¯ construct IR(s)

¯ produce meaningful error messages

¯ attempt error correction

Parser generators mechanize much of the work

15 Front end

Context-free syntax is speciﬁed with a grammar

< baa j

< sheep noise > ::= baa

sheep noise >

This grammar deﬁnes the set of noises that a sheep makes under normal circumstances

The format is called Backus-Naur form (BNF)

Formally, a grammar ´ G = S ; N ; T ; Pµ

S is the start symbol

N is a set of non-terminal symbols

T is a set of terminal symbols

P is a set of productions or rewrite rules (P : N ! N [ T)

16 Front end

Context free syntax can be put to better use

1 < goal> ::= < expr >

2 < expr > ::= < expr< > op< > term>

3 < j term>

4 < term> ::= ÒÙÑbeÖ

5 id j

6 < op> ::= ·

7 ¹ j

This grammar deﬁnes simple expressions with addition and subtraction

over the tokens id and ÒÙÑbeÖ

S = < goal >

T = ÒÙÑbeÖ , id , · , ¹

N = < goal > , < expr > , < term > , < op > P =1,2,3,4,5,6,7

17 Front end

Given a grammar, valid sentences can be derived by repeated substitution.

Prod’n. Result

< goal>

1 < expr >

2 < expr< > op< > term>

5 < expr< > opÝ >

7 < Ý expr¹ >

2 < expr< > op< > Ý term¹ >

4 < expr< Ý > ¹ op¾ >

6 Ý ¹ < ¾ expr· >

3 Ý ¹ < ¾ term· >

Ý ¹ 5 ¾ · Ü

To recognize a valid sentence in some CFG, we reverse this process and build up a parse

18 Front end

A parse can be represented by a tree called a parse or syntax tree

goal

expr

expr op term

exprop term -

term +

Obviously, this contains a lot of unnecessary information 19 Front end

So, compilers often use an abstract syntax tree -

This is much more concise

Abstract syntax trees (ASTs) are often used as an IR between front end and back end

20 Back end

instructionregister machine IR selection allocation code

errors

Responsibilities

¯ translate IR into target machine code

¯ choose instructions for each IR operation

¯ decide what to keep in registers at each point

¯ ensure conformance with system interfaces

Automation has been less successful here 21 Back end

instruction register machine IR selection allocation code

errors

Instruction selection:

¯ produce compact, fast code

¯ use available addressing modes

¯ pattern matching problem

– ad hoc techniques – tree pattern matching – string pattern matching – dynamic programming

22 Back end

instruction register machine IR selection allocation code

errors

¯ have value in a register when used

¯ limited resources

¯ changes instruction choices

¯ can move loads and stores

¯ optimal allocation is difﬁcult

Modern allocators often use an analogy to graph coloring

23 Traditional three pass compiler

source frontIR middle IR back machine codeend end end code

errors

Code Improvement

¯ analyzes and changes IR

¯ goal is to reduce runtime

¯ must preserve values

24 Optimizer (middle end)

IR IR IRopt1 ... opt n IR

errors

Modern optimizers are usually built as a set of passes

Typical passes

¯ constant propagation and folding

¯ code motion

¯ reduction of operator strength

¯ common subexpression elimination

¯ redundant store elimination

¯ dead code elimination

25 The Tiger compiler

Pass 2

Environ-

Tablesments IR Trees Pass 1 Pass 3 Pass 4 IR Trees

Parsing Semantic Translate Canon- Instruction Lex ParseActions Analysis Translate calize Selection Assem Tokens Abstract Syntax Reductions Frame Source Program Frame Layout

Control Data Register Code Flow Flow Allocation Emission Assembler Linker Analysis Analysis

Pass 5Pass 6 Pass 7 Pass 8 Pass 9 Pass 10 Machine Language Relocatable Object Code Assembly Language Register Assignment Flow Graph Interference Graph Assem 26 The Tiger compiler phases

Lex Break source file into individual words, or tokens Parse Analyse the phrase structure of program Parsing Build a piece of abstract syntax tree for each phrase Actions Semantic Determine what each phrase means, relate uses of variables to their Analysis definitions, check types of expressions, request translation of each phrase Frame Place variables, function parameters, etc., into activation records (stack Layout frames) in a machine-dependent way Translate Produce intermediate representation trees (IR trees), a notation that is not tied to any particular source language or target machine Canonicalize Hoist side effects out of expressions, and clean up conditional branches, for convenience of later phases Instruction Group IR-tree nodes into clumps that correspond to actions of target- Selection machine instructions Control Flow Analyse sequence of instructions into control flow graph showing all Analysis possible flows of control program might follow when it runs Data Flow Gather information about flow of data through variables of program; e.g., Analysis liveness analysis calculates places where each variable holds a still- needed (live) value Register Choose registers for variables and temporary values; variables not si- Allocation multaneously live can share same register Code Replace temporary names in each machine instruction with registers Emission

27 A straight-line programming language

¯ A straight-line programming language (no loops or conditionals):

Stm ! Stm ; Stm CompoundStm

Stm id ! := Exp AssignStm

´ Stm ÔÖiÒØ ! ExpList µ PrintStm

Exp id ! IdExp

Exp ÒÙÑ ! NumExp

Exp ! Exp Binop Exp OpExp

Exp ´ ! Stm ; Exp µ EseqExp

ExpList ! Exp ; ExpList PairExpList

ExpList ! Exp LastExpList

Binop· ! Plus

Binop ! Minus

Binop¢ ! Times

Binop= ! Div

¯ e.g.,

a a :; = a ´ 5 · 3; ÔÖiÒØ b ´ := 1; µ µ 10µ a b ´ ¢ ; ÔÖiÒØ

prints:8¼

7 8

28 Tree representation

a a :; = a ´ 5 · 3; ÔÖiÒØ b ´ := 1; µ µ 10µ a b ´ ¢ ; ÔÖiÒØ

CompoundStm

AssignStm CompoundStm

a OpExp AssignStm PrintStm

LastExpList NumExp Plus NumExp b EseqExp IdExp 5 3 b PrintStm OpExp

PairExpList NumExp Times IdExp

IdExp LastExpList 10 a

a OpExp

IdExpMinus NumExp

a1 This is a convenient internal representation for a compiler to use.

29 }

ßhead=h;} hµ Äa×ØEÜÔÄi×Ø´EÜÔ ÔÙbÐic

head; EÜÔ

ß EÜÔÄi×Ø eÜØeÒd× Äa×ØEÜÔÄi×Ø cÐa×× }

} ßid=i;} iµ ÁdEÜÔ´ËØÖiÒg

} ØaiÐ=Ø; head=h; ß id; ËØÖiÒg

Øµ EÜÔÄi×Ø h¸ ÈaiÖEÜÔÄi×Ø´EÜÔ ÔÙbÐic ß EÜÔ eÜØeÒd× ÁdEÜÔ cÐa××

ØaiÐ; EÜÔÄi×Ø head; EÜÔ ß} EÜÔ cÐa×× ab×ØÖacØ

ß EÜÔÄi×Ø eÜØeÒd× ÈaiÖEÜÔÄi×Ø cÐa××

ß} EÜÔÄi×Ø cÐa×× ab×ØÖacØ }

} } eÜÔ×=e; ß

} eÜÔ=e; ×ØÑ=×; ß eµ ÈÖiÒØËØÑ´EÜÔÄi×Ø

eµ EÜÔ ×¸ E×eÕEÜÔ´ËØÑ eÜÔ×; EÜÔÄi×Ø

eÜÔ; EÜÔ ×ØÑ; ËØÑ ß ËØÑ eÜØeÒd× ÈÖiÒØËØÑ cÐa××

ß EÜÔ eÜØeÒd× E×eÕEÜÔ cÐa×× }

} } eÜÔ=e; id=i; ß

} ÖighØ=Ö; ÓÔeÖ=Ó; ÐefØ=Ð; ß eµ EÜÔ i¸ A××igÒËØÑ´ËØÖiÒg

Öµ EÜÔ Ó¸ iÒØ Ð¸ ÇÔEÜÔ´EÜÔ eÜÔ; EÜÔ id; ËØÖiÒg

ÈÐÙ×=½¸ÅiÒÙ×=¾¸ÌiÑe×=¿¸DiÚ=4; ß

iÒØ ×ØaØic fiÒaÐ ËØÑ eÜØeÒd× A××igÒËØÑ cÐa××

ÓÔeÖ; iÒØ ÖighØ; ÐefØ¸ EÜÔ }

ß EÜÔ eÜØeÒd× ÇÔEÜÔ cÐa×× } ×ØÑ¾=×¾; ×ØÑ½=×½; ß

} ×¾µ ËØÑ ×½¸ CÓÑÔÓÙÒdËØÑ´ËØÑ

ßÒÙÑ=Ò;} Òµ ÆÙÑEÜÔ´iÒØ ×ØÑ¾; ×ØÑ½¸ ËØÑ Java classes for trees

ÒÙÑ; iÒØ ËØÑ eÜØeÒd× CÓÑÔÓÙÒdËØÑ cÐa××

ß EÜÔ eÜØeÒd× ÆÙÑEÜÔ cÐa×× ß} ËØÑ cÐa×× ab×ØÖacØ

30 Chapter 2: Lexical Analysis

31 Scanner

source tokens code scanner parser IR

errors

¯ maps characters into tokens – the basic unit of syntax

Ý; · Ü = Ü

becomes ¯

< < = id,> Ü < · id,> Ü ; id,> Ý

character string value for a token is a lexeme ¯

¯ typical tokens: number, id, · , ¹ , ¶ , » , dÓ , eÒd eliminates white space (tabs, blanks, comments )

¯ a key issue is speed for personal or classroom use is granted without fee provided that copies are not made or distributed for profit orµ commercialuse specialized advantage and that recognizer copies bear this (as notice opposed and full citation to onÐeÜ the first) page. To copy otherwise, to republish, to post on servers, or to redistribute to lists, requires prior specific permission and/or Copyright c 2000 by Antony L. Hosking. Permission to make digital or hard copies of part or all of this work fee. Request permission to publish from [email protected].

32 Specifying patterns

A scanner must recognize various parts of the language’s syntax Some parts are easy:

< j white space ³\Ø³ j

< > ³ < ³ > ws ::= ws ³ ³ j

ws³\Ø³ >

keywords and operators

speciﬁed as literal patterns: dÓ , eÒd

comments

opening and closing delimiters:¶» ¡¡¡ »¶

33 Specifying patterns

A scanner must recognize various parts of the language’s syntax

Other parts are much harder:

identiﬁers alphabetic followed by k alphanumerics ( ,$,&,...)

numbers

integers: 0 or digit from 1-9 followed by digits from 0-9

decimals: integer ³º³ digits from 0-9

reals: (integer or decimal) ³E³ (+ or -) digits from 0-9

complex: ³´³ real ³¸³ real ³µ³

We need a powerful notation to specify these patterns

34 Operations on languages

Operation Deﬁnition

union of L and M L [ Mf = s j s ¾ L or s ¾ M g

written L [ M

concatenation of L and M LMf = st j s ¾ L and t ¾ M g written LM = ∞ £ i Kleene closure of L Ë L L i= 0 written L£ = ∞ · i positive closure of L Ë L L i= 1 written L·

35 Regular expressions

Patterns are often speciﬁed as regular languages

Notations used to describe a regular language (or a regular set) include both regular expressions and regular grammars

Regular expressions (over an alphabet Σ):

ε g 1. ε is a RE denoting the set f

Σ f g 2. if a ¾ , then a is a RE denoting a

3. if r and s are REs, denoting L´ r µ and L´ sµ , then: ´

´ ´ r µ is a RE denoting L´ r µ

´ j µ µ ´ µ ´ µ r s is a RE denoting L r L s ´

r µ´ sµ is a RE denoting L´ r µ L´ sµ

r µ is a RE denoting L´ r µ

£ £

If we adopt a precedence for operators, the extra parentheses can go away. We assume closure, then concatenation, then alternation as the order of precedence.

36 Examples

identiﬁer

letter´ ! a j bj j c::: j z j A j B j j C::: j Z µ

digit´ ! 0 j 1 j 2 j 3 j 4 j 5 j 6 j 7 j 8 j 9µ

id ! letter ´ letter j digit µ

numbers

´ ε µ ´ j j j j ::: j µ j integer j ´· ! 0 1 2 3 9 digit

decimal ! integer . ´ digit µ

real´ ! integer j µ decimal j ´· E µ digit

complex³´³ ! real ¸ real ³µ³

Numbers can get much more complicated

Most programming language tokens can be described with REs

We can use REs to build scanners automatically

37 Algebraic properties of REs

Axiom Description

r j s = sj r j is commutative

r´ j ´ s= j t µ r j sj µ t j is associative

´ µ = ´ µ rs t r st concatenation is associative ´

r ´ s= j t µ rsj rt concatenation distributes over j

sj t µ r = sr j tr ε εr = r is the identity for concatenation ε r = r

´ = £ £ ε ε µ r r j relation between and

= r ££ r £ £ is idempotent

38 Examples

Let f Σ = a ; b g

1. a j b denotes f a ; b g

2. ´ a j b µ´ a j b µ denotes f aa; ab; ba; bbg

i.e., ´ a j b µ´ a j = b µ aaj abj baj bb

ε ; ; ; g ;::: 3. a denotes f a aa aaa

ε 4. ´ a j b µ denotes the set of all strings of a’s and b’s (including )

£ £ £

i.e.,´ ´ a= j b µ a b

5. a j a b denotes f a ; b ; ab; aab; aaab; aaaabg ;:::

39 Recognizers

From a regular expression we can construct a

deterministic ﬁnite automaton (DFA)

Recognizer for identiﬁer:

letter digit

letter other 021

digit other accept 3

error

identiﬁer

letter´ ! a j bj j c::: j z j A j B j j C::: j Z µ

digit´ ! 0 j 1 j 2 j 3 j 4 j 5 j 6 j 7 j 8 j 9µ

id ! letter ´ letter j digit µ

40 Code for the recognizerØÓkeÒ

faÐ×e; dÓÒe

¶» ¼ ×ØaØe fÓÖ cÓde »¶ ¼; ×ØaØe

ÒeÜØ chaÖ chaÖ´µ;

chaÖ cÐa××

f µ dÓÒe ÒÓØ ÛhiÐe´

¶» ×ØÖiÒg eÑÔØÝ »¶ "" ÚaÐÙe ØÓkeÒ

¶» id aÒ bÙiÐdiÒg »¶ ½: ca×e ÒeÜØ ×ØaØe

f ×ÛiØch´×ØaØeµ cÐa××[chaÖ];

×ØaØe[cÐa××¸×ØaØe];

ØÓkeÒ

¶» ×ØaØe acceÔØ »¶ ¾: ca×e ÒeÜØ chaÖ

bÖeak; ØÓkeÒ ÚaÐÙe ØÓkeÒ chaÖ; · ÚaÐÙe

¶» eÖÖÓÖ »¶ ¿: chaÖ´µ; ca×e

bÖeak;

ØÖÙe; = dÓÒe ØÓkeÒ ÖeØÙÖÒ

g ideÒØifieÖ; = ØÝÔe

bÖeak;

ØÖÙe; = dÓÒe

eÖÖÓÖ; = ØÝÔe

ØÝÔe;

41 Tables for the recognizer

Two tables control the recognizer

a z A Z 0 9 other chaÖ cÐa××: value letter letter digit other

class 0 1 2 3 letter 1 1 — — ÒeÜØ ×ØaØe: digit 3 1 — — other 3 2 — —

To change languages, we can just change tables

42 Automatic construction

Scanner generators automatically construct code from regular expression- like descriptions

¯ construct a dfa

¯ use state minimization techniques

¯ emit code for the scanner (table driven or direct code )

A key issue in automation is an interface to the parser

ÐeÜ is a scanner generator supplied with UNIX

¯ emits C code for scanner

¯ provides macro deﬁnitions for each token (used in the parser)

43 Grammars for regular languages

Can we place a restriction on the form of a grammar to ensure that it describes a regular language?

Provable fact:

For any RE r,thereisagrammargsuch that L´ = r µ L´ g µ .

The grammars that generate regular sets are called regular grammars

Deﬁnition:

In a regular grammar, all productions have one of two forms:

1. A ! aA

2. A ! a where A is any non-terminal and a is any terminal symbol

These are also called type 3 grammars (Chomsky)

44 More regular languages

Example: the set of strings containing an even number of zeros and an even number of ones

s0 s1 1 0000 1

s2 s3 1

The RE is ´ 00 j 11´´ µ 01 j 10µ´ 00 j 11´ µ 01 j 10µ´ 00 j 11µ µ

£ £ £ £

45 More regular expressions

What about the RE ´ a j b µ abb ?

a j b

abb s0 s1 s2 s3

State s0 has multiple transitions on a!

µ nondeterministic ﬁnite automaton

a b

s0 f s0 ; s1 g f s0 g

s1 – f s2 g

s2 – f s3 g

46 Finite automata A non-deterministic ﬁnite automaton (NFA) consists of:

1. a set of states f S = s0 ;:::; sng 2. a set of input symbols Σ (the alphabet) 3. a transition function move mapping state-symbol pairs to sets of states

4. a distinguished start state s0 5. a set of distinguished accepting or ﬁnal states F

A Deterministic Finite Automaton (DFA) is a special case of an NFA:

1. no state has a ε-transition, and

2. for each state s and input symbol a, there is at most one edge labelled a leaving s.

A DFA accepts x iff. there exists a unique path through the transition graph from the s0 to an accepting state such that the labels along the edges spell x. 47 DFAs and NFAs are equivalent

1. DFAs are clearly a subset of NFAs

2. Any NFA can be converted into a DFA, by simulating sets of simulta- neous states:

¯ each DFA state corresponds to a set of NFA states

¯ possible exponential blowup

48 NFA to DFA using the subset construction: example 1

a j b

abb s0 s1 s2 s3

ab f

f g f ; g f g s0s0s1s0 f

s0; s1g f s0; s1g f s0; s2g f

s0; s2g f s0; s1g f s0; s3g

s0; s3g f s0; s1g f s0g

b a

abb

f s0g f s0; s1g f s0; s2g f s0; s3g

a a

49 Constructing a DFA from a regular expression

DFA minimized

RE DFA

NFA ε moves

ε RE ! NFA w/ moves build NFA for each term connect them with ε moves NFA w/ε moves to DFA construct the simulation the “subset” construction

DFA ! minimized DFA merge compatible states

DFA ! RE

k k 1 k 1 k 1 k 1 construct R = R ´ R µ R R ij ik kk £ kj ij

50 RE to NFA

ε µ N ´ a

N ´ a µ

ε N(A) A ε

ε ε N(B) B

N ´ A j B µ

N(A)A N(B) B

N ´ ABµ ε

ε εε N(A) A

N ´ A

51 RE to NFA: example

´ a j b µ abb

a 2 3

ε ε

1 6

εε

4 5

aj b b ε

a 23

εε

ε ε 01 67

εε

45 b

ε ´ aj bµ

£ abb 7 8 9 10 abb

52 NFA to DFA: the subset construction

Input: NFA N Output: A DFA D with states Dstates and transitions Dtrans

such that L ´ Dµ= L´ Nµ Method: Let s be a state in N and T be a set of states, and using the following operations:

Operation Deﬁnition ε ε-closure´ sµ set of NFA states reachable from NFA state s on -transitions alone ε ε -closure´ T µ set of NFA states reachable from some NFA state s in T on - transitions alone

move´ T ; aµ set of NFA states to which there is a transition on input symbol a from some NFA state s in T

ε ´ µ add state T = -closure s0 unmarked to Dstates

while 9 unmarked state T in Dstates mark T for each input symbol a

ε ´ ´ ; µµ U = -closure move T a

if U 6¾ Dstates then add U to Dstates unmarked

Dtrans[ T ; a]= U endfor endwhile ε -closure ´ s0 µ is the start state of D A state of D is accepting if it contains at least one accepting state in N

53 NFA to DFA using subset construction: example 2

a 2 3

ε ε

ε εabb 0 1 6 7 8910

εε

4 5 b

a b A B C

f A= 0; 1; 2; 4; 7g f D= 1; 2; 4; 5; 6; 7; 9g B B D

f B= 1; 2; 3; 4; 6; 7; 8g f E= 1; 2; 4; 5; 6; 7; 10g C B C

f C = 1; 2; 4; 5; 6; 7g D B E E B C

54 Limits of regular languages Not all languages are regular

One cannot construct DFAs to recognize these languages:

¯ k k ¯ f L = p q g

g r Σ £ f L = wcw j w ¾

Note: neither of these is a regular expression! (DFAs cannot count!)

But, this is a little subtle. One can construct DFAs for:

¯ alternating 0’s and 1’s

ε j µ´ ´ µ ε j µ ´ 1 01 0

£ sets of pairs of 0’s and 1’s

´ 01 j 10µ

55 So what is hard? Language features that can cause problems:

reserved words PL/I had no reserved words

ØheÒ; = eÐ×e eÐ×e eÐ×e; = ØheÒ ØheÒ ØheÒ if signiﬁcant blanks

½º¾5 = i ½¼ FORTRANdÓ and Algol68 ignore blanks

½¸¾5 = i ½¼ dÓ

string constants special characters in strings

ÒeÛÐiÒe , Øab deÐiÑiØeÖ , ÕÙÓØe , cÓÑÑeÒØ ﬁnite closures some languages limit identiﬁer lengths adds states to count length

FORTRAN 66 ! 6 characters

These can be swept under the rug in the language design

56 CÓÖÔÓÖaØiÓÒ ÁBÅ Óf Zadeck FºÃº DÖº ØÓ dÙe EÜaÑÔÐe

EÆD ½4

FÁÄE´½µ °

cÓÑÑeÒØ a i× Øhi× C

EÆD ½¿

CÇÆÌÁÆÍE ¿¼¼ ½¾

ÁF´Xµ¿¼¼¸¾¼¼ ½½

ÁF´XµÀ=½ ½¼

ÁF´Xµ=½ 9

DÇ9E½=½¸¾ 8

DÇ9E½=½ 7

µ=´¿µ FÇÊÅAÌ´4 ¾¼¼ 6

FÇÊÅAÌ´4Àµ=´¿µ ½¼¼ 5

FÇÊÅAÌ´½¼µ¸ÁF´½¼µ¸DÇ9E½ ÁÆÌEGEÊ How4 bad can it get?

CÀAÊACÌEÊ¶´A¹Bµ´A¹Bµ ÁÅÈÄÁCÁÌ ¿

ÈAÊAÅEÌEÊ´A=6¸B=¾µ ¾

ÁÆÌEGEÊFÍÆCÌÁÇÆA ½

57 Chapter 3: LL Parsing

58 The role of the parser

source tokens code scanner parser IR

errors

Parser

¯ performs context-free syntax analysis

¯ guides context-sensitive analysis

¯ constructs an intermediate representation

¯ produces meaningful error messages

¯ attempts error correction

Forfor personal the next or classroom few use weeks, is granted we without will fee look provided at that parser copies are construction not made or distributed for profit or commercial advantage and that copies bear this notice and full citation on the first page. To copy otherwise, to republish, to post on servers, or to redistribute to lists, requires prior specific permission and/or Copyright c 2000 by Antony L. Hosking. Permission to make digital or hard copies of part or all of this work fee. Request permission to publish from [email protected].

59 Syntax analysis

Context-free syntax is speciﬁed with a context-free grammar .

Formally, a CFG G is a 4-tuple ´ Vt ; Vn ; S ; Pµ , where:

Vt is the set of terminal symbols in the grammar. For our purposes, Vt is the set of tokens returned by the scanner.

Vn, the nonterminals, is a set of syntactic variables that denote sets of (sub)strings occurring in the language. These are used to impose a structure on the grammar.

S is a distinguished nonterminal ´ S ¾ Vn µ denoting the entire set of strings

in L´ Gµ . This is sometimes called a goal symbol . P is a ﬁnite set of productions specifying how terminals and non-terminals can be combined to form strings in the language. Each production must have a single non-terminal on its left hand side.

The set V = Vt [ Vn is called the vocabulary of G 60 Notation and terminology

¯ a ; ¾ b ; c;::: Vt

¯ A; ¾ B; C;::: Vn

¯ U; V¾ ; W;::: V

α; ¾ β; γ;::: ¯ V

£ u; ¾ v; w;::: Vt

γ γ α β µ αγβ ! If A ! then A is a single-step derivation using A

Similarly, µ and µ denote derivations of 0 and 1 steps

£ ·

β β If S µ then is said to be a sentential form of G

Lf ´ G= µ w ¾ V j S µ wg , w ¾ L´ Gµ is called a sentence of G

t ·

£ £

β g\ β ¾ µ Note, Lf ´ G= µ V S V

£ t 61 Syntax analysis

Grammars are often written in Backus-Naur form (BNF).

Example:

1 h goali h ::= expri

2 h expri h ::= exprih oph i expri

3 ÒÙÑ j

4 id j

5 h opi · ::=

6 j

7 £ j

8 = j This describes simple expressions over numbers and identiﬁers.

In a BNF for a grammar, we represent 1. non-terminals with angle brackets or capital letters

2. terminals with ØÝÔeÛÖiØeÖ font or underline 3. productions as in the example

62 Scanning vs. parsing Where do we draw the line?

[ j ] Þ ÞA a ]´[ Þ ÞA term a [ ::= 0 9 ]µ

] 9 ¼ ][ 9 ½ 0[ j

= j £ j op j · ::=

expr´ ::= term opµ term

Regular expressions are used to classify:

¯ identiﬁers, numbers, keywords

¯ REs are more concise and simpler for tokens than a grammar

¯ more efﬁcient scanners can be built from REs (DFAs) than grammars

Context-free grammars are used to count:

¯ brackets: ´µ , begiÒ ...eÒd , if ...ØheÒ ...eÐ×e imparting structure: expressions

Syntactic analysis is complicated enough: grammar for C has around 200 productions. Factoring out lexical analysis as a separate phase makes compiler more manageable.

63 Derivations

We can view the productions of a CFG as rewriting rules.

Using our example CFG:

h µ

h h goalµ i expri h µ

h µ exprih oph i expri

ih h i ih h i i expr op expr op expr h µ

id,ih Ü opih exprh i opih expri h µ

h · id,i Ü exprh i oph i expri h µ

h µ h · id,i Ü num,h i ¾ oph i expri

h · id,i Ü num, h i£ ¾ expri

h · id,i Ü num, h i£ ¾ id,i Ý

We have derived the sentenceÝ £ ¾ · Ü .

We denoteid £ ÒÙÑ this· h id goaliµ .

Such a sequence of rewrites is a derivation or a parse.

The process of discovering a derivation is called parsing.

64 Derivations

At each step, we chose a non-terminal to replace.

This choice can lead to different derivations.

Two are of particular interest:

leftmost derivation the leftmost non-terminal is replaced at each step rightmost derivation the rightmost non-terminal is replaced at each step

The previous example was a leftmost derivation.

65 Rightmost derivation

For theÝ string£ ¾ · Ü :

h µ

h h goalµ i expri h µ

exprh i opih expri h µ

h µ exprh i opih id,i Ý

£h i£ i Ý expr id, h µ

exprh i opih expr h i£ id,i Ý h µ

exprh i opih num, h i£ ¾ id,i Ý h µ

exprh · i num, h i£ ¾ id,i Ý

h · id,i Ü num, h i£ ¾ id,i Ý

id £ Again,ÒÙÑ · h id goaliµ .

66 Precedence

goal

expr

expr op expr

expr op expr *

Treewalk evaluation computes¾ · (Ü Ý ) £ — the “wrong” answer!

Should be· Ü Ý £ (¾ )

67 Precedence

These two derivations point out a problem with the grammar.

It has no notion of precedence, or implied order of evaluation. To add precedence takes additional machinery:

1 h goali h ::= expri

2 h expri h ::= exprh · i term i

3 h j expr h i term i

4 h j term i

5 h term i h ::= term i£h factori

6 h j termh = i factori

7 h j factori

8 h factori ÒÙÑ ::=

9 id j

This grammar enforces a precedence on the derivation:

¯ terms must be derived from expressions

¯ forces the “correct” tree

68 Precedence

Now, for theÝ string£ ¾ · Ü :

h µ

h h goalµ i expri h µ

exprh · i term i h µ

h µ exprh · i term i£h factori

h · i i£h i Ý expr term id, h µ

exprh · i factori£h id,i Ý h µ

exprh · i num, h i£ ¾ id,i Ý h µ

h µ termh · i num, h i£ ¾ id,i Ý

factorh · i num, h i£ ¾ id,i Ý

h · id,i Ü num, h i£ ¾ id,i Ý

id £ Again,ÒÙÑ · h id goaliµ , but this time, we build the desired tree.

69 Precedence

goal

expr

expr + term

term term * factor

factor factor

Treewalk evaluation computes· Ü Ý £ (¾ )

70 Ambiguity

If a grammar has more than one derivation for a single sentential form, then it is ambiguous

Example: h if j

h stmti ::=h if h exprØheÒ i stmti ×ØÑØ× ÓØheÖ j

h exprØheÒ i h stmteÐ×e i stmti

Consider deriving the sentential form:

if if E1 ØheÒ E2 ØheÒ S1 eÐ×e S2

It has two derivations.

This ambiguity is purely grammatical.

It is a context-free ambiguity.

71 Ambiguity

May be able to eliminate ambiguitiesh by rearrangingj the grammar:

h h i h i stmt ::= matched h

i unmatched ×ØÑØ× ÓØheÖ j

matchedi ::=h if h exprØheÒ i matchedh eÐ×e i matchedi

h if j

unmatched i ::=h if h exprØheÒ i stmti

h exprØheÒ i matchedh eÐ×e i unmatched i

This generates the same language as the ambiguous grammar, but applies the common sense rule:

match each eÐ×e with the closest unmatched ØheÒ

This is most likely the language designer’s intent.

72 Ambiguity

Ambiguity is often due to confusion in the context-free speciﬁcation.

Context-sensitive confusions can arise from overloading.

Example:

f´½7µ = a

In many Algol-like languages, f could be a function or subscripted variable.

Disambiguating this statement requires context:

¯ need values of declarations

¯ not context-free

¯ really an issue of type

Rather than complicate parsing, we will handle this separately.

73 Parsing: the big picture

tokens

parser grammar parser generator

code IR

Our goal is a ﬂexible parser generator system

74 Top-down versus bottom-up

Top-down parsers

¯ start at the root of derivation tree and ﬁll in

¯ picks a production and tries to match the input

¯ may require backtracking

¯ some grammars are backtrack-free (predictive)

Bottom-up parsers

¯ start at the leaves and ﬁll in

¯ start in a state valid for legal ﬁrst tokens

¯ as input is consumed, change state to encode possibilities (recognize valid preﬁxes )

¯ use a stack to store both state and sentential forms

75 Top-down parsing

A top-down parser starts with the root of the parse tree, labelled with the start or goal symbol of the grammar. To build a parse, it repeats the following steps until the fringe of the parse tree matches the input string

α 1. At a node labelled A, select a production A ! and construct the appropriate child for each symbol of α 2. When a terminal is added to the fringe that doesn’t match the input string, backtrack

3. Find the next node to be expanded (must have a label in Vn)

The key is selecting the right production in step 1

µ should be guided by input string

76 Simple expression grammar

Recall our grammar for simple expressions:

1 h goali h ::= expri

2 h expri h ::= exprh · i term i

3 h j expr h i term i

4 h j term i

5 h term i h ::= term£h i factori

6 h j termh = i factori

7 h j factori

8 h factori ÒÙÑ ::=

9 id j

Consider the inputÝ string£ ¾ Ü

77 Example Prod’n Sentential form Input

– h goalÝ i £ ¾ Ü "

1 h exprÝ i £ ¾ Ü "

2 h exprh Ý · i £ term¾ i Ü "

4 h termh Ý · i £ term¾ i Ü "

7 h factorÝ h · i £ term¾ i Ü "

9h · id Ý term£ i ¾ Ü "

–h · id Ý term£ i ¾ " Ü

–h exprÝ i £ ¾ Ü "

3 h exprh Ý i £ term¾ i Ü "

4 h termh Ý i £ term¾ i Ü "

7 h factorÝ h i £ term¾ i Ü "

9h id Ý term£ i ¾ Ü "

–h id Ý term£ i ¾ " Ü

7h id Ý factor£ i ¾ " Ü

8ÒÙÑ id Ý £ ¾ " Ü

–ÒÙÑ id Ý "£ ¾ Ü

–h id Ý term£ i ¾ " Ü

5h id Ý termh £ £ i ¾ factor" i Ü

7h id Ý factorh £ £ i ¾ " factor i Ü

h £ 8ÒÙÑ id Ý £ factor¾ " i Ü

h £ –ÒÙÑ id Ý "£ factor¾ i Ü

h £ –ÒÙÑ id Ý " £ factor¾ i Ü

id £ 9ÒÙÑ id Ý " £ ¾ Ü

id £ –ÒÙÑ " id Ý £ ¾ Ü 78 Example

Another possible parseÝ £ ¾ for Ü

Prod’n Sentential form Input

– h goali Ý £ ¾ Ü "

1 h expri Ý £ ¾ Ü "

2 h exprh · i term i Ý £ ¾ Ü "

2 h exprh · i termh · i Ý £ term¾ i Ü "

2 h exprh · i term¡¡¡ · i Ý £ ¾ Ü "

2h exprh · i term¡¡¡ · i Ý £ ¾ Ü "

2¡¡¡ Ý £ ¾ Ü "

If the parser makes the wrong choices, expansion doesn’t terminate. This isn’t a good property for a parser to have.

(Parsers should terminate!)

79 Left-recursion

Top-down parsers cannot handle left-recursion in a grammar

Formally, a grammar is left-recursive if

α α 9 A ¾ Vn such that A µ A for some string

Our simple expression grammar is left-recursive

80 Eliminating left-recursion

To remove left-recursion, we can transform the grammar

Consider the grammar fragment: α h foo i h ::= foo i

j β α β where and do not start with h foo i

We can rewrite this as:

β h i j h foo i ::= bar

αh i bari ::= bar ε

where h bari is a new non-terminal

This fragment contains no left-recursion

81 Example Our expression grammar contains two cases of left-recursion

h j

h j h expri h ::= exprh · i term i

h i i expr term h

term i h j

term i h ::= term i£h factori h j

termh = i factori

factori

Applying the transformation gives h

i j

h expri h ::= term ih expr

i i

¼ ¼

exprh · ::= termh i expr ε h

i h

h j termh i expr

i j

term i h ::= factorih term

i i

¼ ¼

term £h ::= factorih term ε

h = j factorih term With this grammar, a top-down parser will

¯ terminate

¯ backtrack on some inputs

82 Example

This cleaner grammar deﬁnes the same language

1 h goali h ::= expri

2 h expri h ::= termh · i expri

3 h j term i h expri

4 h j term i

5 h term i h ::= factori£h term i

6 h j factorh = i term i

7 h j factori

8 h factori ÒÙÑ ::=

9 id j

It is

¯ right-recursive ε ¯ free of productions

Unfortunately, it generates different associativity Same syntax, different meaning 83 Example

Our long-suffering expression grammar:

1 h goali h ::= expri

2 h expri h ::= term ih expr

i i

¼ ¼

3 h exprh · ::= term ih expr

4 h j term ih expr ε 5 j

6 h term i h ::= factorih term

i i

¼ ¼

7 h term £h ::= factorih term

8 h = j factorih term ε 9 j

10 h factori ÒÙÑ ::=

11 id j

Recall, we factored out left-recursion 84 How much lookahead is needed?

We saw that top-down parsers may need to backtrack when they select the wrong production Do we need arbitrary lookahead to parse CFGs?

¯ in general, yes

¯ use the Earley or Cocke-Younger, Kasami algorithms Aho, Hopcroft, and Ullman, Problem 2.34 Parsing, Translation and Compiling, Chapter 4

Fortunately

¯ large subclasses of CFGs can be parsed with limited lookahead

¯ most programming language constructs can be expressed in a grammar that falls in these subclasses

Among the interesting subclasses are: LL(1): left to right scan, left-most derivation, 1-token lookahead; and LR(1): left to right scan, right-most derivation, 1-token lookahead

85 Predictive parsing

Basic idea:

α j β For any two productions A ! , we would like a distinct way of choosing the correct production to expand.

α αµ For some RHS ¾ G, deﬁne FIRST´ as the set of tokens that appear ﬁrst in some string derived from α

αµ α µ γ That is, for some w ¾ V , w ¾ FIRST ´ iff. w .

t £

Key property:

α ! β Whenever two productions A ! and A both appear in the grammar, we would like

= βµ φ α\ µ ´ FIRST ´ FIRST

This would allow the parser to make a correct choice with a lookahead of only one symbol!

The example grammar has this property!

86 Left factoring

What if a grammar does not have this property?

Sometimes, we can transform a grammar to have this property.

For each non-terminal A ﬁnd the longest preﬁx α common to two or more of its alternatives.

ε if α 6= then replace all of the A productions

αβ j αβj ¡¡¡ j αβ A ! 1 2 n with

α ¼ A ! A

! ¼ β j β ¡¡¡ j A β1 j 2 n

where A¼ is a new non-terminal.

Repeat until no two alternatives for a single non-terminal have a common preﬁx.

87 Example

Consider a right-recursive version of the expression grammar:

1 h goali h ::= expri

2 h expri h ::= termh · i expri

3 h j term i h expri

4 h j term i

5 h term i h ::= factori£h term i

6 h j factorh = i term i

7 h j factori

8 h factori ÒÙÑ ::=

9 id j

To choose between productions 2, 3, & 4, the parser must see past the ÒÙÑ

or id and look at the · , , £ ,or = . φ FIRST ´ \ 2µ FIRST ´ \ 3µ FIRST ´ 6= 4µ

This grammar fails the test.

Note: This grammar is right-associative.

88 Example

There are two nonterminals that must be left factored:

h j

h i h = h · i i expr :: term expr h j

term i h expri

term i

h j

term i h ::= factori£h term i h j

factorh = i term i

factori

Applying the transformation gives us: h

h expri h ::= term ih expr h j

exprh · ::= expri j

expri

ε h

h = j h term i h ::= factorih term

£h = i term :: term j

term i ε

89 Example

Substituting back into the grammar yields

1 h goali h ::= expri

2 h expri h ::= term ih expr

3 h exprh · ::= expri

4 h j expri ε 5 j

6 h term i h ::= factorih term

7 h term £h ::= term i

8 h = j term i ε 9 j

10 h factori ÒÙÑ ::=

11 id j

Now, selection requires only a single token lookahead.

Note: This grammar is still right-associative.

90 Example

Sentential form Input

– h goali Ý £ ¾ Ü "

1 h expri Ý £ ¾ Ü "

2 h termih expr Ý £ ¾ Ü "

ih i

¼ ¼

6 h factorih term expr Ý £ ¾ Ü "

ih i

¼ ¼

11 h id term expr Ý £ ¾ Ü "

ih i

¼ ¼

– h id term expr Ý £ ¾ ¹ " Ü

ε h ¾ ¹ " Ü 9 id expr

h 4 id expri Ý £ ¾ ¹ " Ü

h – id expri Ý £ ¾ " Ü

h 2 id termih expr Ý £ ¾ " Ü

ih i

¼ ¼

h 6 id factorih term exprÝ £ ¾ " Ü

ih i

¼ ¼

h 10ÒÙÑ id term expr Ý £ ¾ " Ü

ih i

¼ ¼

h ÒÙÑ – id term expr Ý ¶ " ¾ Ü

h £ ÒÙÑ 7 id termih expr Ý ¶ " ¾ Ü

h £ ÒÙÑ – id termih expr Ý " £ ¾ Ü

ih i

¼ ¼

h £ ÒÙÑ 6 id factorih termÝ " expr£ ¾ Ü

ih i

¼ ¼

h id £ 11ÒÙÑ id term exprÝ " £ ¾ Ü

ih i

¼ ¼

h id £ ÒÙÑ – id term expr" Ý £ ¾ Ü

h id £ ÒÙÑ 9 id expr " Ý £ ¾ Ü

id £ ÒÙÑ 5 id " Ý £ ¾ Ü

The next symbol determined each choice correctly.

91 Back to left-recursion elimination

Given a left-factored CFG, to eliminate left-recursion:

α if 9 A ! A then replace all of the A productions

αj j β::: j γ A ! A with

A ! NA

j β::: j γ N !

! j A¼ αA¼ ε

where N and A¼ are new productions.

Repeat until there are no left-recursive productions.

92 Generality

Question:

By left factoring and eliminating left-recursion , can we transform an arbitrary context-free grammar to a form where it can be pre- dictively parsed with a single token lookahead?

Answer:

Given a context-free grammar that doesn’t meet our conditions, it is undecidable whether an equivalent grammar exists that does meet our conditions.

Many context-free languages do not have such a grammar:

n n n 2n

f a 0b j n f 1 g a 1b j n 1 g

[ Must look past an arbitrary number of a’s to discover the 0 or the 1 and so determine the derivation.

93 eÜÔÖ ÖeØÙÖÒ eÐ×e

Recursive descent parsingEÊÊÇÊ; ÖeØÙÖÒ

ØheÒ EÊÊÇÊµ = ´ØeÖÑ´µ if Now, we can produce a simple recursive descent parser from the (right-

eÜÔÖ: associative) grammar.

ÒeÜØ ØÓkeÒ EÊÊÇÊ; ÖeØÙÖÒ

ØheÒ gÓaÐ: EÇFµ 6= ØÓkeÒ j EÊÊÇÊ = ´eÜÔÖ´µ if

ØÓkeÒ´µ;

eÜÔÖ

ÒeÜØ ØÓkeÒ

ÔÖiÑe´µ;

ØheÒ ÈÄÍËµ = ´ØÓkeÒ if ÒeÜØ ØÓkeÒ

ÔÖiÑe: ØheÒ ÅÁÆÍËµ = ´ØÓkeÒ if eÐ×e

eÜÔÖ´µ; ÖeØÙÖÒ

ØÓkeÒ´µ;

ÇÃ; ÖeØÙÖÒ eÐ×e

eÜÔÖ´µ; ÖeØÙÖÒ

ØÓkeÒ´µ;

94 ØeÖÑ ÖeØÙÖÒ eÐ×e

EÊÊÇÊ; ÖeØÙÖÒ Recursive descent parsing

ØheÒ EÊÊÇÊµ = ´facØÓÖ´µ if

ØeÖÑ:

ØeÖÑ

ÒeÜØ ØÓkeÒ

ÔÖiÑe´µ;

ØheÒ ÅÍÄÌµ = ´ØÓkeÒ if ÒeÜØ ØÓkeÒ

ÒeÜØ ØÓkeÒ

ÔÖiÑe: ØheÒ DÁÎµ = ´ØÓkeÒ if eÐ×e

ØheÒ ÆÍÅµ = ´ØÓkeÒ if

ØeÖÑ´µ; ÖeØÙÖÒ

facØÓÖ:

ØÓkeÒ´µ;

ÇÃ; ÖeØÙÖÒ eÐ×e

ØeÖÑ´µ; ÖeØÙÖÒ

ØÓkeÒ´µ;

ÒeÜØ ØÓkeÒ

ØheÒ ÁDµ = ´ØÓkeÒ if eÐ×e

ÇÃ; ÖeØÙÖÒ

ØÓkeÒ´µ;

EÊÊÇÊ; ÖeØÙÖÒ eÐ×e

ÇÃ; ÖeØÙÖÒ

ØÓkeÒ´µ;

95 Building the tree

One of the key jobs of the parser is to build an intermediate representation of the source code. To build an abstract syntax tree, we can simply insert code at the appropriate points:

ØeÖÑ ¯

ØeÖÑ´µ ¯ facØÓÖ´µ ¯ can stack nodes id , ÒÙÑ

ÔÖiÑe´µ can stack nodes £ , =

can pop 3, build and push subtree eÜÔÖ´µ ¯

eÜÔÖ ¯ ÔÖiÑe´µ can stack nodes · , can pop 3, build and push subtree

gÓaÐ´µ ¯ can pop and return tree

96 Non-recursive predictive parsing

Observation:

Our recursive descent parser encodes state information in its run- time stack, or call stack.

Using recursive procedure calls to implement a stack abstraction may not be particularly efﬁcient.

This suggests other implementation methods:

¯ explicit stack, hand-coded parser

¯ stack-based, table-driven parser

97 Non-recursive predictive parsing

Now, a predictive parser looks like:

stack

source tokens table-driven scanner IR code parser

parsing tables

Rather than writing code, we build tables.

Building tables can be automated!

98 Table-driven parsers

A parser generator system often looks like:

stack

source tokens table-driven scanner IR code parser

parser parsing grammar generator tables

This is true for both top-down (LL) and bottom-up (LR) parsers

99 Non-recursive predictive parsing

ÒeÜØ ØÓkeÒ

X ÔÓÔ Input:ËØack[··ØÓ×] a string w and a parsing table M for G

ØheÒ ØÓkeÒ = X if

EÇF ËØack[ØÓ×]

ØheÒ EÇF ÓÖ ØeÖÑiÒaÐ a i× X if

¼ ØÓ×

ËØack[ØÓ×] X

Start Symbol ÖeÔeaØ

ÒeÜØ ØÓkeÒ ØÓkeÒ´µ

»¶ eÐ×e

eÖÖÓÖ´µ eÐ×e

ØÓkeÒ´µ

if ÔÙ×h

X is a non-terminal ¶» X ÔÓÔ

= M [X¸ØÓkeÒ] X ! Y1Y2 ¡¡¡ YkØheÒ

EÇF = X ÙÒØiÐ Y ; Y; ¡¡¡ ; Y k k 1 1

eÖÖÓÖ´µ eÐ×e

100 Non-recursive predictive parsing

What we need now is a parsing table M. Our expression grammar: Its parse table:

1 h goali h ::= expri †

i id ÒÙÑ · £ = $

2 h expri h ::= term ih expr

h i

¼ goal 1 1 – – – – –

3 h exprh · ::= expri

h expri 2 2 – – – – –

h j i 4 expr i

¼ ε h expr – – 3 4 – – 5 5 j

h i ¼ term 6 6 – – – – –

h i h = ih

6 term :: factor term i

¼ i

h ¼ term – – 9 9 7 8 9 7 h term £h ::= term i

h factori 11 10 – – – – – 8 h = j term i ε 9 j

10 h factori ÒÙÑ ::=

11 id j

† we use $ to represent EÇF

101 FIRST

αµ For a string of grammar symbols α, deﬁne FIRST ´ as:

¯ α ¯ the set of terminal symbols that begin strings derived from :

α µ βg f a ¾ Vt j a

ε ε ¾ ´ αµ If α µ then FIRST

£ α αµ FIRST ´ contains the set of tokens valid in the initial position in

To build FIRST ´ X µ :

1. If X ¾ Vt then FIRST ´ X µ is f X g

ε ε ´ µ 2. If X ! then add to FIRST X .

3. If X ! Y1Y2 ¡¡¡ Yk:

εg ´ µ (a) Put FIRST ´ Y f 1 µ in FIRST X

ε¾ \ ¡¡¡ ´ \ µ ´ µ 8 < FIRST FIRST (b) i :1 i k,if Y1 Yi 1 ε ¡¡¡ µ

(i.e., Y Y )

1 i£ 1

εg ´ µ then put FIRST ´ Y f i µ in FIRST X

ε ´ µ (c) If ε ¾ FIRST \ ¡¡¡ ´ Y\ 1 µ FIRST ´ Yk µ then put in FIRST X Repeat until no more additions can be made.

102 FOLLOW

For a non-terminal A, deﬁne FOLLOW´ Aµ as

the set of terminals that can appear immediately to the right of A in some sentential form

Thus, a non-terminal’s FOLLOW set speciﬁes the tokens that can legally appear after it. A terminal symbol has no FOLLOW set.

To build FOLLOW´ Aµ :

1.Put$inFOLLOWh ´ goalµ i α β 2. If A ! B :

εg ´ µ β f µ (a) Put FIRST ´ in FOLLOW B

βµ β µ ε ´ µ α ε¾ ´ ε ! (b) If β = (i.e., A B)or FIRST (i.e., ) then put FOLLOW A

in FOLLOW´ Bµ Repeat until no more additions can be made

103 LL(1) grammars

Previous deﬁnition A grammar G is LL(1) iff. for all non-terminals A, each distinct pair of pro- φ βµ ´ = γµ β ! γ ´ ductions A ! and A satisfy the condition FIRST FIRST .

Ì ε What if A µ ?

£ Revised deﬁnition α α j j α ¡¡¡ j A grammar G is LL(1) iff. for each set of productions A ! 1 2 n:

α µ α ; µ ; ´ α ;::: µ ´ 1. FIRST ´ 1 FIRST 2 FIRST n are all pairwise disjoint

φ8 ; ; 6= ε ´ α µ ´ = µ 2. If α µ then FIRST FOLLOW A 1 j n i j.

£ i j

Ì If G is ε-free, condition 1 is sufﬁcient.

104 LL(1) grammars

Provable facts about LL(1) grammars:

1. No left-recursive grammar is LL(1) 2. No ambiguous grammar is LL(1) 3. Some languages have no LL(1) grammar 4. A ε–free grammar where each alternative expansion for A begins with a distinct terminal is a simple LL(1) grammar.

Example

S ! aS j a

is not LL(1) because FIRST ´ aS= µ FIRSTf ´ = aµ ag

S ! aS

! j S ¼ aS ¼ ε accepts the same language and is LL(1)

105 LL(1) parse table construction

Input: Grammar G Output: Parsing table M Method: α 1. 8 productions A ! :

α [ ; ] αµ ! (a) 8 a ¾ FIRST ´ , add A to M A a

αµ (b) If ε ¾ FIRST ´ :

α [ ; ] i. 8 b ¾ FOLLOW´ Aµ , add A ! to M A b

α [ ; ] ii. If $ ¾ FOLLOW´ Aµ then add A ! to M A $

2. Set each undeﬁned entry of M to eÖÖÓÖ

If 9 M [ A; a ] with multiple entries then grammar is not LL(1).

ε Note: recall a; b ¾ Vt,soa; b6 = 106 Example Our long-suffering expression grammar:

S ! E T ! FT

£ ! ¼ ¼ ε E ! TE T T= j T j

· !

ÒÙÑ j ε id ! E E j E j F

FIRST FOLLOW

g id ; S ÒÙÑ f f $g

g id ; E ÒÙÑ f f $ g

id ÒÙÑ · £ =

g ; ε· ; f g E f $ $

g id ; T ÒÙÑ f ; ; · f $g S S ! E S ! E

¼ ¼ ¼

g ;= ε£ ; ; ; · f g ! ! T f $E E TE E TE

· ! ! ! ¼ ¼ ¼ ¼ ε g id ; FÒÙÑ f ;=; £ ; ; · f $g E E E E E E

¼ ¼

id g id f TT! FT T ! FT

! ! !£ = ! ! ¼ ¼ ε ¼ ε ¼ ¼ ¼ ε ÒÙÑ g ÒÙÑ f T T T T TTTT

£ f£g Fid F! ÒÙÑ F !

= g = f

· g · f

f g

107 A grammar that is not LL(1)

h if j

::: j h stmth i if ::= h exprØheÒ i stmti

h exprØheÒ i h stmteÐ×e i stmti

Left-factored: h

::: j i

h stmth i if ::= h exprØheÒ i stmth i stmt

i ¼ ε h stmt eÐ×e ::= stmtj i

f = µ i

g εeÐ×e ; Now, FIRSTh ´ stmt

; eÐ×e f = µ i

Also, FOLLOWh ´ stmt $ g

µ i 6= g eÐ×e f = µ i

¼ ¼

Ì φ But, FIRSTh ´ stmt FOLLOWh ´ stmt

On seeing eÐ×e , conﬂict between choosing

i i ¼ ¼ ε h h stmt eÐ×e ::= stmti and h stmt ::=

µ grammar is not LL(1)! The ﬁx:

Put priority on h h stmt eÐ×e ::= stmti to associate eÐ×e with clos-

est previous ØheÒ .

108 Error recovery

Key notion:

¯ For each non-terminal, construct a set of terminals on which the parser can synchronize

¯ When an error occurs looking for A, scan until an element of SYNCH´ Aµ is found

Building SYNCH:

1. a ¾ FOLLOW´ Aµ µ a ¾ SYNCH´ Aµ

2. place keywords that start statements in SYNCH´ Aµ

3. add symbols in FIRST ´ Aµ to SYNCH´ Aµ If we can’t match a terminal on top of stack:

1. pop the terminal 2. print a message saying the terminal was inserted 3. continue the parse

(i.e., SYNCH´ = aµ Vt f ag ) 109 Chapter 4: LR Parsing

110 Some deﬁnitions

Recall

α For a grammar G, with start symbol S,anystringαsuch that S µ is

called a sentential form ¯

α ¾ α ´ µ ¯ If Vt , then is called a sentence in L G

Otherwise it is just a sentential form (not a sentence in L´ Gµ )

A left-sentential form is a sentential form that occurs in the leftmost derivation of some sentence.

A right-sentential form is a sentential form that occurs in the rightmost derivation of some sentence. for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and full citation on the first page. To copy otherwise, to republish, to post on servers, or to redistribute to lists, requires prior specific permission and/or Copyright c 2000 by Antony L. Hosking. Permission to make digital or hard copies of part or all of this work fee. Request permission to publish from [email protected].

111 Bottom-up parsing

Goal: starting at the leaves and working to the root.

Given an input string w andagrammarG, construct a parse tree by

The parser repeatedly matches a right-sentential form from the language against the tree’s upper frontier.

At each match, it applies a reduction to build on the frontier:

¯ each reduction matches an upper frontier of the partially built tree to the RHS of some production

¯ each reduction adds a node on top of the frontier

The ﬁnal result is a rightmost derivation, in reverse.

112 Example

Consider the grammar

1 a S ! ABe

2 A ! Abc

3 b j

4 d B !

and the input string abbcde

Prod’n. Sentential Form

3 a b bcde

2 a Abc de

4 a A d e 1 aABe – S

The trick appears to be scanning the input and ﬁnding valid sentential forms.

113 Handles

What are we trying to ﬁnd?

A substring α of the tree’s upper frontier that

α α matches some production A ! where reducing to A is one step in the reverse of a rightmost derivation

We call such a string a handle.

Formally:

β a handle of a right-sentential form γ is a production A ! and a position in γ where β may be found and replaced by A to produce the previous right-sentential form in a rightmost derivation of γ

β α αβ ! α µ i.e., if S µ Aw rm w then A in the position following is a £ rm handle of αβw

Because γ is a right-sentential form, the substring to the right of a handle contains only terminal symbols.

114 Handles

β w β αβ The handle A ! in the parse tree for w

115 Handles

Theorem:

If G is unambiguous then every right-sentential form has a unique handle.

Proof: (by deﬁnition)

1. G is unambiguous µ rightmost derivation is unique

β γ γ µ !

2. a unique production A applied to take i 1 to i

β 3. µ a unique position k at which A ! is applied

β 4. µ a unique handle A !

116 Example

The left-recursive expression grammar (original form )

Prod’n. Sentential Form

– h goali 1 h goali h ::= expri

1 h expri 2 h expri h ::= exprh · i term i

h h i i 3 h j expr h i term i 3 expr term

h h i h £ i i 4 h j term i 5 expr term factor

h i h = i£h i 5 term :: term factor 9 h exprh i termid £ i

h j h = i i 6 term factor 7 h exprh i factorid i £

h j i 7 factor 8 h exprÒÙÑ i id £

h i ÒÙÑ = 8 factor :: 4 id £ h termÒÙÑ i

id j 9 7 id h factor£ ÒÙÑ i

9id £ id ÒÙÑ

117 Handle-pruning

The process to construct a bottom-up parse is called handle-pruning.

To construct a rightmost derivation

µ ¡¡¡ γ µ γµ γ= = γ µ γ µ

S 0 1 2 n 1nw we set i to n and apply the following simple algorithm

= i fÓÖ ½ n dÓÛÒØÓ

γ β iÒ haÒdÐe Øhe 1. fiÒd Ai ! i i

γ ÖeÔÐace β ÛiØh geÒeÖaØe ØÓ

2. i Ai i 1

This takes 2n steps, where n is the length of the derivation

118 Stack implementation

One scheme to implement a handle-pruning, bottom-up parser is called a shift-reduce parser.

Shift-reduce parsers use a stack and an input buffer

1. initialize stack with $

2. Repeat until the top of the stack is the goal symbol and the input token is $ a) ﬁnd the handle if we don’t have a handle on top of the stack, shift an input symbol onto the stack b) prune the handle β if we have a handle A ! on the stack, reduce

β j i) pop j symbols off the stack ii) push A onto the stack

119 Example: backÝ £ ¾ to Ü

Stack Input Action

$ id £ ÒÙÑ id shift

$ id id £ ÒÙÑ reduce 9

$h factori id £ ÒÙÑ reduce 7

1 h goali h ::= expri

$h termi id £ ÒÙÑ reduce 4

2 h expri h ::= exprh · i termi

$h expri id £ ÒÙÑ shift

3 h j expr h i termi

$ h expr i id £ ÒÙÑ shift

4 h j termi

$ h exprÒÙÑ i id £ reduce 8

5 h termi h ::= term h i£ factori

$h exprh i factori id £ reduce 7

6 h j termh = i factori

$h exprh i termi id £ shift

7 h j factori

$ h exprh i term£ i id shift

8 h factori ÒÙÑ ::=

$ h exprh i termid £ i reduce 9

9 id j

$h exprh i termh £ i factori reduce 5

$h exprh i termi reduce 3

$h expri reduce 1

$h goali accept

1. Shift until top of stack is the right end of a handle

2. Find the left end of the handle and reduce

5 shifts + 9 reduces + 1 accept

120 Shift-reduce parsing

Shift-reduce parsers are simple to understand

A shift-reduce parser has just four canonical actions:

1. shift — next input symbol is shifted onto the top of the stack

2. reduce — right end of handle is on top of stack; locate left end of handle within the stack; pop handle off stack and push appropriate non-terminal LHS

3. accept — terminate parsing and signal success

4. error — call an error recovery routine

The key problem: to recognize handles (not covered in this course).

121 LR´ k µ grammars

Informally, we say that a grammar G is LR´ k µ if, given a rightmost derivation

γ µ µ ¡¡¡ γ µ γ= ; γ µ S = 0 1 2 nw we can, for each right-sentential form in the derivation,

1. isolate the handle of each right-sentential form , and 2. determine the production by which to reduce by scanning γi from left to right, going at most k symbols beyond the right end of the handle of γi.

122 LR´ k µ grammars

Formally, a grammar G is LR´ k µ iff.:

α µ αβ 1. S µ Aw rm w, and

rm µ

γ µ αβ 2. S µ Bx rm y, and £ rm

3. FIRSTk ´ w= µ FIRSTk ´ yµ

γ αAy = Bx i.e., Assume sentential forms αβw and αβy, with common preﬁx αβ and αβ common k-symbol lookahead FIRSTk ´ = yµ FIRSTk ´ wµ , such that w reduces to αAw and αβy reduces to γBx.

But, the common preﬁx means αβy also reduces to αAy, for the same result. γ Thus αAy = Bx.

123 Why study LR grammars?

LR(1) grammars are often used to construct parsers.

We call these parsers LR(1) parsers.

¯ everyone’s favorite parser

¯ virtually all context-free programming language constructs can be expressed in an LR(1) form

¯ LR grammars are the most general grammars parsable by a deterministic, bottom-up parser

¯ efﬁcient parsers can be implemented for LR(1) grammars

¯ LR parsers detect an error as soon as possible in a left-to-right scan of the input

¯ LR grammars describe a proper superset of the languages recognized by predictive (i.e., LL) parsers β β LL´ k µ : recognize use of a production A ! seeing ﬁrst k symbols of β LR ´ k µ : recognize occurrence of (the handle) having seen all of what is derived from β plus k symbols of lookahead

124 Left versus right recursion

Right Recursion:

¯ needed for termination in predictive parsers

¯ requires more stack space

¯ right associative operators

Left Recursion:

¯ works ﬁne in bottom-up parsers

¯ limits required stack space

¯ left associative operators

Rule of thumb:

¯ right recursion for top-down parsers

¯ left recursion for bottom-up parsers

125 Parsing review

Recursive descent A hand coded recursive descent parser directly encodes a grammar (typically an LL(1) grammar) into a series of mutually recursive procedures. It has most of the linguistic limitations of LL(1).

LL´ k µ

An LL´ k µ parser must be able to recognize the use of a production after seeing only the ﬁrst k symbols of its right hand side.

LR´ k µ

An LR´ k µ parser must be able to recognize the occurrence of the right hand side of a production after having seen all that is derived from that right hand side with k symbols of lookahead.

126 Chapter 5: JavaCC and JTB

127 The Java Compiler Compiler

¯ Can be thought of as “Lex and Yacc for Java.”

¯ It is based on LL(k) rather than LALR(1).

¯ Grammars are written in EBNF.

¯ The Java Compiler Compiler transforms an EBNF grammar into an LL(k) parser.

¯ The JavaCC grammar can have embedded action code written in Java, just like a Yacc grammar can have embedded action code written in C.

¯ The lookahead can be changed by writing ÄÇÇÃAÀEAD´ºººµ .

¯ The whole input is given in just one ﬁle (not two).

128 The JavaCC input format

One ﬁle:

¯ header

¯ token speciﬁcations for lexical analysis

¯ grammar

129 The JavaCC input format

}

> µ "¼" | ´["¼"¹"9"]µ¶ ["½"¹"9"] ´ ÁÆÌEGEÊ_ÄÁÌEÊAÄ: < Example of a token speciﬁcation:

: ÌÇÃEÆ

}

";" EÜÔÖe××iÓÒ´µ "ÖeØÙÖÒ" µ¶ ËØaØeÑeÒØ´µ ´

ß Example of a production:

ß}

: ËØaØeÑeÒØÄi×ØÊeØÙÖÒ´µ ÚÓid

130 Generating a parser with JavaCC

ÔÖÓgºf ÔÖÓgÖaÑ Øhe ÔaÖ×e× »» ÔÖÓgºf < ÅaiÒ jaÚa

ÔaÖ×eÖ Øhe Óf caÐÐ a cÓÒØaiÒ× ÅaiÒºjaÚa »» ÅaiÒºjaÚa jaÚac

ÒaÑe ×Ôecified a ÛiØh ÔaÖ×eÖ a geÒeÖaØe× »» fÓÖØÖaÒºjj jaÚacc

131 The Visitor Pattern

For object-oriented programming, the Visitor pattern enables the deﬁnition of a new operation on an object structure without changing the classes of the objects.

Gamma, Helm, Johnson, Vlissides: Design Patterns, 1995.

132 Sneak Preview

When using the Visitor pattern,

¯ the set of classes must be ﬁxed in advance, and

¯ each class must have an accept method.

133 }

ØaiÐ; Äi×Ø First Approach: Instanceof and Type Casts

head; iÒØ

ß Äi×Ø iÑÔÐeÑeÒØ× CÓÒ× cÐa×× The running Java example: summing an integer list.

ß} Äi×Ø iÑÔÐeÑeÒØ× ÆiÐ cÐa××

ß} Äi×Ø iÒØeÖface

134 }

}

ca×Ø×! ØÝÔe ØÛÓ Øhe ÆÓØice »»

ÐµºØaiÐ; ´´CÓÒ×µ = Ð

Ðµºhead; ´´CÓÒ×µ · ×ÙÑ = ×ÙÑ

ß CÓÒ×µ iÒ×ØaÒceÓf ´Ð if eÐ×e

faÐ×e; = ÔÖÓceed

ÆiÐµ iÒ×ØaÒceÓf ´Ð if

ß ´ÔÖÓceedµ FirstÛhiÐe Approach: Instanceof and Type Casts

ØÖÙe; = ÔÖÓceed bÓÓÐeaÒ

¼; = ×ÙÑ iÒØ

Äi×Ø¹ÓbjecØ Ìhe »» Ð; Äi×Ø

Advantage: The code is written without touching the classes ÆiÐ and

CÓÒ× .

Drawback: The code constantly uses type casts and iÒ×ØaÒceÓf to determine what class of object it is considering.

135 Second Approach: Dedicated Methods

The ﬁrst approach is not object-oriented!

To access parts of an object, the classical approach is to use dedicated methods which both access and act on the subobjects.

}

×ÙÑ´µ; iÒØ

ß Äi×Ø iÒØeÖface

We can now compute the sum of all components of a given Äi×Ø -object Ð

by writing Ðº×ÙÑ´µ .

136 }

}

ØaiÐº×ÙÑ´µ; · head ÖeØÙÖÒ

ß ×ÙÑ´µ iÒØ ÔÙbÐic

ØaiÐ; Äi×Ø

head; iÒØ

ß Äi×Ø iÑÔÐeÑeÒØ× CÓÒ× cÐa××

}

} Second Approach: Dedicated Methods

¼; ÖeØÙÖÒ

ß ×ÙÑ´µ iÒØ ÔÙbÐic

ß Äi×Ø iÑÔÐeÑeÒØ× ÆiÐ cÐa××

Advantage: The type casts and iÒ×ØaÒceÓf operations have disappeared, and the code can be written in a systematic way.

Disadvantage: For each new operation on Äi×Ø -objects, new dedicated methods have to be written, and all classes must be recompiled.

137 Third Approach: The Visitor Pattern

The Idea:

¯ Divide the code into an object structure and a Visitor (akin to Func- tional Programming!)

}

¯ Insert an acceÔØ method in each class. Each accept method takes a

Üµ; Úi×iØ´CÓÒ× ÚÓid Visitor as argument.

Üµ; Úi×iØ´ÆiÐ ÚÓid

ß Îi×iØÓÖ iÒØeÖface

¯ A Visitor contains a Úi×iØ method for each class (overloading!) A method for a class C takes an argument of type C.

}

Úµ; acceÔØ´Îi×iØÓÖ ÚÓid

ß Äi×Ø iÒØeÖface

138 }

}

ÚºÚi×iØ´Øhi×µ;

ß Úµ acceÔØ´Îi×iØÓÖ ÚÓid ÔÙbÐic

ØaiÐ; Äi×Ø Third Approach: The Visitor Pattern

head; iÒØ

ß Äi×Ø iÑÔÐeÑeÒØ× CÓÒ× cÐa××

¯ The purpose of the acceÔØ methods is to

}

invoke the Úi×iØ method in the Visitor which can handle the current

} object.

ÚºÚi×iØ´Øhi×µ;

ß Úµ acceÔØ´Îi×iØÓÖ ÚÓid ÔÙbÐic

ß Äi×Ø iÑÔÐeÑeÒØ× ÆiÐ cÐa××

139 ËÝ×ØeÑºÓÙØºÔÖiÒØÐÒ´×Úº×ÙÑµ;

ÐºacceÔØ´×Úµ;

ËÙÑÎi×iØÓÖ´µ; ÒeÛ = ×Ú ËÙÑÎi×iØÓÖ

ººººº

Third} Approach: The Visitor Pattern

}

ÜºØaiÐºacceÔØ´Øhi×µ;

Üºhead; · ×ÙÑ = ×ÙÑ ¯ The control ﬂow goes back and forth between the Úi×iØ methods in

ß Üµ Úi×iØ´CÓÒ× ÚÓid ÔÙbÐic the Visitor and the acceÔØ methods in the object structure.

ß} Üµ Úi×iØ´ÆiÐ ÚÓid ÔÙbÐic

¼; = ×ÙÑ iÒØ

ß Îi×iØÓÖ iÑÔÐeÑeÒØ× ËÙÑÎi×iØÓÖ cÐa××

Notice: The Úi×iØ methods describe both 1) actions, and 2) access of subobjects. 140 Comparison

The Visitor pattern combines the advantages of the two other approaches.

Frequent Frequent type casts? recompilation? Instanceof and type casts Ye s No Dedicated methods No Ye s The Visitor pattern No No

The advantage of Visitors: New methods without recompilation! Requirement for using Visitors: All classes must have an accept method.

Tools that use the Visitor pattern:

¯ JJTree (from Sun Microsystems) and the Java Tree Builder (from Pur- due University), both frontends for The Java Compiler Compiler from Sun Microsystems.

141 Visitors: Summary

¯ Visitor makes adding new operations easy. Simply write a new visitor.

¯ A visitor gathers related operations. It also separates unrelated ones.

¯ Adding new classes to the object structure is hard. Key consid- eration: are you most likely to change the algorithm applied over an object structure, or are you most like to change the classes of objects that make up the structure.

¯ Visitors can accumulate state.

¯ Visitor can break encapsulation. Visitor’s approach assumes that the interface of the data structure classes is powerful enough to let visitors do their job. As a result, the pattern often forces you to provide public operations that access internal state, which may compromise its encapsulation.

142 The Java Tree Builder

The Java Tree Builder (JTB) has been developed here at Purdue in my group.

JTB is a frontend for The Java Compiler Compiler.

JTB supports the building of syntax trees which can be traversed using visitors.

JTB transforms a bare JavaCC grammar into three components:

¯ a JavaCC grammar with embedded Java code for building a syntax tree;

¯ one class for every form of syntax tree node; and

¯ a default visitor which can do a depth-ﬁrst traversal of a syntax tree.

143 The Java Tree Builder The produced JavaCC grammar can then be processed by the Java Com- piler Compiler to give a parser which produces syntax trees.

The produced syntax trees can now be traversed by a Java program by writing subclasses of the default visitor.

Program

? JavaCC ¹ JTB¹ JavaCC grammar ¹ Java Compiler ¹ Parser grammar with embedded Compiler Java code

? ¹ Syntax-tree-node Syntax tree classes with accept methods

¹ Default visitor

144 Úi×iØÓÖ× Øhe eÜecÙØe×

aÒd ÔÖÓgºf¸ fÓÖ ØÖee ×ÝÒØaÜ a bÙiÐd× »» ÔÖÓgºf < ÅaiÒ jaÚa

Úi×iØÓÖ× ØÓ caÐÐ× aÒd Using JTB

ÔaÖ×eÖ Øhe Óf caÐÐ a cÓÒØaiÒ× ÅaiÒºjaÚa »» ÅaiÒºjaÚa jaÚac

ÒaÑe ×Ôecified a ÛiØh ÔaÖ×eÖ a geÒeÖaØe× »» jØbºÓÙØºjj jaÚacc

jØbºÓÙØºjj geÒeÖaØe× »» fÓÖØÖaÒºjj jØb

145 Example (simpliﬁed)

} } EÜÔÖe××iÓÒ´µ For example, consider the Java 1.1 production

} A××igÒÑeÒØ´Ò¼¸Ò½¸Ò¾µ; ÒeÛ ÖeØÙÖÒ ß

A××igÒÑeÒØÇÔeÖaØÓÖ´µ ÈÖiÑaÖÝEÜÔÖe××iÓÒ´µ ß

Ò¾=EÜÔÖe××iÓÒ´µ

ß} : A××igÒÑeÒØ´µ ÚÓid

Ò½=A××igÒÑeÒØÇÔeÖaØÓÖ´µ

Ò¼=ÈÖiÑaÖÝEÜÔÖe××iÓÒ´µ ß

} ß} Ò¾; EÜÔÖe××iÓÒ

Ò½; A××igÒÑeÒØÇÔeÖaØÓÖ JTB produces:

Ò¼; ÈÖiÑaÖÝEÜÔÖe××iÓÒ ß

: ´µ A××igÒÑeÒØ A××igÒÑeÒØ

Notice that the production returns a syntax tree represented as an

A××igÒÑeÒØ object.

146 }

}

ÚºÚi×iØ´Øhi×µ;

ß Úµ acceÔØ´Úi×iØÓÖºÎi×iØÓÖ ÚÓid ÔÙbÐic

} Ò¾; = f¾ Ò½; = f½ Ò¼; = f¼ ß

Ò¾µ EÜÔÖe××iÓÒ

Ò½¸ A××igÒÑeÒØÇÔeÖaØÓÖ

Ò¼¸ A××igÒÑeÒØ´ÈÖiÑaÖÝEÜÔÖe××iÓÒ ÔÙbÐic Example (simpliﬁed)

A××igÒÑeÒØ f¾; EÜÔÖe××iÓÒ JTB produces a syntax-tree-node class for :

f½; A××igÒÑeÒØÇÔeÖaØÓÖ f¼; ÈÖiÑaÖÝEÜÔÖe××iÓÒ

ß ÆÓde iÑÔÐeÑeÒØ× A××igÒÑeÒØ cÐa×× ÔÙbÐic

Notice the acceÔØ method; it invokes the method Úi×iØ for A××igÒÑeÒØ in the default visitor. 147 }

}

Òºf¾ºacceÔØ´Øhi×µ;

Òºf½ºacceÔØ´Øhi×µ;

Òºf¼ºacceÔØ´Øhi×µ;

ß Òµ Úi×iØ´A××igÒÑeÒØ ÚÓid ÔÙbÐic

»»

EÜÔÖe××iÓÒ´µ ¹> f¾ »»

A××igÒÑeÒØÇÔeÖaØÓÖ´µ ¹> f½ »» Example (simpliﬁed)

ÈÖiÑaÖÝEÜÔÖe××iÓÒ´µ ¹> f¼ »»

»» The default visitor looks like this:

ººº

ß Îi×iØÓÖ iÑÔÐeÑeÒØ× DeÔØhFiÖ×ØÎi×iØÓÖ cÐa×× ÔÙbÐic

Notice the body of the method which visits each of the three subtrees of

the A××igÒÑeÒØ node.

148 } Example (simpliﬁed)

}

Òºf¾ºacceÔØ´Øhi×µ; Here is an example of a program which operates on syntax trees for Java

ÚºÓÙØºÔÖiÒØÐÒ´µ; Òºf¾ºacceÔØ´Úµ; 1.1 programs. The program prints the right-hand side of every assignment.

ÎÈÖeØØÝÈÖiÒØeÖ´µ; ÒeÛ = Ú ÎÈÖeØØÝÈÖiÒØeÖ The entire program is six lines:

ß Òµ Úi×iØ´A××igÒÑeÒØ ÚÓid

ß DeÔØhFiÖ×ØÎi×iØÓÖ eÜØeÒd× ÎÔÖiÒØA××igÒÊÀË cÐa×× ÔÙbÐic

When this visitor is passed to the root of the syntax tree, the depth-ﬁrst

traversal will begin, and when A××igÒÑeÒØ nodes are reached, the method

Úi×iØ in ÎÔÖiÒØA××igÒÊÀË is executed.

Notice the use of ÎÈÖeØØÝÈÖiÒØeÖ . It is a visitor which pretty prints Java 1.1 programs.

JTB is bootstrapped.

149 Chapter 6: Semantic Analysis

150 Semantic Analysis

The compilation process is driven by the syntactic structure of the program as discovered by the parser

Semantic routines:

¯ interpret meaning of the program based on its syntactic structure

¯ two purposes: – ﬁnish analysis by deriving context-sensitive information – begin synthesis by generating the IR or target code

¯ associated with individual productions of a context free grammar or subtrees of a syntax tree for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and full citation on the first page. To copy otherwise, to republish, to post on servers, or to redistribute to lists, requires prior specific permission and/or Copyright c 2000 by Antony L. Hosking. Permission to make digital or hard copies of part or all of this work fee. Request permission to publish from [email protected].

151 Context-sensitive analysis

What context-sensitive questions might the compiler ask?

1. Is Ü scalar, an array, or a function?

2. Is Ü declared before it is used? 3. Are any names declared but not used?

4. Which declaration of Ü does this reference? 5. Is an expression type-consistent? 6. Does the dimension of a reference match the declaration?

7. Where can Ü be stored? (heap, stack, ::: )

8. Does ¶Ô reference the result of a malloc()?

9. Is Ü deﬁned before it is used? 10. Is an array reference in bounds ?

11. Does function fÓÓ produce a constant value?

12. Can Ô be implemented as a memo-function?

These cannot be answered with a context-free grammar

152 Context-sensitive analysis

Why is context-sensitive analysis hard?

¯ answers depend on values, not syntax

¯ questions and answers involve non-local information

¯ answers may involve computation

Several alternatives: abstract syntax tree specify non-local computations (attribute grammars ) automatic evaluators

symbol tables central store for facts express checking code

language design simplify language avoid problems

153 Symbol tables

For compile-time efﬁciency, compilers often use a symbol table :

¯ associates lexical names (symbols) with their attributes

What items should be entered?

¯ variable names

¯ deﬁned constants

¯ procedure and function names

¯ literal constants and strings

¯ source text labels

¯ compiler-generated temporaries (we’ll get there )

Separate table for structure layouts (types) (ﬁeld offsets and lengths )

A symbol table is a compile-time structure

154 Symbol table information

What kind of information might the compiler need?

¯ textual name

¯ data type

¯ dimension information (for aggregates )

¯ declaring procedure

¯ lexical level of declaration

¯ storage class (base address )

¯ offset in storage

¯ if record, pointer to structure table

¯ if parameter, by-reference or by-value?

¯ can it be aliased? to what other names?

¯ number and type of arguments to functions

155 Nested scopes: block-structured symbol tables

What information is needed?

¯ when we ask about a name, we want the most recent declaration

¯ the declaration may be from the current scope or some enclosing scope

¯ innermost scope overrides declarations from outer scopes

Key point: new declarations (usually) occur only in current scope

What operations do we need?

ÚaÐÙeµ ÇbjecØ keÝ¸ ´ËÝÑbÓÐ ÔÙØ ÚÓid ¯ – binds key to value

keÝµ geØ´ËÝÑbÓÐ ÇbjecØ ¯ – returns value bound to key

begiÒËcÓÔe´µ ÚÓid ¯ – remembers current state of table

eÒdËcÓÔe´µ ÚÓid ¯ – restores table to state at most recent scope that has not been ended

May need to preserve list of locals for the debugger

156 Attribute information

Attributes are internal representation of declarations

Symbol table associates names with attributes

Names may have different attributes depending on their meaning:

¯ variables: type, procedure level, frame offset

¯ types: type descriptor, data size/alignment

¯ constants: type, value

¯ procedures: formals (names/types), result type, block information (local decls.), frame size

157 Type expressions

Type expressions are a textual representation for types:

1. basic types: boolean, char, integer, real, etc. 2. type names 3. constructed types (constructors applied to type expressions):

(a) array´ I ; T µ denotes array of elements type T, index type I

e.g., array´ 1 ::: 10; integer µ

(b) T1 ¢ T2 denotes Cartesian product of type expressions T1 and T2 (c) records: ﬁelds have names

e.g., record¢ a ´´ integer¢ b ´ ; µ real µµ

(d) pointer ´ T µ denotes the type “pointer to object of type T”

(e) D ! R denotes type of function mapping domain D to range R

e.g., integer ¢ integer ! integer

158 Type descriptors

Type descriptors are compile-time structures representing type expressions

e.g., char ¢ char ! pointer ´ integer µ

pointer or pointer

char char integer char integer

159 Type compatibility

Type checking needs to determine type equivalence

Two approaches:

Name equivalence : each type name is a distinct type

Structural equivalence : two types are equivalent iff. they have the same structure (after substituting type expressions for type names)

¯ s t iff. s and t are the same basic types

¯ array´ s1 ; s 2 µ array´ t1 ; t2 µ iff. s1 t1 and s2 t2

¯ s1 ¢ s2 t1 ¢ t2 iff. s1 t1 and s2 t2

¯ pointer ´ sµ pointer ´ t µ iff. s t

¯ s1 ! s2 t1 ! t2 iff. s1 t1 and s2 t2

160 ceÐÐ; " : Ö Õ¸ Type compatibility: example

ceÐÐ; " : Ô

ÐiÒk; : Ða×Ø Consider:

ÐiÒk; : ÒeÜØ ÚaÖ

ceÐÐ; " = ÐiÒk ØÝÔe

Under name equivalence:

ÒeÜØ ¯ and Ða×Ø have the same type

Ô ¯ , Õ and Ö have the same type

Ô ¯ and ÒeÜØ have different type

Under structural equivalence all variables have the same type

Ada/Pascal/Modula-2/Tiger are somewhat confusing: they treat distinct type deﬁnitions as distinct types, so

Ô has different type from Õ and Ö

161 Type compatibility: Pascal-style name equivalence

Build compile-time structure called a type graph :

link

¯ each constructor or basic type creates a node

¯ each name creates a leaf (associated with the type’s descriptor)

last next p q r

= pointer pointer pointer

cell

Type expressions are equivalent if they are represented by the same node in the graph

162 eÒd;

ÐiÒk; : ÒeÜØ Type compatibility: recursive types

iÒØegeÖ; : iÒfÓ

ÖecÓÖd = ceÐÐ Consider:

ceÐÐ; " = ÐiÒk ØÝÔe

info

We may want to eliminate the names from the type graph

Eliminating name ÐiÒk from type graph for record:

= recordcell

integer pointer

cell

163 info

Type compatibility: recursive types

Allowing cycles in the type graph eliminates ceÐÐ :

= recordcell

integer pointer

164 Chapter 7: Translation and Simpliﬁcation

165 Tiger IR trees: Expressions CONST Integer constant i i NAME Symbolic constant n [a code label] n TEMP Temporary t [one of any number of “registers”] t BINOP Application of binary operator o: oe1 e2 PLUS, MINUS, MUL, DIV AND, OR, XOR [bitwise logical] LSHIFT, RSHIFT [logical shifts] ARSHIFT [arithmetic right-shift] to integer operands e1 (evaluated ﬁrst) and e2 (evaluated second) MEM Contents of a word of memory starting at address e e CALL

Procedure call; expression f is evaluated before arguments ; e1 ;::: en

f ; [ e1 ;::: en] ESEQ for personal or classroomExpression use is granted sequence; without fee provided evaluate that copiess for are side-effects, not made or distributed then e for for result profit or commercialse advantage and that copies bear this notice and full citation on the first page. To copy otherwise, to republish, to post on servers, or to redistribute to lists, requires prior specific permission and/or Copyright c 2000 by Antony L. Hosking. Permission to make digital or hard copies of part or all of this work fee. Request permission to publish from [email protected].

166 Tiger IR trees: Statements MOVE TEMP e Evaluate e into temporary t t MOVE

MEM e2 Evaluate e1 yielding address a, e2 into word at a

e1 EXP Evaluate e and discard result e JUMP

Transfer control to address e ; ; l1 ;::: lnare all possible values for e

e ; [ l1 ;::: ln] CJUMP Evaluate e1 then e2, yielding a and b, respectively; compare a with b oe1 e2tf using relational operator o: EQ, NE [signed and unsigned integers] LT, GT, LE, GE [signed] ULT, ULE, UGT, UGE [unsigned] jump to t if true, f if false

SEQ Statement s1 followed by s2 s1 s2 LABEL

Deﬁne constant value of name n as current code address; NAME ´ n µ n can be used as target of jumps, calls, etc. 167 Kinds of expressions

Expression kinds indicate “how expression might be used” Ex(exp) expressions that compute a value Nx(stm) statements: expressions that compute no value Cx conditionals (jump to true and false destinations) RelCx(op, left, right) IfThenElseExp expression/statement depending on use Conversion operators allow use of one form in context of another: unEx convert to tree expression that computes value of inner tree unNx convert to tree statement that computes inner tree but returns no value unCx(t, f) convert to statement that evaluates inner tree and branches to true destination if non-zero, false destination otherwise

168 Translating Tiger

Simple variables: fetch with a MEM: MEM

Ex(MEM(· (TEMP fp,CONSTk))) BINOP

PLUS TEMP fp CONST k where fp is home frame of variable, found by following static links; k is offset of variable in that level Tiger array variables: Tiger arrays are pointers to array base, so fetch with a MEM like any other variable:

Ex(MEM(· (TEMP fp,CONSTk)))

Thus, for e[ i] :

Ex(MEM(· (e.unEx, ¢ (i.unEx, CONST w)))) i is index expression and w is word size – all values are word-sized (scalar) in Tiger

Note: must ﬁrst check array index i < size ´ e µ ; runtime will put size in word preceding array base

169 Translating Tiger

Tiger record variables: Again, records are pointers to record base, so fetch like other

variables. For e.f :

Ex(MEM( · (e.unEx, CONST o)))

where o is the byte offset of the ﬁeld f in the record Note: must check record pointer is non-nil (i.e., non-zero) String literals: Statically allocated, so just use the string’s label Ex(NAME(label)) where the literal will be emitted as:

ÛÓÖÐd" "heÐÐÓ ºa×cii ÐabeÐ:

½½ ºÛÓÖd

Record creation: f Ø f1 = e1 ; f2 = e2 ;::: fn= eng in the (preferably GC’d) heap, ﬁrst allocate the space then initialize it:

Ex( ESEQ(SEQ(MOVE(TEMP r, externalCall(”allocRecord”, [CONST n])), SEQ(MOVE(MEM(TEMP r), e1.unEx)), SEQ(...,

MOVE(MEM(+(TEMP r, CONST ´ n 1µ w)), en.unEx))), TEMP r))

where w is the word size

Array creation: [ Ø eÓf 1 ] e2: Ex(externalCall(”initArray”, [e1.unEx, e2.unEx])) 170 Control structures

Basic blocks :

¯ a sequence of straight-line code

¯ if one instruction executes then they all execute

¯ a maximal sequence of instructions without branches

¯ a label starts a new basic block Overview of control structure translation:

¯ control ﬂow links up the basic blocks

¯ ideas are simple

¯ implementation requires bookkeeping

¯ some care is needed for good code

171 while loops while c do s: 1. evaluate c 2. if false jump to next statement after loop 3. if true fall into loop body 4. branch to top of loop e.g., test: if not(c)jumpdone s done jump test : Nx( SEQ(SEQ(SEQ(LABEL test, c.unCx(body, done)), SEQ(SEQ(LABEL body, s.unNx), JUMP(NAME test))), LABEL done))

repeat e1 until e2 µ evaluate/compare/branch at bottom of loop 172 for loops

for i := e1 to e2 do s 1. evaluate lower bound into index variable 2. evaluate upper bound into limit variable

3. if index > limit jump to next statement after loop 4. fall through to loop body 5. increment index

6. if index limit jump to top of loop body

t1 e1

body t2 e2

if t1 > t2 jump done : s

done t1 t1 · 1

if t1 t2 jump body : For break statements:

¯ when translating a loop push the done label on some stack

¯ break simply jumps to label on top of stack

¯ when done translating loop and its body, pop the label

173 Function calls

f ; ´ e1 ;::: enµ :

Ex(CALL(NAME label f ,[sl,e1,...en])) where sl is the static link for the callee f , found by following n static links from the caller, n being the difference between the levels of the caller and the callee

174 Comparisons

Translate a op b as:

RelCx(op, a.unEx, b.unEx)

When used as a conditional unCx´ t ; f µ yields:

CJUMP(op, a.unEx, b.unEx, t, f ) where t and f are labels.

When used as a value unEx yields:

ESEQ(SEQ(MOVE(TEMP r, CONST 1), SEQ(unCx(t, f), SEQ(LABEL f, SEQ(MOVE(TEMP r, CONST 0), LABEL t)))), TEMP r)

175 Conditionals

The short-circuiting Boolean operators have already been transformed into if-expressions in Tiger abstract syntax:

e.g., x < 5 & a > b turns into if x < 5 then a > b else 0

Translate if e1 then e2 else e3 into: IfThenElseExp(e1, e2, e3) When used as a value unEx yields: ESEQ(SEQ(SEQ(e1.unCx(t, f), SEQ(SEQ(LABEL t, SEQ(MOVE(TEMP r, e2.unEx), JUMP join)), SEQ(LABEL f, SEQ(MOVE(TEMP r, e3.unEx), JUMP join)))), LABEL join), TEMP r)

As a conditional unCx´ t ; f µ yields: SEQ(e1.unCx(tt,ff), SEQ(SEQ(LABEL tt, e2.unCx(t, f )), SEQ(LABEL ff, e3.unCx(t, f ))))

176 Conditionals: Example

Applying unCx´ t ; f µ to if x < 5 then a > b else 0:

SEQ(CJUMP(LT, x.unEx, CONST 5, tt, ff), SEQ(SEQ(LABEL tt, CJUMP(GT, a.unEx, b.unEx, t, f )), SEQ(LABEL ff, JUMP f ))) or more optimally:

SEQ(CJUMP(LT, x.unEx, CONST 5, tt, f ), SEQ(LABEL tt, CJUMP(GT, a.unEx, b.uneX, t, f )))

177 One-dimensionala[e] ﬁxed arrays

ººº

iÒØegeÖ; Óf [¾ºº5] AÊÊAY : a ÚaÖ

translates to:

MEM(+(TEMP fp, +(CONST k 2w, ¢ (CONST w, e.unEx))))

where k is offset of static array from fp, w is word size

In Pascal, multidimensional arrays are treated as arrays of arrays, so A[i¸j] is equivalent to A[i][j], so can translate as above.

178 Multidimensional arrays

Array allocation: constant bounds – allocate in static area, stack, or heap – no run-time descriptor is needed dynamic arrays: bounds ﬁxed at run-time – allocate in stack or heap – descriptor is needed dynamic arrays: bounds can change at run-time – allocate in heap – descriptor is needed

179 Multidimensional arrays

Array layout:

¯ Contiguous: 1. Row major

ººº A[¾¸¾]¸ RightmostA[¾¸½]¸ subscript varies most quickly:

ººº A[½¸¾]¸ A[½¸½]¸

Used in PL/1, Algol, Pascal, C, Ada, Modula-3 2. Column major

ººº A[¾¸¾]¸ LeftmostA[½¸¾]¸ subscript varies most quickly:

ººº A[¾¸½]¸ A[½¸½]¸

Used in FORTRAN

¯ By vectors Contiguous vector of pointers to (non-contiguous) subarrays

180 Multi-dimensional arrays: row-major layout

Ì Óf [½ººÅ] aÖÖaÝ Óf [½ººÆ] aÖÖaÝ

Ì Óf [½ººÆ¸½ººÅ] aÖÖaÝ

no. of elt’s in dimension j:

D j = Uj L j · 1

position of A[ ººº¸ i1 ¸ in ] :

·´

´ in Ln µ

in 1 Ln 1 Dn

·´ ·´ µ

in 2 Ln 2 DnDn 1

¡¡¡ ·

i1 L1 µ Dn ¡¡¡ D2 which can be rewritten as variable part

Þ }| ß

¡¡¡ · ¡¡¡ · ¡¡¡ · ·

i1D2 Dni2D3Dnin 1Dnin

´ ¡¡¡ · ¡¡¡ · ¡¡¡ · · µ

L1D2DnL2D3DnLn 1DnLn

ßÞ } constant part

address of A[ ººº¸ i1 ¸ in ] :

address(A ) + ((variable part constant part) ¢ element size)

181 case statements case E of V1: S1 ...Vn: Sn end 1. evaluate the expression 2. ﬁnd value in case list equal to value of expression 3. execute statement associated with value found 4. jump to next statement after case Key issue: ﬁnding the right case

¯ ¯ sequence of conditional jumps (small case set)

Oj ´ casesµ j

binary search of an ordered jump¯ table (sparse case set)

O ´ log2 j casesµ j hash table (dense case set)

O ´ 1 µ

182 case statements case E of V1: S1 ...Vn: Sn end One translation approach: t := expr jump test

LfÓÖ 1: cÓde S1 jump next L2: code for S2 jump next ... Ln: code for Sn jump next

test: if t = V1 jump L1

if t = V2 jump L2 ...

if t = Vn jump Ln code to raise run-time exception next:

183 Simpliﬁcation

¯ Goal 1: No SEQ or ESEQ.

¯ Goal 2: CALL can only be subtree of EXP(...) orMOVE(TEMPt,...). Transformations:

¯ lift ESEQs up tree until they can become SEQs

¯ turn SEQs into linear list

ESEQ(s1, ESEQ(s2, e)) = ESEQ(SEQ(s1,s2), e)

BINOP(op, ESEQ(s, e1), e2) = ESEQ(s,BINOP(op, e1, e2))

MEM(ESEQ(s, e1)) = ESEQ(s,MEM(e1))

JUMP(ESEQ(s, e1)) = SEQ(s,JUMP(e1))

CJUMP(op, = SEQ(s,CJUMP(op, e1, e2, l1, l2)) ESEQ(s, e1), e2, l1, l2)

BINOP(op, e1, ESEQ(s, e2)) = ESEQ(MOVE(TEMP t, e1), ESEQ(s, BINOP(op,TEMPt,e2)))

CJUMP(op, = SEQ(MOVE(TEMP t, e1), e1, ESEQ(s, e2), l1, l2) SEQ(s, CJUMP(op,TEMPt,e2,l1,l2)))

MOVE(ESEQ(s, e1), e2) = SEQ(s, MOVE(e1, e2))

CALL( f , a) = ESEQ(MOVE(TEMP t, CALL( f , a)), TEMP(t))

184 Chapter 8: Liveness Analysis and Register Allocation

185 Register allocation

instruction register machine IR selection allocation code

errors

¯ have value in a register when used

¯ limited resources

¯ changes instruction choices

¯ can move loads and stores

¯ optimal allocation is difﬁcult

µ NP-complete for k 1 registers work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and full citation on the first page. To copy otherwise, to republish, to post on servers, or to redistribute to lists, requires prior specific permission

Copyright c 2000 by Antony L. Hosking. Permission to make digital or hard copies of part or all of this and/or fee. Request permission to publish from [email protected].

186 Liveness analysis

Problem:

¯ IR contains an unbounded number of temporaries

¯ machine has bounded number of registers Approach:

¯ temporaries with disjoint live ranges can map to same register

¯ if not enough registers then spill some temporaries (i.e., keep them in memory) The compiler must perform liveness analysis for each temporary: It is live if it holds a value that may be needed in future

187 Control ﬂow analysis

Before performing liveness analysis, need to understand the control ﬂow by building a control ﬂow graph (CFG):

¯ nodes may be individual program statements or basic blocks

¯ edges represent potential ﬂow of control

Out-edges from node n lead to successor nodes, succ[ n]

In-edges to node n come from predecessor nodes, pred[ n ]

Example:

a 0

L1 : b a · 1

c c · b

a b ¢ 2

if a < N goto L1 return c

188 Liveness analysis

Gathering liveness information is a form of data ﬂow analysis operating over the CFG:

¯ liveness of variables “ﬂows” around the edges of the graph

¯ assignments deﬁne avariable,v:

– def´ = vµ set of graph nodes that deﬁne v

– def[ n= ] set of variables deﬁned by n

¯ occurrences of v in expressions use it:

– use´ = vµ set of nodes that use v

– use[ n= ] set of variables used in n Liveness: v is live on edge e if there is a directed path from e to a use of v

that does not pass through any def´ vµ v is live-in at node n if live on any of n’s in-edges v is live-out at n if live on any of n’s out-edges

v ¾ use[ nµ ] v live-in at n

v live-in at n µ v live-out at all m ¾ pred[ n ]

v live-out at n ; v 6¾ def[ nµ ] v live-in at n

189 Liveness analysis

Deﬁne:

in[ n] : variables live-in at n

in[ n] : variables live-out at n Then:

out[ n= ] in[ s]

[

s ¾ succ´ nµ

φ µ [ = ] φ succ[ n= ] out n

Note:

in[ n ] use[ n]

in[ n ] out[ n ] def[ n ]

use[ n] and def[ n ] are constant (independent of control ﬂow)

Now, v ¾ in[ n] iff. v ¾ use[ n] or v ¾ out[ n ] def[ n ]

Thus, in[ n= ] use´ [ n[ ] out[ n ] def[ n ]µ

190 Iterative solution for liveness

φ [ ] φ g foreach n f in [ n ] ; out n repeat foreach n

[

in n ] in [ n ] ;

[

out n ] out [ n] ;

in [ n ] use´ [ n[ ] out [ n ] def [ n]µ

out [ n ] in [ s]

[

s ¾ succ[ n]

[ [

¼ ¼

until in n= ] in [ n^ ] out n= ] out [ 8 n; ] n

Notes:

¯ should order computation of inner loop to follow the “ﬂow”

¯ liveness ﬂows backward along control-ﬂow arcs, from out to in

¯ nodes can just as easily be basic blocks to reduce CFG size

¯ could do one variable at a time, from uses back to defs, noting liveness along the way

191 Iterative solution for liveness

Complexity: for input program of size N

¯ N nodes in CFG

µ N variables

µ N elements per in/out

µ O´ N µ time per set-union

¯ for loop performs constant number of set operations per node 2 µ O´ N µ time for for loop

¯ each iteration of repeat loop can only add to each set sets can contain at most every variable 2 µ sizes of all in and out sets sum to 2N ,

bounding the number of iterations of the repeat loop ¯ 4 µ worst-case complexity of O´ N µ ordering can cut repeat loop down to 2-3 iterations 2 µ O´ N µ or O´ N µ in practice

192 Iterative solution for liveness

Least fixed points There is often more than one solution for a given dataflow problem (see example). Any solution to dataflow equations is a conservative approximation :

¯ v has some later use downstream from n

µ v ¾ out´ n µ but not the converse Conservatively assuming a variable is live does not break the program; just means more registers may be needed. Assuming a variable is dead when it is really live will break things. May be many possible solutions but want the “smallest”: the least ﬁxpoint. The iterative liveness computation computes this least ﬁxpoint.

193 Register allocation

instruction register machine IR selection allocation code

errors

¯ have value in a register when used

¯ limited resources

¯ changes instruction choices

¯ can move loads and stores

¯ optimal allocation is difﬁcult

Copyright c 2000 by Antony L. Hosking. Permission to make digital or hard copies of part or all of this and/or fee. Request permission to publish from [email protected].

194 Register allocation by simpliﬁcation

1. Build interference graph G: for each program point (a) compute set of temporaries simultaneously live (b) add edge to graph for each pair in set 2. Simplify: Color graph using a simple heuristic

(a) suppose G has node m with degree < K

(b) if G G f mg can be colored then so can G, since nodes adjacent

to m have at most K 1 colors (c) each such simpliﬁcation will reduce degree of remaining nodes leading to more opportunity for simpliﬁcation (d) leads to recursive coloring algorithm

3. Spill: suppose 9 6 m of degree < K (a) target some node (temporary) for spilling (optimistically, spilling node will allow coloring of remaining nodes) (b) remove and continue simplifying

195 Register allocation by simpliﬁcation (continued)

4. Select: assign colors to nodes (a) start with empty graph (b) if adding non-spill node there must be a color for it as that was the basis for its removal (c) if adding a spill node and no color available (neighbors already K- colored) then mark as an actual spill (d) repeat select 5. Start over : if select has no actual spills then ﬁnished, otherwise (a) rewrite program to fetch actual spills before each use and store after each deﬁnition (b) recalculate liveness and repeat

196 Coalescing

¯ Can delete a move instruction when source s and destination d do not interfere: – coalesce them into a new node whose edges are the union of those of s and d

¯ In principle, any pair of non-interfering nodes can be coalesced – unfortunately, the union is more constrained and new graph may no longer be K-colorable – overly aggressive

197 Simpliﬁcation with aggressive coalescing

build

aggressive coalesce done

coalesce any

simplify

done spill

spill any select

198 Conservative coalescing

Apply tests for coalescing that preserve colorability. Suppose a and b are candidates for coalescing into node ab

Briggs: coalesce only if ab has < K neighbors of signiﬁcant degree K

¯ simplify will ﬁrst remove all insigniﬁcant-degree neighbors

¯ ab will then be adjacent to < K neighbors

¯ simplify can then remove ab George: coalesce only if all signiﬁcant-degree neighbors of a already interfere with b

¯ simplify can remove all insigniﬁcant-degree neighbors of a

¯ remaining signiﬁcant-degree neighbors of a already interfere with b so coalescing does not increase the degree of any node

199 Iterated register coalescing

Interleave simpliﬁcation with coalescing to eliminate most moves while without extra spills

1. Build interference graph G; distinguish move-related from non-move-related nodes 2. Simplify: remove non-move-related nodes of low degree one at a time 3. Coalesce: conservatively coalesce move-related nodes

¯ remove associated move instruction

¯ if resulting node is non-move-related it can now be simpliﬁed

¯ repeat simplify and coalesce until only significant-degree or uncoalesced moves 4. Freeze: if unable to simplify or coalesce (a) look for move-related node of low-degree (b) freeze its associated moves (give up hope of coalescing them) (c) now treat as a non-move-related and resume iteration of simplify and coalesce 5. Spill: if no low-degree nodes (a) select candidate for spilling (b) remove to stack and continue simplifying 6. Select: pop stack assigning colors (including actual spills) 7. Start over : if select has no actual spills then finished, otherwise (a) rewrite code to fetch actual spills before each use and store after each definition (b) recalculate liveness and repeat

200 Iterated register coalescing

SSA constant propagation (optional)

build

simplify

conservative coalesce

freeze

potential spill

select

done actual spill spills

any

201 Spilling

¯ Spills require repeating build and simplify on the whole program

¯ To avoid increasing number of spills in future rounds of build can simply discard coalescences

¯ Alternatively, preserve coalescences from before ﬁrst potential spill, discard those after that point

¯ Move-related spilled temporaries can be aggressively coalesced, since (unlike registers) there is no limit on the number of stack-frame loca- tions

202 Precolored nodes

Precolored nodes correspond to machine registers (e.g., stack pointer, arguments, return address, return value)

¯ select and coalesce can give an ordinary temporary the same color as a precolored register, if they don’t interfere

¯ e.g., argument registers can be reused inside procedures for a temporary

¯ simplify, freeze and spill cannot be performed on them

¯ also, precolored nodes interfere with other precolored nodes So, treat precolored nodes as having inﬁnite degree This also avoids needing to store large adjacency lists for precolored nodes; coalescing can use the George criterion

203 Temporary copies of machine registers

Since precolored nodes don’t spill, their live ranges must be kept short: 1. use move instructions 2. move callee-save registers to fresh temporaries on procedure entry, and back on exit, spilling between as necessary 3. register pressure will spill the fresh temporaries as necessary, otherwise they can be coalesced with their precolored counterpart and the moves deleted

204 Caller-save and callee-save registers

Variables whose live ranges span calls should go to callee-save registers, otherwise to caller-save This is easy for graph coloring allocation with spilling

¯ calls interfere with caller-save registers

¯ a cross-call variable interferes with all precolored caller-save registers, as well as with the fresh temporaries created for callee-save copies, forcing a spill

¯ choose nodes with high degree but few uses, to spill the fresh callee- save temporary instead of the cross-call variable

¯ this makes the original callee-save register available for coloring the cross-call variable

205 ¯

] ÓÙØ ÐiÚe Ö¿ Ö½¸ [ ÖeØÙÖÒ

c := Ö¿

d := Ö½

ÐÓÓÔ gÓØÓ ¼ > e if

½ ¹ e := e

b · d := d

ÐÓÓÔ:

a := e

¼ := d

Ö¾ := b

Ö½ := a Example

Ö¿ := c

eÒØeÖ:

Temporaries are a , b , c , d , e

Assume target machine with K = 3 registers: Ö½ , Ö¾ (caller-save/argument/resul

Ö¿ (callee-save)

¯ The code generator has already made arrangements to save Ö¿ ex-

plicitly by copying into temporary a and back again

206 Example (cont.)

¯ Interference graph: r3 c

b e r2

r1 a d

¯ No opportunity for simplify or freeze (all non-precolored nodes have

signiﬁcant degree K)

¯ Any coalesce will produce a new node adjacent to K signiﬁcant- degree nodes

¯ Must spill based on priorities:

Node uses · defs uses · defs degree priority outside loop inside loop

´ a 2 · 10¢ 0 = µ 4 = 0.50

´ b 1 · 10¢ 1 = µ 4 = 2.75

´ c 2 · 10¢ 0 = µ 6 = 0.33

´ d 2 · 10¢ 2 = µ 4 = 5.50

´ e 1 · 10¢ 3 = µ 3 = 10.30

¯ Node c has lowest priority so spill it

207 Example (cont.)

¯ Interference graph with c removed: r3

b e r2

r1 a d

¯ Only possibility is to coalesce a and e : ae will have < K signiﬁcant-

degree neighbors (after coalescing d will be low-degree, though high- degree before) r3

b r2

r1 ae d

208 Example (cont.)

¯ Can now coalesce b with Ö¾ (or coalesce ae and Ö½ ): r3

r2b

r1ae d

¯ Coalescing ae and Ö½ (could also coalesce d with Ö½ ): r3

r2b

r1ae d

209 Example (cont.)

¯ Cannot coalesce Ö½ae with d because the move is constrained:the

nodes interfere. Must simplify d : r3

r2b

r1ae

¯ Graph now has only precolored nodes, so pop nodes from stack coloring along the way

Ö¿ – d

– a , b , e have colors by coalescing

– c must spill since no color can be found for it

¯ Introduce new temporaries c½ and c¾ for each use/def, add loads before each use and stores after each def

210 ] ÓÙØ ÐiÚe Ö¿ Ö½¸ [ ÖeØÙÖÒ

c¾ := Ö¿

Å[c_ÐÓc] := c¾

d := Ö½

ÐÓÓÔ gÓØÓ ¼ > e if

½ ¹ e := e

b · d := d

ÐÓÓÔ:

a := e

¼ := d

Ö¾ := b

Ö½ := a c½ := Å[c_ÐÓc] Example (cont.)

Ö¿ := c½

eÒØeÖ:

211 Example (cont.)

¯ New interference graph: r3

c1 b e r2 c2

r1 a d

¯ Coalesce c½ with Ö¿ , then c¾ with Ö¿ : r3c1c2

b e r2

r1 a d

¯ As before, coalesce a with e , then b with Ö¾ : r3c1c2

r2b

r1ae d

212 Example (cont.)

¯ As before, coalesce ae with Ö½ and simplify d : r3c1c2

r2b

r1ae

¯ Pop d from stack: select Ö¿ . All other nodes were coalesced or precolored. So, the coloring is:

Ö½ – a

Ö¾ – b

Ö¿ – c

Ö¿ – d

Ö½ – e

213 ] ÓÙØ ÐiÚe Ö¿ Ö½¸ [ ÖeØÙÖÒ

Ö¿ := Ö¿

Å[c_ÐÓc] := Ö¿

Ö¿ := Ö½

ÐÓÓÔ gÓØÓ ¼ > Ö½ if

½ ¹ Ö½ := Ö½

Ö¾ · Ö¿ := Ö¾

ÐÓÓÔ:

Ö½ := Ö½

¼ := Ö¿

Ö¾ := Ö¾

Ö½ := ExampleÖ½ (cont.)

Ö¿ := Å[c_ÐÓc]

¯ Rewrite the program with this assignment:

Ö¿ := Ö¿

eÒØeÖ:

214 ¯

] ÓÙØ ÐiÚe Ö¿ Ö½¸ [ ÖeØÙÖÒ

Å[c_ÐÓc] := Ö¿

Ö¿ := Ö½

ÐÓÓÔ gÓØÓ ¼ > Ö½ if

½ ¹ Ö½ := Ö½

Ö¾ · Ö¿ := Ö¾

ExampleÐÓÓÔ: (cont.)

¼ := Ö¿

¯ Delete moves with source and destination the same (coalesced):

Ö¿ := Å[c_ÐÓc]

eÒØeÖ:

One uncoalesced move remains

215 Chapter 9: Activation Records

216 The procedure abstraction

Separate compilation:

¯ allows us to build large programs

¯ keeps compile times reasonable

¯ requires independent procedures The linkage convention:

¯ a social contract

¯ machine dependent

¯ division of responsibility The linkage convention ensures that procedures inherit a valid run-time environment and that they restore one for their parents. Linkages execute at run time Codefor personal to or make classroom the use linkage is granted without is generated fee provided that at copiescompile are not made time or distributed for profit or commercial advantage and that copies bear this notice and full citation on the first page. To copy otherwise, to republish, to post on servers, or to redistribute to lists, requires prior specific permission and/or Copyright c 2000 by Antony L. Hosking. Permission to make digital or hard copies of part or all of this work fee. Request permission to publish from [email protected].

217 The procedure abstraction The essentials:

¯ on entry , establish Ô ’s environment

¯ atacall, preserve Ô ’s environment

¯ on exit , tear down Ô ’s environment

¯ in between , addressability and proper lifetimes procedure P procedure Q prologue prologue

pre−call call post−call

epilogue epilogue

Each system has a standard linkage

218 Procedure linkages

higher addresses

rvosfaenextframe argument n previous frame . . .

incoming argument 2 arguments

frame argument 1 run time pointer local Assume that each procedure activation has variables an associated activation record or frame (at return address

) current frame Assumptions: temporaries

saved registers ¯ RISC architecture

¯ can always expand an allocated block argument m .

¯ locals stored in frame . .

outgoing argument 2 arguments

stack argument 1 pointer

lower addresses

219 Procedure linkages The linkage divides responsibility between caller and callee

Caller Callee Call pre-call prologue 1. allocate basic frame 1. save registers, state 2. evaluate & store params. 2. store FP (dynamic link) 3. store return address 3. set new FP 4. jump to child 4. store static link 5. extend basic frame (for local data) 6. initialize locals 7. fall through to code

Return post-call epilogue 1. copy return value 1. store return value 2. deallocate basic frame 2. restore state 3. restore parameters 3. cut back to basic frame (if copy out) 4. restore parent’s FP 5. jump to return address

At compile time, generate the code to do this At run time, that code manipulates the frame & data areas

220 Run-time storage organization

To maintain the illusion of procedures, the compiler can adopt some con- ventions to govern memory use.

Code space

¯ ﬁxed size

¯ statically allocated (link time )

Data space

¯ ﬁxed-sized data may be statically allocated

¯ variable-sized data must be dynamically allocated

¯ some data is dynamically allocated in code

Control stack

¯ dynamic slice of activation tree

¯ return addresses

¯ may be implemented in hardware

221 Run-time storage organization

Typical memory layout

high address

stack

free memory

heap static data

code

low address

The classical scheme

¯ allows both stack and heap maximal freedom

¯ code and static data may be separate or intermingled

222 Run-time storage organization

Where do local variables go? When can we allocate them on a stack? Key issue is lifetime of local names

Downward exposure:

¯ called procedures may reference my variables

¯ dynamic scoping

¯ lexical scoping

Upward exposure:

¯ can I return a reference to my variables?

¯ functions that return functions

¯ continuation-passing style

With only downward exposure , the compiler can allocate the frames on the run-time call stack 223 Storage classes

Each variable must be assigned a storage class (base address )

Static variables:

¯ addresses compiled into code (relocatable)

¯ (usually) allocated at compile-time

¯ limited to ﬁxed size objects

¯ control access with naming scheme

Global variables:

¯ almost identical to static variables

¯ layout may be important (exposed)

¯ naming scheme ensures universal access

Link editor must handle duplicate deﬁnitions

224 Storage classes (cont.)

Procedure local variables Put them on the stack —

¯ if sizes are ﬁxed

¯ if lifetimes are limited

¯ if values are not preserved

Dynamically allocated variables Must be treated differently —

¯ call-by-reference, pointers, lead to non-local lifetimes

¯ (usually) an explicit allocation

¯ explicit or implicit deallocation

225 Access to non-local data

How does the code ﬁnd non-local data at run-time?

Real globals

¯ visible everywhere

¯ naming convention gives an address

¯ initialization requires cooperation

Lexical nesting

¯ view variables as (level,offset) pairs (compile-time)

¯ chain of non-local access links

¯ more expensive to ﬁnd (at run-time )

226 Access to non-local data

Two important problems arise

¯ How do we map a name into a (level,offset) pair? Use a block-structured symbol table (remember last lecture?) – look up a name, want its most recent declaration – declaration may be at current level or any lower level

¯ Given a (level,offset) pair, what’s the address? Two classic approaches – access links (or static links ) – displays

227 Access to non-local data

To ﬁnd the value speciﬁed by ´ l ; o µ need current procedure level, k

¯ k = l µ local value

¯ k > l µ ﬁnd l’s activation record

¯ k < l cannot occur

Maintaining access links: (static links )

¯ calling level k · 1 procedure 1. pass my FP as access link 2. my backward chain will work for lower levels

¯ calling procedure at level l < k

1. ﬁnd link to level l 1 and pass it 2. its access link will work for lower levels

228 The display

To improve run-time access costs, use a display:

¯ table of access links for lower levels

¯ lookup is index from known offset

¯ takes slight amount of time at call

¯ a single display or one per frame

¯ for level k procedure, need k 1 slots

Access with the display ¯

¯ assume a value described by ´ l ; o µ

ﬁnd[ slot as di×ÔÐaÝ l ]

add offset to pointer from[ slot (di×ÔÐaÝ l ][ o ] ) “Setting up the basic frame” now includes display manipulation

229 Calls: Saving and restoring registers caller’s registers callee’s registers all registers callee saves 1 3 5 caller saves 2 4 6 1. Call includes bitmap of caller’s registers to be saved/restored (best with save/restore instructions to interpret bitmap directly)

2. Caller saves and restores its own registers Unstructured returns (e.g., non-local gotos, exceptions) create some problems, since code to restore must be located and executed

3. Backpatch code to save registers used in callee on entry, restore on exit e.g., VAX places bitmap in callee’s stack frame for use on call/return/non-local goto/exception Non-local gotos and exceptions must unwind dynamic chain restoring callee-saved registers

4. Bitmap in callee’s stack frame is used by caller to save/restore (best with save/restore instructions to interpret bitmap directly) Unwind dynamic chain as for 3

5. Easy Non-local gotos and exceptions must restore all registers from “outermost callee”

6. Easy (use utility routine to keep calls compact) Non-local gotos and exceptions need only restore original registers from caller

Top-left is best: saves fewer registers, compact calling sequences

230 Call/return

Assuming callee saves: 1. caller pushes space for return value 2. caller pushes SP 3. caller pushes space for: return address, static chain, saved registers 4. caller evaluates and pushes actuals onto stack 5. caller sets return address, callee’s static chain, performs call 6. callee saves registers in register-save area 7. callee copies by-value arrays/records using addresses passed as actuals 8. callee allocates dynamic arrays as needed 9. on return, callee restores saved registers 10. jumps to return address Caller must allocate much of stack frame, because it computes the actual parameters Alternative is to put actuals below callee’s stack frame in caller’s: common when hardware supports stack management (e.g., VAX)

231 MIPS procedure call convention

Registers: Number Name Usage

0 ÞeÖÓ Constant 0 1 at Reserved for assembler 2, 3 v0, v1 Expression evaluation, scalar function results 4–7 a0–a3 ﬁrst 4 scalar arguments 8–15 t0–t7 Temporaries, caller-saved; caller must save to preserve across calls 16–23 s0–s7 Callee-saved; must be preserved across calls 24, 25 t8, t9 Temporaries, caller-saved; caller must save to preserve across calls 26, 27 k0, k1 Reserved for OS kernel 28 gp Pointer to global area 29 sp Stack pointer 30 s8 (fp) Callee-saved; must be preserved across calls 31 ra Expression evaluation, pass return address in calls

232 MIPS procedure call convention

Philosophy: Use full, general calling sequence only when necessary; omit por- tions of it where possible (e.g., avoid using fp register whenever possible) Classify routines as:

¯ non-leaf routines: routines that call other routines

¯ leaf routines: routines that do not themselves call other routines – leaf routines that require stack storage for locals – leaf routines that do not require stack storage for locals

233 MIPS procedure call convention

The stack frame high memory argument n

argument 1 virtual frame pointer ($fp) static link

locals framesize

frame offset saved $ra temporaries

other saved registers

argument build stack pointer ($sp) low memory

234 MIPS procedure call convention

Pre-call: 1. Pass arguments: use registers a0 . . . a3; remaining arguments are pushedonthestackalongwithsavespacefora0...a3 2. Save caller-saved registers if necessary

3. Execute a jaÐ instruction: jumps to target address (callee’s ﬁrst instruction), saves return address in register ra

235 MIPS procedure call convention

Prologue: 1. Leaf procedures that use the stack and non-leaf procedures: (a) Allocate all stack space needed by routine:

¯ local variables

¯ saved registers

¯ sufﬁcient space for arguments to routines called by this routine

°×Ô¸fÖaÑe×iÞe ×ÙbÙ

(b)°½6¸fÖaÑe×iÞe·fÖaÑeÓff×eØ¹8´°×Ôµ ×Û Save registers (ra, etc.)

°½7¸fÖaÑe×iÞe·fÖaÑeÓff×eØ¹4´°×Ôµ e.g.,×Û

°¿½¸fÖaÑe×iÞe·fÖaÑeÓff×eØ´°×Ôµ ×Û

where fÖaÑe×iÞe and fÖaÑeÓff×eØ (usually negative) are compile- time constants 2. Emit code for routine

236 MIPS procedure call convention

Epilogue: 1. Copy return values into result registers (if not already there) 2. Restore saved registers

Öeg¸fÖaÑe×iÞe·fÖaÑeÓff×eØ¹Æ´°×Ôµ ÐÛ 3. Get return address

°¿½¸fÖaÑe×iÞe·fÖaÑeÓff×eØ´°×Ôµ ÐÛ 4. Clean up stack

°×Ô¸fÖaÑe×iÞe addÙ 5. Return

°¿½ j

237