The Procedure Abstraction Part III: Symbol Tables, Storage
Copyright 2003, Keith D. Cooper, Ken Kennedy & Linda Torczon, all rights reserved. Students enrolled in Comp 412 at Rice University have explicit permission to make copies of these materials for their personal use. §5.7 in EaC Lexically-scoped Symbol Tables
The problem • The compiler needs a distinct record for each declaration • Nested lexical scopes admit duplicate declarations
The interface • insert(name, level ) – creates record for name at level • lookup(name, level ) – returns pointer or index • delete(level ) – removes all names declared at level
Many implementation schemes have been proposed (see § B.4) • We’ll stay at the conceptual level • Hash table implementation is tricky, detailed, & fun
Symbol tables are compile-time structures the compiler use to resolve references to names. We’ll see the corresponding run-time structures that are used to establish addressability later. Example procedure p { B0: { int a, b, c int a, b, c procedure q { B1: { int v, b, x, w int v, b, x, w procedure r { B2: { int x, y, z int x, y, z …. …. } } procedure s { B3: { int x, a, v int x, a, v … … } } … r … s … } } … q … … } } Example procedure p { B0: { int a, b, c int a0, b1, c2 procedure q { B1: { int v, b, x, w int v3, b4, x5, w6 procedure r { B2: { int x, y, z int x7, y8, z9 …. …. } } procedure s { B3: { int x, a, v int x10, a11, v12 c … … } } b, w … r … s … a11, b4, c2, x, a, v } } … q … … v12, w6,, x10, } } no y or z
Picturing it as a series of Using the rewrite rules from Algol-like procedures COMP 210 for lexical scopes Lexically-scoped Symbol Tables
High-level idea • Create a new table for each scope • Chain them together for lookup
“Sheaf of tables” implementation ... • insert() may need to create table a • it always inserts at current level s q p b • lookup() walks chain of tables & x returns first occurrence of name • v ... • delete() throws away table for level p, if it is top table in the chain b c a If the compiler must preserve the x table (for, say, the debugger ), this w idea is actually practical. v Individual tables can be hash tables. Lexically-scoped Symbol Tables
High-level idea • Create a new table for each scope • Chain them together for lookup
Remember ... a a11, b5, c2, s q the names p v , w , x , b 12 6 , 10 visible in s x no y or z • v ... If we add the subscripts from the a b c COMP 210 rewrite rules, the x relationship between the code and w the table becomes clear v Lexically-scoped Symbol Tables
High-level idea a11, b4, c2, • Create a new table for each scope v12, w6,, x10, • Chain them together for lookup no y or z
procedure p { ... int a0, b1, c2 a procedure q { 0 int v , b , x , w s q 3 4 5 6 p procedure r { b1 x int x7, y8, z9 • 10 …. v3 ... } procedure s { a b c 11 4 2 int x10, a11, v12 x5 … w6 } … v } 12 … } Implementing Lexically Scoped Symbol Tables
Stack organization
Implementation • insert () creates new level growth pointer if needed and inserts at nextFree • lookup () searches linearly nextFree from nextFree–1 forward v a • delete () sets nextFree to s (level 2) x the equal the start location w of the level deleted. x Advantage b • Uses much less space q (level 1) v c Disadvantage b • Lookups can be expensive p (level 0) a Implementing Lexically Scoped Symbol Tables
Stack organization a11, b4, c2, Implementation v12, w6,, x10, • insert () creates new level no y or z growth pointer if needed and inserts at nextFree • lookup () searches linearly nextFree from nextFree–1 backward v 12 a 11 • delete () sets nextFree to s (level 2) x 10 the equal the start location w 6 of the level deleted. x 5 Advantage b 4 • Uses much less space q (level 1) v 3 c 2 Disadvantage b 1 • Lookups can be expensive p (level 0) a 0 Implementing Lexically Scoped Symbol Tables
Threaded stack organization
Implementation • • insert () puts new entry at the growth head of the list for the name • • lookup () goes direct to location • delete () processes each element • v in level being deleted to remove a from head of list h(x) s • x Advantage w • lookup is fast • x b Disadvantage q v • delete takes time proportional to c number of declared variables in b level (though inserts+deletes are • a p still O(n)) Implementing Lexically Scoped Symbol Tables
Threaded stack organization a , b , c , Implementation 11 4 2 • v12, w6,, x10, • insert () puts new entry at the no y or z growth head of the list for the name • • lookup () goes direct to location • delete () processes each element • v 12 in level being deleted to remove a 11 from head of list h(x) s • x 10 Advantage w 6 • lookup is fast • x 5 b 4 Disadvantage q v 3 • delete takes time proportional to c 2 number of declared variables in b 1 level (though inserts+deletes are • a 0 p still O(n)) The Procedure as an External Interface
OS needs a way to start the program’s execution • Programmer needs a way to indicate where it begins The “main” procedure in most languaages • When user invokes “grep” at a command line UNIX/Linux specific discussion OS finds the executable OS creates a process and arranges for it to run “grep” “grep” is code from the compiler, linked with run-time system Starts the run-time environment & calls “main” After main, it shuts down run-time environment & returns • When “grep” needs system services It makes a system call, such as fopen() Where Do All These Variables Go?
Automatic & Local • Keep them in the procedure activation record or in a register • Automatic lifetime matches procedure’s lifetime Static • Procedure scope storage area affixed with procedure name &_p.x • File scope storage area affixed with file name • Lifetime is entire execution Global • One or more named global data areas • One per variable, or per file, or per program, … • Lifetime is entire execution Placing Run-time Data Structures
Classic Organization • Better utilization if stack & heap grow S G S C t l H toward each other t o a & o e a • Very old result (Knuth) d t b a c e i a p • Code & data separate or k c l interleaved 0 high • Uses address space, Single Logical Address Space not allocated memory
• Code, static, & global data have known size Use symbolic labels in the code • Heap & stack both grow & shrink over time • This is a virtual address space How Does This Really Work?
The Big Picture virtual address Compiler’s view spaces
S G S G S G S G S S S S C t l H C t l H C t l H C t l H t t t t o a & o e o a & o e o a & o e o a & o e a a a a d t b a d t b a d t b a ... d t b a c c c c e i a p e i a p e i a p e i a p k k k k c l c l c l c l
OS’s view ...
0 high
Hardware’s view Physical address space_ Where Do Local Variables Live?
A Simplistic model • Allocate a data area for each distinct scope • One data area per “sheaf” in scoped table What about recursion? • Need a data area per invocation (or activation) of a scope • We call this the scope’s activation record • The compiler can also store control information there ! More complex scheme • One activation record (AR) per procedure instance • All the procedure’s scopes share a single AR (may share space) • Static relationship between scopes in single procedure
Used this way, “static” means knowable at compile time (and, therefore, fixed). Translating Local Names
How does the compiler represent a specific instance of x ? • Name is translated into a static coordinate < level,offset > pair “level” is lexical nesting level of the procedure “offset” is unique within that scope • Subsequent code will use the static coordinate to generate addresses and references • “level” is a function of the table in which x is found Stored in the entry for each x • “offset” must be assigned and stored in the symbol table Assigned at compile time Known at compile time Used to generate code that executes at run-time Storage for Blocks within a Single Procedure
B0: { • Fixed length data can always be at a int a, b, c B1: { constant offset from the beginning int v, b, x, w of a procedure B2: { In our example, the a declared at int x, y, z level 0 will always be the first data …. element, stored at byte 0 in the } fixed-length data area B3: { The x declared at level 1 will always int x, a, v be the sixth data item, stored at … byte 20 in the fixed data area } … The x declared at level 2 will always } be the eighth data item, stored at … byte 28 in the fixed data area } But what about the a declared in the second block at level 2? Variable-length Data
B0: { Arrays int a, b If size is fixed at compile time, store in … assign value to a fixed-length data area B1: { If size is variable, store descriptor in int v(a), b, x B2: { fixed length area, with pointer to variable int x, y(8) length area …. Variable-length data area is assigned at } the end of the fixed length area for block in which it is allocated
a xx v(a) b v b y(8)
Includes variable length data for Variable-length data all blocks in the procedure … Activation Record Basics
Space for parameters to the current routine parameters Saved register contents register If function, space for save area return value return value Address to resume caller return address addressability ARP Help with non-local access caller’s ARP To restore caller’s AR on a local return variables Space for local values & variables (including spills)
One AR for each invocation of a procedure Activation Record Details
How does the compiler find the variables? • They are at known offsets from the AR pointer • The static coordinate leads to a “loadAI” operation Level specifies an ARP, offset is the constant
Variable-length data • If AR can be extended, put it after local variables • Leave a pointer at a known offset from ARP • Otherwise, put variable-length data on the heap
Initializing local variables • Must generate explicit code to store the values • Among the procedure’s first actions Activation Record Details Where do activation records live? • If lifetime of AR matches lifetime of invocation, AND
S G • If code normally executes a “return” S C t l H t o a & o e a AR d t b a Keep s on a stack c e i a p k c l • If a procedure can outlive its caller, OR Yes! This stack. • If it can return an object that can reference its execution state ARs must be kept in the heap
• If a procedure makes no calls AR can be allocated statically Efficiency prefers static, stack, then heap Communicating Between Procedures
Most languages provide a parameter passing mechanism Expression used at “call site” becomes variable in callee
Two common binding mechanisms • Call-by-reference passes a pointer to actual parameter Requires slot in the AR (for address of parameter) Multiple names with the same address? • Call-by-value passes a copy of its value at time of call Requires slot in the AR call fee(x,x,x); Each name gets a unique location (may have same value) Arrays are mostly passed by reference, not value
• Can always use global variables … Extra Slides Start Here Establishing Addressability
Must create base addresses • Global & static variables Construct a label by mangling names (i.e., &_fee)
• Local variables Convert to static data coordinate and use ARP + offset
• Local variables of other procedures Convert to static coordinates Find appropriate ARP Use that ARP + offset Must find the right AR { Need links to nameable ARs Establishing Addressability
Using access links • Each AR has a pointer to AR of lexical ancestor • Lexical ancestor need not be the caller
parameters parameters Some setup cost parameters register register save area on each call register save area save area return value return value return value return address return address return address access link access link access link caller’s ARP A caller’s ARP RP caller’s ARP local local variables local variables variables
• Reference to
Using access links Assume • Current lexical level is 2 SC Generated Code • Access link is at ARP -4 <2,8> loadAI r0, 8 r2 Maintaining access link <1,12> loadAI r0, -4 r1 • Calling level k+1 loadAI r1, 12 r2 Use current ARP as link <0,16> loadAI r0, -4 r1 • Calling level j < k loadAI r1, -4 r1 loadAI r1, 16 r2 Find ARP for j –1 Use that ARP as link Access & maintenance cost varies with level
All accesses are relative to ARP (r0 ) Establishing Addressability
Using a display • Global array of pointer to nameable ARs Some setup cost • Needed ARP is an array access away on each call Display level 0 parameters level 1 parameters register level 2 save area parameters register level 3 save area return value register return address save area return value return value return address saved ptr. saved ptr. caller’s ARP ARP return address saved ptr. caller’s ARP local variables caller’s ARP local variables local variables
• Reference to
<2,8> loadAI r0, 8 r2 Maintaining access link <1,12> loadI _disp r1 • On entry to level j loadAI r1, 4 r1 Save level j entry into AR loadAI r1, 12 r2 (Saved Ptr field) <0,16> loadI _disp r1 Store ARP in level j slot loadAI r1, 16 r2 • On exit from level j Restore level j entry Desired AR is at _disp + 4 x level
Access & maintenance costs are fixed Address of display may consume a register Establishing Addressability Access links versus Display • Each adds some overhead to each call • Access links costs vary with level of reference Overhead only incurred on references & calls If ARs outlive the procedure, access links still work • Display costs are fixed for all references References & calls must load display address Typically, this requires a register (rematerialization) Your mileage will vary • Depends on ratio of non-local accesses to calls • Extra register can make a difference in overall speed For either scheme to work, the compiler must insert code into each procedure call & return Procedure Linkages
How do procedure calls actually work? • At compile time, callee may not be available for inspection Different calls may be in different compilation units Compiler may not know system code from user code All calls must use the same protocol
Compiler must use a standard sequence of operations • Enforces control & data abstractions • Divides responsibility between caller & callee Usually a system-wide agreement (for interoperability) Procedure Linkages
Standard procedure linkage
procedure p Procedure has • standard prolog prolog procedure q • standard epilog prolog Each call involves a
pre-call • pre-call sequence • post-return sequence post-return These are completely epilog predictable from the call site depend on the number & type of epilog the actual parameters Procedure Linkages
Pre-call Sequence • Sets up callee’s basic AR • Helps preserve its own environment The Details • Allocate space for the callee’s AR except space for local variables • Evaluates each parameter & stores value or address • Saves return address, caller’s ARP into callee’s AR • If access links are used Find appropriate lexical ancestor & copy into callee’s AR • Save any caller-save registers Save into space in caller’s AR • Jump to address of callee’s prolog code Procedure Linkages
Post-return Sequence • Finish restoring caller’s environment • Place any value back where it belongs
The Details • Copy return value from callee’s AR, if necessary • Free the callee’s AR • Restore any caller-save registers • Restore any call-by-reference parameters to registers, if needed Also copy back call-by-value/result parameters • Continue execution after the call Procedure Linkages Prolog Code • Finish setting up the callee’s environment • Preserve parts of the caller’s environment that will be disturbed With heap allocated AR, The Details may need to use a • Preserve any callee-save registers separate heap object for local variables • If display is being used Save display entry for current lexical level Store current ARP into display for current lexical level • Allocate space for local data Easiest scenario is to extend the AR • Find any static data areas referenced in the callee • Handle any local variable initializations Procedure Linkages
Epilog Code • Wind up the business of the callee If ARs are stack allocated, this may not be necessary. • Start restoring the caller’s environment (Caller can reset stacktop to its pre-call value.) The Details • Store return value? No, this happens on the return statement • Restore callee-save registers • Free space for local data, if necessary (on the heap) • Load return address from AR • Restore caller’s ARP • Jump to the return address