GNAT: The GNU Ada Compiler June, 2004 Copyright (C) Javier Miranda and Edmond Schonberg [email protected], [email protected] Applied Microelectronics Research Institute University of Las Palmas de Gran Canaria Canary Islands Spain Computer Science Department New York University U.S.A. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any latter published by the Free Software Foundation; with the Invariant Sections being “GNU General Public Documentation License”, the Front-Cover text being “GNAT: The GNU Ada Compiler”, and the Back-Cover text being “Copies published by the Free Software Foundation raise funds for GNU development.” ii Contents I First Part: Introduction 7 1 The GNAT Project 9 1.1 GCC . 10 1.2 The GNAT Compiler . 10 1.3 Compilation Model . 12 1.3.1 Traditional Compilation Model . 13 1.3.2 GNAT Compilation Model . 13 1.4 Summary . 17 2 Overview of the Front-end Architecture 19 2.1 The Scanner . 19 2.2 The Parser . 20 2.2.1 The Abstract Syntax Tree . 22 2.3 The Semantic Analyzer . 24 2.4 The Expander . 27 2.5 Summary . 28 3 Error Recovery 31 3.1 Scanner Error Recovery . 31 3.1.1 Use of the Casing of Identifiers . 32 iii iv CONTENTS 3.2 Parser Error Recovery . 32 3.2.1 The Parser Scope-Stack . 33 3.2.2 Example 1: Use of indentation . 34 3.2.3 Example 2: Handling Semicolon Used in Place of 'is' . 34 3.2.4 Example 3: Handling 'is' Used in Place of Semicolon . 36 3.3 Summary . 37 II Second Part: Semantic Analysis 39 4 Scopes and Visibility 41 4.1 Flags and Data Structures . 41 4.2 Analysis of Records, Tasks and Protected Types. 45 4.3 Analysis of Packages . 46 4.4 Analysis of Private Types . 47 4.4.1 Private Entities Visibility . 48 4.4.2 Private Type Declarations . 49 4.4.3 Deferred Constants and Incomplete Types . 51 4.4.4 Limited Types . 51 4.4.5 Analysis of Child Units . 51 4.4.6 Analysis of Subunits . 52 4.5 Name Resolution . 52 4.6 Summary . 53 5 Overload Resolution 55 5.1 Resolution Algorithm . 56 5.1.1 Additional Details for the Bottom-Up Pass . 57 CONTENTS v 5.1.2 Data Structures . 59 5.1.3 Additional Details for the Top-Down Pass . 60 5.2 Summary . 61 6 Analysis of Discriminants 63 6.1 Analysis of Discriminants . 64 6.2 Analysis of Discriminants in Derived Types . 67 6.3 Discriminals . 68 6.4 Summary . 69 7 Generic Units 71 7.1 Generic Units . 72 7.1.1 Analysis of Generic Units . 72 7.1.2 Instantiation of Generic Units . 75 7.1.3 Parameter Matching . 75 7.1.4 Private Types . 80 7.2 Nested Generic Units . 80 7.2.1 Analysis of Nested Generic Units . 80 7.2.2 Instantiation of Nested Generic Units . 81 7.3 Generic Child Units . 84 7.3.1 Analysis of Generic Child Units . 84 7.3.2 Instantiation of Child Generic Units . 85 7.4 Delayed Instantiation of Bodies . 89 7.5 Detection of Instantiation Circularities . 90 7.6 Summary . 90 8 Freezing Analysis 93 vi CONTENTS 8.1 Freezing Types and Subtypes . 94 8.2 Freezing Expressions . 95 8.3 Freezing Objects . 96 8.4 Freezing Subprograms . 96 8.5 Freezing Packages . 97 8.6 Freezing Generic Units . 97 8.7 Summary . 98 III Third Part: Expansion 99 9 Expansion of Tasks 101 9.1 Task Creation . 102 9.2 Task Termination . 103 9.3 Task Abortion . 103 9.4 Expansion of Task Type Declarations . 104 9.5 Task Body Expansion . 105 9.6 Example of Task Expansion . 106 9.7 Summary . 108 10 Expansion of Rendezvous and related Constructs 109 10.1 Entry Identification . 110 10.2 Entry-Call Expansion . 110 10.2.1 Expansion of Simple Entry-Calls . 112 10.2.2 Expansion of Conditional and Timed Entry-Calls . 112 10.3 Asynchronous Transfer of Control . 113 10.3.1 ATC Implementation Models . 114 CONTENTS vii 10.3.2 Expansion of ATC . 116 10.4 Expansion of Accept Statements . 117 10.4.1 Simple Accept Expansion . 117 10.4.2 Timed and Selective Accept . 118 10.4.3 Count Attribute Expansion . 120 10.5 Summary . 120 11 Expansion of Protected Objects 121 11.1 The Proxy Model . 123 11.1.1 Implementation . 124 11.2 Expansion of Protected Types . 125 11.2.1 Expansion of Protected-Type Specification . 125 11.2.2 Expansion of Protected Subprograms . 127 11.2.3 Expansion of Entry Barriers . 129 11.2.4 Expansion of Entry bodies . 129 11.2.5 Table to Barriers and Entry-Bodies . 130 11.2.6 Expansion of Entry Families . 130 11.3 Optimizations . 130 11.4 Summary . 131 12 Expansion of Controlled-Types 133 12.1 Implementation Overview . 134 12.1.1 Exceptional Block Exit . 135 12.1.2 Finalization of Anonymous Objects . 136 12.1.3 Finalization of Dynamically Allocated Objects . 136 12.1.4 Problems Related to Mutable Objects . 137 12.1.5 Controlled Class-Wide Objects . 137 viii CONTENTS 12.2 Expansion Activities for Controlled-Types . 138 12.2.1 Expansion of Assignment . 139 12.2.2 Expansion of Anonymous Controlled Objects . 140 12.2.3 Objects with Controlled Components . 141 12.3 Summary . 143 13 Expansion of Tagged-Types 145 13.1 Tagged and Polymorphic Objects . 146 13.2 The Dispatch Table . 147 13.3 Primitive Operations . 149 13.4 Summary . 152 IV Fourth Part: Run-Time 155 14 Tasking 157 14.1 The Ada Task Control Block . 157 14.2 Task States . 158 14.3 Task Creation and Termination . 159 14.4 Run-Time Subprograms for Task Creation and Termination . 163 14.4.1 GNARL.Enter Master . 165 14.4.2 GNARL.Create Task . 165 14.4.3 GNARL.Activate Tasks . 166 14.4.4 GNARL.Tasks Wrapper . 168 14.4.5 GNARL.Complete Activation . 168 14.4.6 GNARL.Complete Task . 169 14.4.7 GNARL.Complete Master . 169 CONTENTS ix 14.5 Summary . 170 15 The Rendezvous 171 15.1 The Entry-Call Record . 172 15.2 Entries and Queues . 173 15.3 Accepted-Calls Stack . 174 15.4 Selective Accept . 174 15.5 Run-Time Rendezvous Subprograms . 176 15.5.1 GNARL.Call Simple . 176 15.5.2 GNARL.Call Synchronous . 176 15.5.3 GNARL.Task Do Or Queue . 177 15.5.4 GNARL.Task Entry Call . 177 15.5.5 GNARL.Accept Trivial . 177 15.5.6 GNARL.Accept Call . 178 15.5.7 GNARL.Complete Rendezvous . 178 15.5.8 GNARL.Exceptional Complete Rendezvous . 178 15.5.9 GNARL.Selective Wait . 179 15.5.10 GNARL.Task Count . 180 15.6 Summary . 180 16 Protected Objects 181 16.1 The Lock . ..