1 ================================ 2 Source Level Debugging with LLVM 3 ================================ 4 5 .. contents:: 6 :local: 7 8 Introduction 9 ============ 10 11 This document is the central repository for all information pertaining to debug 12 information in LLVM. It describes the :ref:`actual format that the LLVM debug 13 information takes <format>`, which is useful for those interested in creating 14 front-ends or dealing directly with the information. Further, this document 15 provides specific examples of what debug information for C/C++ looks like. 16 17 Philosophy behind LLVM debugging information 18 -------------------------------------------- 19 20 The idea of the LLVM debugging information is to capture how the important 21 pieces of the source-language's Abstract Syntax Tree map onto LLVM code. 22 Several design aspects have shaped the solution that appears here. The 23 important ones are: 24 25 * Debugging information should have very little impact on the rest of the 26 compiler. No transformations, analyses, or code generators should need to 27 be modified because of debugging information. 28 29 * LLVM optimizations should interact in :ref:`well-defined and easily described 30 ways <intro_debugopt>` with the debugging information. 31 32 * Because LLVM is designed to support arbitrary programming languages, 33 LLVM-to-LLVM tools should not need to know anything about the semantics of 34 the source-level-language. 35 36 * Source-level languages are often **widely** different from one another. 37 LLVM should not put any restrictions of the flavor of the source-language, 38 and the debugging information should work with any language. 39 40 * With code generator support, it should be possible to use an LLVM compiler 41 to compile a program to native machine code and standard debugging 42 formats. This allows compatibility with traditional machine-code level 43 debuggers, like GDB or DBX. 44 45 The approach used by the LLVM implementation is to use a small set of 46 :ref:`intrinsic functions <format_common_intrinsics>` to define a mapping 47 between LLVM program objects and the source-level objects. The description of 48 the source-level program is maintained in LLVM metadata in an 49 :ref:`implementation-defined format <ccxx_frontend>` (the C/C++ front-end 50 currently uses working draft 7 of the `DWARF 3 standard 51 <http://www.eagercon.com/dwarf/dwarf3std.htm>`_). 52 53 When a program is being debugged, a debugger interacts with the user and turns 54 the stored debug information into source-language specific information. As 55 such, a debugger must be aware of the source-language, and is thus tied to a 56 specific language or family of languages. 57 58 Debug information consumers 59 --------------------------- 60 61 The role of debug information is to provide meta information normally stripped 62 away during the compilation process. This meta information provides an LLVM 63 user a relationship between generated code and the original program source 64 code. 65 66 Currently, debug information is consumed by DwarfDebug to produce dwarf 67 information used by the gdb debugger. Other targets could use the same 68 information to produce stabs or other debug forms. 69 70 It would also be reasonable to use debug information to feed profiling tools 71 for analysis of generated code, or, tools for reconstructing the original 72 source from generated code. 73 74 TODO - expound a bit more. 75 76 .. _intro_debugopt: 77 78 Debugging optimized code 79 ------------------------ 80 81 An extremely high priority of LLVM debugging information is to make it interact 82 well with optimizations and analysis. In particular, the LLVM debug 83 information provides the following guarantees: 84 85 * LLVM debug information **always provides information to accurately read 86 the source-level state of the program**, regardless of which LLVM 87 optimizations have been run, and without any modification to the 88 optimizations themselves. However, some optimizations may impact the 89 ability to modify the current state of the program with a debugger, such 90 as setting program variables, or calling functions that have been 91 deleted. 92 93 * As desired, LLVM optimizations can be upgraded to be aware of the LLVM 94 debugging information, allowing them to update the debugging information 95 as they perform aggressive optimizations. This means that, with effort, 96 the LLVM optimizers could optimize debug code just as well as non-debug 97 code. 98 99 * LLVM debug information does not prevent optimizations from 100 happening (for example inlining, basic block reordering/merging/cleanup, 101 tail duplication, etc). 102 103 * LLVM debug information is automatically optimized along with the rest of 104 the program, using existing facilities. For example, duplicate 105 information is automatically merged by the linker, and unused information 106 is automatically removed. 107 108 Basically, the debug information allows you to compile a program with 109 "``-O0 -g``" and get full debug information, allowing you to arbitrarily modify 110 the program as it executes from a debugger. Compiling a program with 111 "``-O3 -g``" gives you full debug information that is always available and 112 accurate for reading (e.g., you get accurate stack traces despite tail call 113 elimination and inlining), but you might lose the ability to modify the program 114 and call functions where were optimized out of the program, or inlined away 115 completely. 116 117 :ref:`LLVM test suite <test-suite-quickstart>` provides a framework to test 118 optimizer's handling of debugging information. It can be run like this: 119 120 .. code-block:: bash 121 122 % cd llvm/projects/test-suite/MultiSource/Benchmarks # or some other level 123 % make TEST=dbgopt 124 125 This will test impact of debugging information on optimization passes. If 126 debugging information influences optimization passes then it will be reported 127 as a failure. See :doc:`TestingGuide` for more information on LLVM test 128 infrastructure and how to run various tests. 129 130 .. _format: 131 132 Debugging information format 133 ============================ 134 135 LLVM debugging information has been carefully designed to make it possible for 136 the optimizer to optimize the program and debugging information without 137 necessarily having to know anything about debugging information. In 138 particular, the use of metadata avoids duplicated debugging information from 139 the beginning, and the global dead code elimination pass automatically deletes 140 debugging information for a function if it decides to delete the function. 141 142 To do this, most of the debugging information (descriptors for types, 143 variables, functions, source files, etc) is inserted by the language front-end 144 in the form of LLVM metadata. 145 146 Debug information is designed to be agnostic about the target debugger and 147 debugging information representation (e.g. DWARF/Stabs/etc). It uses a generic 148 pass to decode the information that represents variables, types, functions, 149 namespaces, etc: this allows for arbitrary source-language semantics and 150 type-systems to be used, as long as there is a module written for the target 151 debugger to interpret the information. 152 153 To provide basic functionality, the LLVM debugger does have to make some 154 assumptions about the source-level language being debugged, though it keeps 155 these to a minimum. The only common features that the LLVM debugger assumes 156 exist are `source files <LangRef.html#difile>`_, and `program objects 157 <LangRef.html#diglobalvariable>`_. These abstract objects are used by a 158 debugger to form stack traces, show information about local variables, etc. 159 160 This section of the documentation first describes the representation aspects 161 common to any source-language. :ref:`ccxx_frontend` describes the data layout 162 conventions used by the C and C++ front-ends. 163 164 Debug information descriptors are `specialized metadata nodes 165 <LangRef.html#specialized-metadata>`_, first-class subclasses of ``Metadata``. 166 167 .. _format_common_intrinsics: 168 169 Debugger intrinsic functions 170 ---------------------------- 171 172 LLVM uses several intrinsic functions (name prefixed with "``llvm.dbg``") to 173 provide debug information at various points in generated code. 174 175 ``llvm.dbg.declare`` 176 ^^^^^^^^^^^^^^^^^^^^ 177 178 .. code-block:: llvm 179 180 void @llvm.dbg.declare(metadata, metadata, metadata) 181 182 This intrinsic provides information about a local element (e.g., variable). 183 The first argument is metadata holding the alloca for the variable. The second 184 argument is a `local variable <LangRef.html#dilocalvariable>`_ containing a 185 description of the variable. The third argument is a `complex expression 186 <LangRef.html#diexpression>`_. 187 188 ``llvm.dbg.value`` 189 ^^^^^^^^^^^^^^^^^^ 190 191 .. code-block:: llvm 192 193 void @llvm.dbg.value(metadata, i64, metadata, metadata) 194 195 This intrinsic provides information when a user source variable is set to a new 196 value. The first argument is the new value (wrapped as metadata). The second 197 argument is the offset in the user source variable where the new value is 198 written. The third argument is a `local variable 199 <LangRef.html#dilocalvariable>`_ containing a description of the variable. The 200 third argument is a `complex expression <LangRef.html#diexpression>`_. 201 202 Object lifetimes and scoping 203 ============================ 204 205 In many languages, the local variables in functions can have their lifetimes or 206 scopes limited to a subset of a function. In the C family of languages, for 207 example, variables are only live (readable and writable) within the source 208 block that they are defined in. In functional languages, values are only 209 readable after they have been defined. Though this is a very obvious concept, 210 it is non-trivial to model in LLVM, because it has no notion of scoping in this 211 sense, and does not want to be tied to a language's scoping rules. 212 213 In order to handle this, the LLVM debug format uses the metadata attached to 214 llvm instructions to encode line number and scoping information. Consider the 215 following C fragment, for example: 216 217 .. code-block:: c 218 219 1. void foo() { 220 2. int X = 21; 221 3. int Y = 22; 222 4. { 223 5. int Z = 23; 224 6. Z = X; 225 7. } 226 8. X = Y; 227 9. } 228 229 Compiled to LLVM, this function would be represented like this: 230 231 .. code-block:: llvm 232 233 ; Function Attrs: nounwind ssp uwtable 234 define void @foo() #0 !dbg !4 { 235 entry: 236 %X = alloca i32, align 4 237 %Y = alloca i32, align 4 238 %Z = alloca i32, align 4 239 call void @llvm.dbg.declare(metadata i32* %X, metadata !11, metadata !13), !dbg !14 240 store i32 21, i32* %X, align 4, !dbg !14 241 call void @llvm.dbg.declare(metadata i32* %Y, metadata !15, metadata !13), !dbg !16 242 store i32 22, i32* %Y, align 4, !dbg !16 243 call void @llvm.dbg.declare(metadata i32* %Z, metadata !17, metadata !13), !dbg !19 244 store i32 23, i32* %Z, align 4, !dbg !19 245 %0 = load i32, i32* %X, align 4, !dbg !20 246 store i32 %0, i32* %Z, align 4, !dbg !21 247 %1 = load i32, i32* %Y, align 4, !dbg !22 248 store i32 %1, i32* %X, align 4, !dbg !23 249 ret void, !dbg !24 250 } 251 252 ; Function Attrs: nounwind readnone 253 declare void @llvm.dbg.declare(metadata, metadata, metadata) #1 254 255 attributes #0 = { nounwind ssp uwtable "less-precise-fpmad"="false" "no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf" "no-infs-fp-math"="false" "no-nans-fp-math"="false" "stack-protector-buffer-size"="8" "unsafe-fp-math"="false" "use-soft-float"="false" } 256 attributes #1 = { nounwind readnone } 257 258 !llvm.dbg.cu = !{!0} 259 !llvm.module.flags = !{!7, !8, !9} 260 !llvm.ident = !{!10} 261 262 !0 = !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "clang version 3.7.0 (trunk 231150) (llvm/trunk 231154)", isOptimized: false, runtimeVersion: 0, emissionKind: 1, enums: !2, retainedTypes: !2, subprograms: !3, globals: !2, imports: !2) 263 !1 = !DIFile(filename: "/dev/stdin", directory: "/Users/dexonsmith/data/llvm/debug-info") 264 !2 = !{} 265 !3 = !{!4} 266 !4 = distinct !DISubprogram(name: "foo", scope: !1, file: !1, line: 1, type: !5, isLocal: false, isDefinition: true, scopeLine: 1, isOptimized: false, variables: !2) 267 !5 = !DISubroutineType(types: !6) 268 !6 = !{null} 269 !7 = !{i32 2, !"Dwarf Version", i32 2} 270 !8 = !{i32 2, !"Debug Info Version", i32 3} 271 !9 = !{i32 1, !"PIC Level", i32 2} 272 !10 = !{!"clang version 3.7.0 (trunk 231150) (llvm/trunk 231154)"} 273 !11 = !DILocalVariable(name: "X", scope: !4, file: !1, line: 2, type: !12) 274 !12 = !DIBasicType(name: "int", size: 32, align: 32, encoding: DW_ATE_signed) 275 !13 = !DIExpression() 276 !14 = !DILocation(line: 2, column: 9, scope: !4) 277 !15 = !DILocalVariable(name: "Y", scope: !4, file: !1, line: 3, type: !12) 278 !16 = !DILocation(line: 3, column: 9, scope: !4) 279 !17 = !DILocalVariable(name: "Z", scope: !18, file: !1, line: 5, type: !12) 280 !18 = distinct !DILexicalBlock(scope: !4, file: !1, line: 4, column: 5) 281 !19 = !DILocation(line: 5, column: 11, scope: !18) 282 !20 = !DILocation(line: 6, column: 11, scope: !18) 283 !21 = !DILocation(line: 6, column: 9, scope: !18) 284 !22 = !DILocation(line: 8, column: 9, scope: !4) 285 !23 = !DILocation(line: 8, column: 7, scope: !4) 286 !24 = !DILocation(line: 9, column: 3, scope: !4) 287 288 289 This example illustrates a few important details about LLVM debugging 290 information. In particular, it shows how the ``llvm.dbg.declare`` intrinsic and 291 location information, which are attached to an instruction, are applied 292 together to allow a debugger to analyze the relationship between statements, 293 variable definitions, and the code used to implement the function. 294 295 .. code-block:: llvm 296 297 call void @llvm.dbg.declare(metadata i32* %X, metadata !11, metadata !13), !dbg !14 298 ; [debug line = 2:7] [debug variable = X] 299 300 The first intrinsic ``%llvm.dbg.declare`` encodes debugging information for the 301 variable ``X``. The metadata ``!dbg !14`` attached to the intrinsic provides 302 scope information for the variable ``X``. 303 304 .. code-block:: llvm 305 306 !14 = !DILocation(line: 2, column: 9, scope: !4) 307 !4 = distinct !DISubprogram(name: "foo", scope: !1, file: !1, line: 1, type: !5, 308 isLocal: false, isDefinition: true, scopeLine: 1, 309 isOptimized: false, variables: !2) 310 311 Here ``!14`` is metadata providing `location information 312 <LangRef.html#dilocation>`_. In this example, scope is encoded by ``!4``, a 313 `subprogram descriptor <LangRef.html#disubprogram>`_. This way the location 314 information attached to the intrinsics indicates that the variable ``X`` is 315 declared at line number 2 at a function level scope in function ``foo``. 316 317 Now lets take another example. 318 319 .. code-block:: llvm 320 321 call void @llvm.dbg.declare(metadata i32* %Z, metadata !17, metadata !13), !dbg !19 322 ; [debug line = 5:9] [debug variable = Z] 323 324 The third intrinsic ``%llvm.dbg.declare`` encodes debugging information for 325 variable ``Z``. The metadata ``!dbg !19`` attached to the intrinsic provides 326 scope information for the variable ``Z``. 327 328 .. code-block:: llvm 329 330 !18 = distinct !DILexicalBlock(scope: !4, file: !1, line: 4, column: 5) 331 !19 = !DILocation(line: 5, column: 11, scope: !18) 332 333 Here ``!19`` indicates that ``Z`` is declared at line number 5 and column 334 number 0 inside of lexical scope ``!18``. The lexical scope itself resides 335 inside of subprogram ``!4`` described above. 336 337 The scope information attached with each instruction provides a straightforward 338 way to find instructions covered by a scope. 339 340 .. _ccxx_frontend: 341 342 C/C++ front-end specific debug information 343 ========================================== 344 345 The C and C++ front-ends represent information about the program in a format 346 that is effectively identical to `DWARF 3.0 347 <http://www.eagercon.com/dwarf/dwarf3std.htm>`_ in terms of information 348 content. This allows code generators to trivially support native debuggers by 349 generating standard dwarf information, and contains enough information for 350 non-dwarf targets to translate it as needed. 351 352 This section describes the forms used to represent C and C++ programs. Other 353 languages could pattern themselves after this (which itself is tuned to 354 representing programs in the same way that DWARF 3 does), or they could choose 355 to provide completely different forms if they don't fit into the DWARF model. 356 As support for debugging information gets added to the various LLVM 357 source-language front-ends, the information used should be documented here. 358 359 The following sections provide examples of a few C/C++ constructs and the debug 360 information that would best describe those constructs. The canonical 361 references are the ``DIDescriptor`` classes defined in 362 ``include/llvm/IR/DebugInfo.h`` and the implementations of the helper functions 363 in ``lib/IR/DIBuilder.cpp``. 364 365 C/C++ source file information 366 ----------------------------- 367 368 ``llvm::Instruction`` provides easy access to metadata attached with an 369 instruction. One can extract line number information encoded in LLVM IR using 370 ``Instruction::getDebugLoc()`` and ``DILocation::getLine()``. 371 372 .. code-block:: c++ 373 374 if (DILocation *Loc = I->getDebugLoc()) { // Here I is an LLVM instruction 375 unsigned Line = Loc->getLine(); 376 StringRef File = Loc->getFilename(); 377 StringRef Dir = Loc->getDirectory(); 378 } 379 380 C/C++ global variable information 381 --------------------------------- 382 383 Given an integer global variable declared as follows: 384 385 .. code-block:: c 386 387 int MyGlobal = 100; 388 389 a C/C++ front-end would generate the following descriptors: 390 391 .. code-block:: llvm 392 393 ;; 394 ;; Define the global itself. 395 ;; 396 @MyGlobal = global i32 100, align 4 397 398 ;; 399 ;; List of debug info of globals 400 ;; 401 !llvm.dbg.cu = !{!0} 402 403 ;; Some unrelated metadata. 404 !llvm.module.flags = !{!6, !7} 405 406 ;; Define the compile unit. 407 !0 = !DICompileUnit(language: DW_LANG_C99, file: !1, 408 producer: 409 "clang version 3.7.0 (trunk 231150) (llvm/trunk 231154)", 410 isOptimized: false, runtimeVersion: 0, emissionKind: 1, 411 enums: !2, retainedTypes: !2, subprograms: !2, globals: 412 !3, imports: !2) 413 414 ;; 415 ;; Define the file 416 ;; 417 !1 = !DIFile(filename: "/dev/stdin", 418 directory: "/Users/dexonsmith/data/llvm/debug-info") 419 420 ;; An empty array. 421 !2 = !{} 422 423 ;; The Array of Global Variables 424 !3 = !{!4} 425 426 ;; 427 ;; Define the global variable itself. 428 ;; 429 !4 = !DIGlobalVariable(name: "MyGlobal", scope: !0, file: !1, line: 1, 430 type: !5, isLocal: false, isDefinition: true, 431 variable: i32* @MyGlobal) 432 433 ;; 434 ;; Define the type 435 ;; 436 !5 = !DIBasicType(name: "int", size: 32, align: 32, encoding: DW_ATE_signed) 437 438 ;; Dwarf version to output. 439 !6 = !{i32 2, !"Dwarf Version", i32 2} 440 441 ;; Debug info schema version. 442 !7 = !{i32 2, !"Debug Info Version", i32 3} 443 444 C/C++ function information 445 -------------------------- 446 447 Given a function declared as follows: 448 449 .. code-block:: c 450 451 int main(int argc, char *argv[]) { 452 return 0; 453 } 454 455 a C/C++ front-end would generate the following descriptors: 456 457 .. code-block:: llvm 458 459 ;; 460 ;; Define the anchor for subprograms. 461 ;; 462 !4 = !DISubprogram(name: "main", scope: !1, file: !1, line: 1, type: !5, 463 isLocal: false, isDefinition: true, scopeLine: 1, 464 flags: DIFlagPrototyped, isOptimized: false, 465 variables: !2) 466 467 ;; 468 ;; Define the subprogram itself. 469 ;; 470 define i32 @main(i32 %argc, i8** %argv) !dbg !4 { 471 ... 472 } 473 474 Debugging information format 475 ============================ 476 477 Debugging Information Extension for Objective C Properties 478 ---------------------------------------------------------- 479 480 Introduction 481 ^^^^^^^^^^^^ 482 483 Objective C provides a simpler way to declare and define accessor methods using 484 declared properties. The language provides features to declare a property and 485 to let compiler synthesize accessor methods. 486 487 The debugger lets developer inspect Objective C interfaces and their instance 488 variables and class variables. However, the debugger does not know anything 489 about the properties defined in Objective C interfaces. The debugger consumes 490 information generated by compiler in DWARF format. The format does not support 491 encoding of Objective C properties. This proposal describes DWARF extensions to 492 encode Objective C properties, which the debugger can use to let developers 493 inspect Objective C properties. 494 495 Proposal 496 ^^^^^^^^ 497 498 Objective C properties exist separately from class members. A property can be 499 defined only by "setter" and "getter" selectors, and be calculated anew on each 500 access. Or a property can just be a direct access to some declared ivar. 501 Finally it can have an ivar "automatically synthesized" for it by the compiler, 502 in which case the property can be referred to in user code directly using the 503 standard C dereference syntax as well as through the property "dot" syntax, but 504 there is no entry in the ``@interface`` declaration corresponding to this ivar. 505 506 To facilitate debugging, these properties we will add a new DWARF TAG into the 507 ``DW_TAG_structure_type`` definition for the class to hold the description of a 508 given property, and a set of DWARF attributes that provide said description. 509 The property tag will also contain the name and declared type of the property. 510 511 If there is a related ivar, there will also be a DWARF property attribute placed 512 in the ``DW_TAG_member`` DIE for that ivar referring back to the property TAG 513 for that property. And in the case where the compiler synthesizes the ivar 514 directly, the compiler is expected to generate a ``DW_TAG_member`` for that 515 ivar (with the ``DW_AT_artificial`` set to 1), whose name will be the name used 516 to access this ivar directly in code, and with the property attribute pointing 517 back to the property it is backing. 518 519 The following examples will serve as illustration for our discussion: 520 521 .. code-block:: objc 522 523 @interface I1 { 524 int n2; 525 } 526 527 @property int p1; 528 @property int p2; 529 @end 530 531 @implementation I1 532 @synthesize p1; 533 @synthesize p2 = n2; 534 @end 535 536 This produces the following DWARF (this is a "pseudo dwarfdump" output): 537 538 .. code-block:: none 539 540 0x00000100: TAG_structure_type [7] * 541 AT_APPLE_runtime_class( 0x10 ) 542 AT_name( "I1" ) 543 AT_decl_file( "Objc_Property.m" ) 544 AT_decl_line( 3 ) 545 546 0x00000110 TAG_APPLE_property 547 AT_name ( "p1" ) 548 AT_type ( {0x00000150} ( int ) ) 549 550 0x00000120: TAG_APPLE_property 551 AT_name ( "p2" ) 552 AT_type ( {0x00000150} ( int ) ) 553 554 0x00000130: TAG_member [8] 555 AT_name( "_p1" ) 556 AT_APPLE_property ( {0x00000110} "p1" ) 557 AT_type( {0x00000150} ( int ) ) 558 AT_artificial ( 0x1 ) 559 560 0x00000140: TAG_member [8] 561 AT_name( "n2" ) 562 AT_APPLE_property ( {0x00000120} "p2" ) 563 AT_type( {0x00000150} ( int ) ) 564 565 0x00000150: AT_type( ( int ) ) 566 567 Note, the current convention is that the name of the ivar for an 568 auto-synthesized property is the name of the property from which it derives 569 with an underscore prepended, as is shown in the example. But we actually 570 don't need to know this convention, since we are given the name of the ivar 571 directly. 572 573 Also, it is common practice in ObjC to have different property declarations in 574 the @interface and @implementation - e.g. to provide a read-only property in 575 the interface,and a read-write interface in the implementation. In that case, 576 the compiler should emit whichever property declaration will be in force in the 577 current translation unit. 578 579 Developers can decorate a property with attributes which are encoded using 580 ``DW_AT_APPLE_property_attribute``. 581 582 .. code-block:: objc 583 584 @property (readonly, nonatomic) int pr; 585 586 .. code-block:: none 587 588 TAG_APPLE_property [8] 589 AT_name( "pr" ) 590 AT_type ( {0x00000147} (int) ) 591 AT_APPLE_property_attribute (DW_APPLE_PROPERTY_readonly, DW_APPLE_PROPERTY_nonatomic) 592 593 The setter and getter method names are attached to the property using 594 ``DW_AT_APPLE_property_setter`` and ``DW_AT_APPLE_property_getter`` attributes. 595 596 .. code-block:: objc 597 598 @interface I1 599 @property (setter=myOwnP3Setter:) int p3; 600 -(void)myOwnP3Setter:(int)a; 601 @end 602 603 @implementation I1 604 @synthesize p3; 605 -(void)myOwnP3Setter:(int)a{ } 606 @end 607 608 The DWARF for this would be: 609 610 .. code-block:: none 611 612 0x000003bd: TAG_structure_type [7] * 613 AT_APPLE_runtime_class( 0x10 ) 614 AT_name( "I1" ) 615 AT_decl_file( "Objc_Property.m" ) 616 AT_decl_line( 3 ) 617 618 0x000003cd TAG_APPLE_property 619 AT_name ( "p3" ) 620 AT_APPLE_property_setter ( "myOwnP3Setter:" ) 621 AT_type( {0x00000147} ( int ) ) 622 623 0x000003f3: TAG_member [8] 624 AT_name( "_p3" ) 625 AT_type ( {0x00000147} ( int ) ) 626 AT_APPLE_property ( {0x000003cd} ) 627 AT_artificial ( 0x1 ) 628 629 New DWARF Tags 630 ^^^^^^^^^^^^^^ 631 632 +-----------------------+--------+ 633 | TAG | Value | 634 +=======================+========+ 635 | DW_TAG_APPLE_property | 0x4200 | 636 +-----------------------+--------+ 637 638 New DWARF Attributes 639 ^^^^^^^^^^^^^^^^^^^^ 640 641 +--------------------------------+--------+-----------+ 642 | Attribute | Value | Classes | 643 +================================+========+===========+ 644 | DW_AT_APPLE_property | 0x3fed | Reference | 645 +--------------------------------+--------+-----------+ 646 | DW_AT_APPLE_property_getter | 0x3fe9 | String | 647 +--------------------------------+--------+-----------+ 648 | DW_AT_APPLE_property_setter | 0x3fea | String | 649 +--------------------------------+--------+-----------+ 650 | DW_AT_APPLE_property_attribute | 0x3feb | Constant | 651 +--------------------------------+--------+-----------+ 652 653 New DWARF Constants 654 ^^^^^^^^^^^^^^^^^^^ 655 656 +--------------------------------------+-------+ 657 | Name | Value | 658 +======================================+=======+ 659 | DW_APPLE_PROPERTY_readonly | 0x01 | 660 +--------------------------------------+-------+ 661 | DW_APPLE_PROPERTY_getter | 0x02 | 662 +--------------------------------------+-------+ 663 | DW_APPLE_PROPERTY_assign | 0x04 | 664 +--------------------------------------+-------+ 665 | DW_APPLE_PROPERTY_readwrite | 0x08 | 666 +--------------------------------------+-------+ 667 | DW_APPLE_PROPERTY_retain | 0x10 | 668 +--------------------------------------+-------+ 669 | DW_APPLE_PROPERTY_copy | 0x20 | 670 +--------------------------------------+-------+ 671 | DW_APPLE_PROPERTY_nonatomic | 0x40 | 672 +--------------------------------------+-------+ 673 | DW_APPLE_PROPERTY_setter | 0x80 | 674 +--------------------------------------+-------+ 675 | DW_APPLE_PROPERTY_atomic | 0x100 | 676 +--------------------------------------+-------+ 677 | DW_APPLE_PROPERTY_weak | 0x200 | 678 +--------------------------------------+-------+ 679 | DW_APPLE_PROPERTY_strong | 0x400 | 680 +--------------------------------------+-------+ 681 | DW_APPLE_PROPERTY_unsafe_unretained | 0x800 | 682 +--------------------------------+-----+-------+ 683 684 Name Accelerator Tables 685 ----------------------- 686 687 Introduction 688 ^^^^^^^^^^^^ 689 690 The "``.debug_pubnames``" and "``.debug_pubtypes``" formats are not what a 691 debugger needs. The "``pub``" in the section name indicates that the entries 692 in the table are publicly visible names only. This means no static or hidden 693 functions show up in the "``.debug_pubnames``". No static variables or private 694 class variables are in the "``.debug_pubtypes``". Many compilers add different 695 things to these tables, so we can't rely upon the contents between gcc, icc, or 696 clang. 697 698 The typical query given by users tends not to match up with the contents of 699 these tables. For example, the DWARF spec states that "In the case of the name 700 of a function member or static data member of a C++ structure, class or union, 701 the name presented in the "``.debug_pubnames``" section is not the simple name 702 given by the ``DW_AT_name attribute`` of the referenced debugging information 703 entry, but rather the fully qualified name of the data or function member." 704 So the only names in these tables for complex C++ entries is a fully 705 qualified name. Debugger users tend not to enter their search strings as 706 "``a::b::c(int,const Foo&) const``", but rather as "``c``", "``b::c``" , or 707 "``a::b::c``". So the name entered in the name table must be demangled in 708 order to chop it up appropriately and additional names must be manually entered 709 into the table to make it effective as a name lookup table for debuggers to 710 use. 711 712 All debuggers currently ignore the "``.debug_pubnames``" table as a result of 713 its inconsistent and useless public-only name content making it a waste of 714 space in the object file. These tables, when they are written to disk, are not 715 sorted in any way, leaving every debugger to do its own parsing and sorting. 716 These tables also include an inlined copy of the string values in the table 717 itself making the tables much larger than they need to be on disk, especially 718 for large C++ programs. 719 720 Can't we just fix the sections by adding all of the names we need to this 721 table? No, because that is not what the tables are defined to contain and we 722 won't know the difference between the old bad tables and the new good tables. 723 At best we could make our own renamed sections that contain all of the data we 724 need. 725 726 These tables are also insufficient for what a debugger like LLDB needs. LLDB 727 uses clang for its expression parsing where LLDB acts as a PCH. LLDB is then 728 often asked to look for type "``foo``" or namespace "``bar``", or list items in 729 namespace "``baz``". Namespaces are not included in the pubnames or pubtypes 730 tables. Since clang asks a lot of questions when it is parsing an expression, 731 we need to be very fast when looking up names, as it happens a lot. Having new 732 accelerator tables that are optimized for very quick lookups will benefit this 733 type of debugging experience greatly. 734 735 We would like to generate name lookup tables that can be mapped into memory 736 from disk, and used as is, with little or no up-front parsing. We would also 737 be able to control the exact content of these different tables so they contain 738 exactly what we need. The Name Accelerator Tables were designed to fix these 739 issues. In order to solve these issues we need to: 740 741 * Have a format that can be mapped into memory from disk and used as is 742 * Lookups should be very fast 743 * Extensible table format so these tables can be made by many producers 744 * Contain all of the names needed for typical lookups out of the box 745 * Strict rules for the contents of tables 746 747 Table size is important and the accelerator table format should allow the reuse 748 of strings from common string tables so the strings for the names are not 749 duplicated. We also want to make sure the table is ready to be used as-is by 750 simply mapping the table into memory with minimal header parsing. 751 752 The name lookups need to be fast and optimized for the kinds of lookups that 753 debuggers tend to do. Optimally we would like to touch as few parts of the 754 mapped table as possible when doing a name lookup and be able to quickly find 755 the name entry we are looking for, or discover there are no matches. In the 756 case of debuggers we optimized for lookups that fail most of the time. 757 758 Each table that is defined should have strict rules on exactly what is in the 759 accelerator tables and documented so clients can rely on the content. 760 761 Hash Tables 762 ^^^^^^^^^^^ 763 764 Standard Hash Tables 765 """""""""""""""""""" 766 767 Typical hash tables have a header, buckets, and each bucket points to the 768 bucket contents: 769 770 .. code-block:: none 771 772 .------------. 773 | HEADER | 774 |------------| 775 | BUCKETS | 776 |------------| 777 | DATA | 778 `------------' 779 780 The BUCKETS are an array of offsets to DATA for each hash: 781 782 .. code-block:: none 783 784 .------------. 785 | 0x00001000 | BUCKETS[0] 786 | 0x00002000 | BUCKETS[1] 787 | 0x00002200 | BUCKETS[2] 788 | 0x000034f0 | BUCKETS[3] 789 | | ... 790 | 0xXXXXXXXX | BUCKETS[n_buckets] 791 '------------' 792 793 So for ``bucket[3]`` in the example above, we have an offset into the table 794 0x000034f0 which points to a chain of entries for the bucket. Each bucket must 795 contain a next pointer, full 32 bit hash value, the string itself, and the data 796 for the current string value. 797 798 .. code-block:: none 799 800 .------------. 801 0x000034f0: | 0x00003500 | next pointer 802 | 0x12345678 | 32 bit hash 803 | "erase" | string value 804 | data[n] | HashData for this bucket 805 |------------| 806 0x00003500: | 0x00003550 | next pointer 807 | 0x29273623 | 32 bit hash 808 | "dump" | string value 809 | data[n] | HashData for this bucket 810 |------------| 811 0x00003550: | 0x00000000 | next pointer 812 | 0x82638293 | 32 bit hash 813 | "main" | string value 814 | data[n] | HashData for this bucket 815 `------------' 816 817 The problem with this layout for debuggers is that we need to optimize for the 818 negative lookup case where the symbol we're searching for is not present. So 819 if we were to lookup "``printf``" in the table above, we would make a 32 hash 820 for "``printf``", it might match ``bucket[3]``. We would need to go to the 821 offset 0x000034f0 and start looking to see if our 32 bit hash matches. To do 822 so, we need to read the next pointer, then read the hash, compare it, and skip 823 to the next bucket. Each time we are skipping many bytes in memory and 824 touching new cache pages just to do the compare on the full 32 bit hash. All 825 of these accesses then tell us that we didn't have a match. 826 827 Name Hash Tables 828 """""""""""""""" 829 830 To solve the issues mentioned above we have structured the hash tables a bit 831 differently: a header, buckets, an array of all unique 32 bit hash values, 832 followed by an array of hash value data offsets, one for each hash value, then 833 the data for all hash values: 834 835 .. code-block:: none 836 837 .-------------. 838 | HEADER | 839 |-------------| 840 | BUCKETS | 841 |-------------| 842 | HASHES | 843 |-------------| 844 | OFFSETS | 845 |-------------| 846 | DATA | 847 `-------------' 848 849 The ``BUCKETS`` in the name tables are an index into the ``HASHES`` array. By 850 making all of the full 32 bit hash values contiguous in memory, we allow 851 ourselves to efficiently check for a match while touching as little memory as 852 possible. Most often checking the 32 bit hash values is as far as the lookup 853 goes. If it does match, it usually is a match with no collisions. So for a 854 table with "``n_buckets``" buckets, and "``n_hashes``" unique 32 bit hash 855 values, we can clarify the contents of the ``BUCKETS``, ``HASHES`` and 856 ``OFFSETS`` as: 857 858 .. code-block:: none 859 860 .-------------------------. 861 | HEADER.magic | uint32_t 862 | HEADER.version | uint16_t 863 | HEADER.hash_function | uint16_t 864 | HEADER.bucket_count | uint32_t 865 | HEADER.hashes_count | uint32_t 866 | HEADER.header_data_len | uint32_t 867 | HEADER_DATA | HeaderData 868 |-------------------------| 869 | BUCKETS | uint32_t[n_buckets] // 32 bit hash indexes 870 |-------------------------| 871 | HASHES | uint32_t[n_hashes] // 32 bit hash values 872 |-------------------------| 873 | OFFSETS | uint32_t[n_hashes] // 32 bit offsets to hash value data 874 |-------------------------| 875 | ALL HASH DATA | 876 `-------------------------' 877 878 So taking the exact same data from the standard hash example above we end up 879 with: 880 881 .. code-block:: none 882 883 .------------. 884 | HEADER | 885 |------------| 886 | 0 | BUCKETS[0] 887 | 2 | BUCKETS[1] 888 | 5 | BUCKETS[2] 889 | 6 | BUCKETS[3] 890 | | ... 891 | ... | BUCKETS[n_buckets] 892 |------------| 893 | 0x........ | HASHES[0] 894 | 0x........ | HASHES[1] 895 | 0x........ | HASHES[2] 896 | 0x........ | HASHES[3] 897 | 0x........ | HASHES[4] 898 | 0x........ | HASHES[5] 899 | 0x12345678 | HASHES[6] hash for BUCKETS[3] 900 | 0x29273623 | HASHES[7] hash for BUCKETS[3] 901 | 0x82638293 | HASHES[8] hash for BUCKETS[3] 902 | 0x........ | HASHES[9] 903 | 0x........ | HASHES[10] 904 | 0x........ | HASHES[11] 905 | 0x........ | HASHES[12] 906 | 0x........ | HASHES[13] 907 | 0x........ | HASHES[n_hashes] 908 |------------| 909 | 0x........ | OFFSETS[0] 910 | 0x........ | OFFSETS[1] 911 | 0x........ | OFFSETS[2] 912 | 0x........ | OFFSETS[3] 913 | 0x........ | OFFSETS[4] 914 | 0x........ | OFFSETS[5] 915 | 0x000034f0 | OFFSETS[6] offset for BUCKETS[3] 916 | 0x00003500 | OFFSETS[7] offset for BUCKETS[3] 917 | 0x00003550 | OFFSETS[8] offset for BUCKETS[3] 918 | 0x........ | OFFSETS[9] 919 | 0x........ | OFFSETS[10] 920 | 0x........ | OFFSETS[11] 921 | 0x........ | OFFSETS[12] 922 | 0x........ | OFFSETS[13] 923 | 0x........ | OFFSETS[n_hashes] 924 |------------| 925 | | 926 | | 927 | | 928 | | 929 | | 930 |------------| 931 0x000034f0: | 0x00001203 | .debug_str ("erase") 932 | 0x00000004 | A 32 bit array count - number of HashData with name "erase" 933 | 0x........ | HashData[0] 934 | 0x........ | HashData[1] 935 | 0x........ | HashData[2] 936 | 0x........ | HashData[3] 937 | 0x00000000 | String offset into .debug_str (terminate data for hash) 938 |------------| 939 0x00003500: | 0x00001203 | String offset into .debug_str ("collision") 940 | 0x00000002 | A 32 bit array count - number of HashData with name "collision" 941 | 0x........ | HashData[0] 942 | 0x........ | HashData[1] 943 | 0x00001203 | String offset into .debug_str ("dump") 944 | 0x00000003 | A 32 bit array count - number of HashData with name "dump" 945 | 0x........ | HashData[0] 946 | 0x........ | HashData[1] 947 | 0x........ | HashData[2] 948 | 0x00000000 | String offset into .debug_str (terminate data for hash) 949 |------------| 950 0x00003550: | 0x00001203 | String offset into .debug_str ("main") 951 | 0x00000009 | A 32 bit array count - number of HashData with name "main" 952 | 0x........ | HashData[0] 953 | 0x........ | HashData[1] 954 | 0x........ | HashData[2] 955 | 0x........ | HashData[3] 956 | 0x........ | HashData[4] 957 | 0x........ | HashData[5] 958 | 0x........ | HashData[6] 959 | 0x........ | HashData[7] 960 | 0x........ | HashData[8] 961 | 0x00000000 | String offset into .debug_str (terminate data for hash) 962 `------------' 963 964 So we still have all of the same data, we just organize it more efficiently for 965 debugger lookup. If we repeat the same "``printf``" lookup from above, we 966 would hash "``printf``" and find it matches ``BUCKETS[3]`` by taking the 32 bit 967 hash value and modulo it by ``n_buckets``. ``BUCKETS[3]`` contains "6" which 968 is the index into the ``HASHES`` table. We would then compare any consecutive 969 32 bit hashes values in the ``HASHES`` array as long as the hashes would be in 970 ``BUCKETS[3]``. We do this by verifying that each subsequent hash value modulo 971 ``n_buckets`` is still 3. In the case of a failed lookup we would access the 972 memory for ``BUCKETS[3]``, and then compare a few consecutive 32 bit hashes 973 before we know that we have no match. We don't end up marching through 974 multiple words of memory and we really keep the number of processor data cache 975 lines being accessed as small as possible. 976 977 The string hash that is used for these lookup tables is the Daniel J. 978 Bernstein hash which is also used in the ELF ``GNU_HASH`` sections. It is a 979 very good hash for all kinds of names in programs with very few hash 980 collisions. 981 982 Empty buckets are designated by using an invalid hash index of ``UINT32_MAX``. 983 984 Details 985 ^^^^^^^ 986 987 These name hash tables are designed to be generic where specializations of the 988 table get to define additional data that goes into the header ("``HeaderData``"), 989 how the string value is stored ("``KeyType``") and the content of the data for each 990 hash value. 991 992 Header Layout 993 """"""""""""" 994 995 The header has a fixed part, and the specialized part. The exact format of the 996 header is: 997 998 .. code-block:: c 999 1000 struct Header 1001 { 1002 uint32_t magic; // 'HASH' magic value to allow endian detection 1003 uint16_t version; // Version number 1004 uint16_t hash_function; // The hash function enumeration that was used 1005 uint32_t bucket_count; // The number of buckets in this hash table 1006 uint32_t hashes_count; // The total number of unique hash values and hash data offsets in this table 1007 uint32_t header_data_len; // The bytes to skip to get to the hash indexes (buckets) for correct alignment 1008 // Specifically the length of the following HeaderData field - this does not 1009 // include the size of the preceding fields 1010 HeaderData header_data; // Implementation specific header data 1011 }; 1012 1013 The header starts with a 32 bit "``magic``" value which must be ``'HASH'`` 1014 encoded as an ASCII integer. This allows the detection of the start of the 1015 hash table and also allows the table's byte order to be determined so the table 1016 can be correctly extracted. The "``magic``" value is followed by a 16 bit 1017 ``version`` number which allows the table to be revised and modified in the 1018 future. The current version number is 1. ``hash_function`` is a ``uint16_t`` 1019 enumeration that specifies which hash function was used to produce this table. 1020 The current values for the hash function enumerations include: 1021 1022 .. code-block:: c 1023 1024 enum HashFunctionType 1025 { 1026 eHashFunctionDJB = 0u, // Daniel J Bernstein hash function 1027 }; 1028 1029 ``bucket_count`` is a 32 bit unsigned integer that represents how many buckets 1030 are in the ``BUCKETS`` array. ``hashes_count`` is the number of unique 32 bit 1031 hash values that are in the ``HASHES`` array, and is the same number of offsets 1032 are contained in the ``OFFSETS`` array. ``header_data_len`` specifies the size 1033 in bytes of the ``HeaderData`` that is filled in by specialized versions of 1034 this table. 1035 1036 Fixed Lookup 1037 """""""""""" 1038 1039 The header is followed by the buckets, hashes, offsets, and hash value data. 1040 1041 .. code-block:: c 1042 1043 struct FixedTable 1044 { 1045 uint32_t buckets[Header.bucket_count]; // An array of hash indexes into the "hashes[]" array below 1046 uint32_t hashes [Header.hashes_count]; // Every unique 32 bit hash for the entire table is in this table 1047 uint32_t offsets[Header.hashes_count]; // An offset that corresponds to each item in the "hashes[]" array above 1048 }; 1049 1050 ``buckets`` is an array of 32 bit indexes into the ``hashes`` array. The 1051 ``hashes`` array contains all of the 32 bit hash values for all names in the 1052 hash table. Each hash in the ``hashes`` table has an offset in the ``offsets`` 1053 array that points to the data for the hash value. 1054 1055 This table setup makes it very easy to repurpose these tables to contain 1056 different data, while keeping the lookup mechanism the same for all tables. 1057 This layout also makes it possible to save the table to disk and map it in 1058 later and do very efficient name lookups with little or no parsing. 1059 1060 DWARF lookup tables can be implemented in a variety of ways and can store a lot 1061 of information for each name. We want to make the DWARF tables extensible and 1062 able to store the data efficiently so we have used some of the DWARF features 1063 that enable efficient data storage to define exactly what kind of data we store 1064 for each name. 1065 1066 The ``HeaderData`` contains a definition of the contents of each HashData chunk. 1067 We might want to store an offset to all of the debug information entries (DIEs) 1068 for each name. To keep things extensible, we create a list of items, or 1069 Atoms, that are contained in the data for each name. First comes the type of 1070 the data in each atom: 1071 1072 .. code-block:: c 1073 1074 enum AtomType 1075 { 1076 eAtomTypeNULL = 0u, 1077 eAtomTypeDIEOffset = 1u, // DIE offset, check form for encoding 1078 eAtomTypeCUOffset = 2u, // DIE offset of the compiler unit header that contains the item in question 1079 eAtomTypeTag = 3u, // DW_TAG_xxx value, should be encoded as DW_FORM_data1 (if no tags exceed 255) or DW_FORM_data2 1080 eAtomTypeNameFlags = 4u, // Flags from enum NameFlags 1081 eAtomTypeTypeFlags = 5u, // Flags from enum TypeFlags 1082 }; 1083 1084 The enumeration values and their meanings are: 1085 1086 .. code-block:: none 1087 1088 eAtomTypeNULL - a termination atom that specifies the end of the atom list 1089 eAtomTypeDIEOffset - an offset into the .debug_info section for the DWARF DIE for this name 1090 eAtomTypeCUOffset - an offset into the .debug_info section for the CU that contains the DIE 1091 eAtomTypeDIETag - The DW_TAG_XXX enumeration value so you don't have to parse the DWARF to see what it is 1092 eAtomTypeNameFlags - Flags for functions and global variables (isFunction, isInlined, isExternal...) 1093 eAtomTypeTypeFlags - Flags for types (isCXXClass, isObjCClass, ...) 1094 1095 Then we allow each atom type to define the atom type and how the data for each 1096 atom type data is encoded: 1097 1098 .. code-block:: c 1099 1100 struct Atom 1101 { 1102 uint16_t type; // AtomType enum value 1103 uint16_t form; // DWARF DW_FORM_XXX defines 1104 }; 1105 1106 The ``form`` type above is from the DWARF specification and defines the exact 1107 encoding of the data for the Atom type. See the DWARF specification for the 1108 ``DW_FORM_`` definitions. 1109 1110 .. code-block:: c 1111 1112 struct HeaderData 1113 { 1114 uint32_t die_offset_base; 1115 uint32_t atom_count; 1116 Atoms atoms[atom_count0]; 1117 }; 1118 1119 ``HeaderData`` defines the base DIE offset that should be added to any atoms 1120 that are encoded using the ``DW_FORM_ref1``, ``DW_FORM_ref2``, 1121 ``DW_FORM_ref4``, ``DW_FORM_ref8`` or ``DW_FORM_ref_udata``. It also defines 1122 what is contained in each ``HashData`` object -- ``Atom.form`` tells us how large 1123 each field will be in the ``HashData`` and the ``Atom.type`` tells us how this data 1124 should be interpreted. 1125 1126 For the current implementations of the "``.apple_names``" (all functions + 1127 globals), the "``.apple_types``" (names of all types that are defined), and 1128 the "``.apple_namespaces``" (all namespaces), we currently set the ``Atom`` 1129 array to be: 1130 1131 .. code-block:: c 1132 1133 HeaderData.atom_count = 1; 1134 HeaderData.atoms[0].type = eAtomTypeDIEOffset; 1135 HeaderData.atoms[0].form = DW_FORM_data4; 1136 1137 This defines the contents to be the DIE offset (eAtomTypeDIEOffset) that is 1138 encoded as a 32 bit value (DW_FORM_data4). This allows a single name to have 1139 multiple matching DIEs in a single file, which could come up with an inlined 1140 function for instance. Future tables could include more information about the 1141 DIE such as flags indicating if the DIE is a function, method, block, 1142 or inlined. 1143 1144 The KeyType for the DWARF table is a 32 bit string table offset into the 1145 ".debug_str" table. The ".debug_str" is the string table for the DWARF which 1146 may already contain copies of all of the strings. This helps make sure, with 1147 help from the compiler, that we reuse the strings between all of the DWARF 1148 sections and keeps the hash table size down. Another benefit to having the 1149 compiler generate all strings as DW_FORM_strp in the debug info, is that 1150 DWARF parsing can be made much faster. 1151 1152 After a lookup is made, we get an offset into the hash data. The hash data 1153 needs to be able to deal with 32 bit hash collisions, so the chunk of data 1154 at the offset in the hash data consists of a triple: 1155 1156 .. code-block:: c 1157 1158 uint32_t str_offset 1159 uint32_t hash_data_count 1160 HashData[hash_data_count] 1161 1162 If "str_offset" is zero, then the bucket contents are done. 99.9% of the 1163 hash data chunks contain a single item (no 32 bit hash collision): 1164 1165 .. code-block:: none 1166 1167 .------------. 1168 | 0x00001023 | uint32_t KeyType (.debug_str[0x0001023] => "main") 1169 | 0x00000004 | uint32_t HashData count 1170 | 0x........ | uint32_t HashData[0] DIE offset 1171 | 0x........ | uint32_t HashData[1] DIE offset 1172 | 0x........ | uint32_t HashData[2] DIE offset 1173 | 0x........ | uint32_t HashData[3] DIE offset 1174 | 0x00000000 | uint32_t KeyType (end of hash chain) 1175 `------------' 1176 1177 If there are collisions, you will have multiple valid string offsets: 1178 1179 .. code-block:: none 1180 1181 .------------. 1182 | 0x00001023 | uint32_t KeyType (.debug_str[0x0001023] => "main") 1183 | 0x00000004 | uint32_t HashData count 1184 | 0x........ | uint32_t HashData[0] DIE offset 1185 | 0x........ | uint32_t HashData[1] DIE offset 1186 | 0x........ | uint32_t HashData[2] DIE offset 1187 | 0x........ | uint32_t HashData[3] DIE offset 1188 | 0x00002023 | uint32_t KeyType (.debug_str[0x0002023] => "print") 1189 | 0x00000002 | uint32_t HashData count 1190 | 0x........ | uint32_t HashData[0] DIE offset 1191 | 0x........ | uint32_t HashData[1] DIE offset 1192 | 0x00000000 | uint32_t KeyType (end of hash chain) 1193 `------------' 1194 1195 Current testing with real world C++ binaries has shown that there is around 1 1196 32 bit hash collision per 100,000 name entries. 1197 1198 Contents 1199 ^^^^^^^^ 1200 1201 As we said, we want to strictly define exactly what is included in the 1202 different tables. For DWARF, we have 3 tables: "``.apple_names``", 1203 "``.apple_types``", and "``.apple_namespaces``". 1204 1205 "``.apple_names``" sections should contain an entry for each DWARF DIE whose 1206 ``DW_TAG`` is a ``DW_TAG_label``, ``DW_TAG_inlined_subroutine``, or 1207 ``DW_TAG_subprogram`` that has address attributes: ``DW_AT_low_pc``, 1208 ``DW_AT_high_pc``, ``DW_AT_ranges`` or ``DW_AT_entry_pc``. It also contains 1209 ``DW_TAG_variable`` DIEs that have a ``DW_OP_addr`` in the location (global and 1210 static variables). All global and static variables should be included, 1211 including those scoped within functions and classes. For example using the 1212 following code: 1213 1214 .. code-block:: c 1215 1216 static int var = 0; 1217 1218 void f () 1219 { 1220 static int var = 0; 1221 } 1222 1223 Both of the static ``var`` variables would be included in the table. All 1224 functions should emit both their full names and their basenames. For C or C++, 1225 the full name is the mangled name (if available) which is usually in the 1226 ``DW_AT_MIPS_linkage_name`` attribute, and the ``DW_AT_name`` contains the 1227 function basename. If global or static variables have a mangled name in a 1228 ``DW_AT_MIPS_linkage_name`` attribute, this should be emitted along with the 1229 simple name found in the ``DW_AT_name`` attribute. 1230 1231 "``.apple_types``" sections should contain an entry for each DWARF DIE whose 1232 tag is one of: 1233 1234 * DW_TAG_array_type 1235 * DW_TAG_class_type 1236 * DW_TAG_enumeration_type 1237 * DW_TAG_pointer_type 1238 * DW_TAG_reference_type 1239 * DW_TAG_string_type 1240 * DW_TAG_structure_type 1241 * DW_TAG_subroutine_type 1242 * DW_TAG_typedef 1243 * DW_TAG_union_type 1244 * DW_TAG_ptr_to_member_type 1245 * DW_TAG_set_type 1246 * DW_TAG_subrange_type 1247 * DW_TAG_base_type 1248 * DW_TAG_const_type 1249 * DW_TAG_file_type 1250 * DW_TAG_namelist 1251 * DW_TAG_packed_type 1252 * DW_TAG_volatile_type 1253 * DW_TAG_restrict_type 1254 * DW_TAG_interface_type 1255 * DW_TAG_unspecified_type 1256 * DW_TAG_shared_type 1257 1258 Only entries with a ``DW_AT_name`` attribute are included, and the entry must 1259 not be a forward declaration (``DW_AT_declaration`` attribute with a non-zero 1260 value). For example, using the following code: 1261 1262 .. code-block:: c 1263 1264 int main () 1265 { 1266 int *b = 0; 1267 return *b; 1268 } 1269 1270 We get a few type DIEs: 1271 1272 .. code-block:: none 1273 1274 0x00000067: TAG_base_type [5] 1275 AT_encoding( DW_ATE_signed ) 1276 AT_name( "int" ) 1277 AT_byte_size( 0x04 ) 1278 1279 0x0000006e: TAG_pointer_type [6] 1280 AT_type( {0x00000067} ( int ) ) 1281 AT_byte_size( 0x08 ) 1282 1283 The DW_TAG_pointer_type is not included because it does not have a ``DW_AT_name``. 1284 1285 "``.apple_namespaces``" section should contain all ``DW_TAG_namespace`` DIEs. 1286 If we run into a namespace that has no name this is an anonymous namespace, and 1287 the name should be output as "``(anonymous namespace)``" (without the quotes). 1288 Why? This matches the output of the ``abi::cxa_demangle()`` that is in the 1289 standard C++ library that demangles mangled names. 1290 1291 1292 Language Extensions and File Format Changes 1293 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 1294 1295 Objective-C Extensions 1296 """""""""""""""""""""" 1297 1298 "``.apple_objc``" section should contain all ``DW_TAG_subprogram`` DIEs for an 1299 Objective-C class. The name used in the hash table is the name of the 1300 Objective-C class itself. If the Objective-C class has a category, then an 1301 entry is made for both the class name without the category, and for the class 1302 name with the category. So if we have a DIE at offset 0x1234 with a name of 1303 method "``-[NSString(my_additions) stringWithSpecialString:]``", we would add 1304 an entry for "``NSString``" that points to DIE 0x1234, and an entry for 1305 "``NSString(my_additions)``" that points to 0x1234. This allows us to quickly 1306 track down all Objective-C methods for an Objective-C class when doing 1307 expressions. It is needed because of the dynamic nature of Objective-C where 1308 anyone can add methods to a class. The DWARF for Objective-C methods is also 1309 emitted differently from C++ classes where the methods are not usually 1310 contained in the class definition, they are scattered about across one or more 1311 compile units. Categories can also be defined in different shared libraries. 1312 So we need to be able to quickly find all of the methods and class functions 1313 given the Objective-C class name, or quickly find all methods and class 1314 functions for a class + category name. This table does not contain any 1315 selector names, it just maps Objective-C class names (or class names + 1316 category) to all of the methods and class functions. The selectors are added 1317 as function basenames in the "``.debug_names``" section. 1318 1319 In the "``.apple_names``" section for Objective-C functions, the full name is 1320 the entire function name with the brackets ("``-[NSString 1321 stringWithCString:]``") and the basename is the selector only 1322 ("``stringWithCString:``"). 1323 1324 Mach-O Changes 1325 """""""""""""" 1326 1327 The sections names for the apple hash tables are for non-mach-o files. For 1328 mach-o files, the sections should be contained in the ``__DWARF`` segment with 1329 names as follows: 1330 1331 * "``.apple_names``" -> "``__apple_names``" 1332 * "``.apple_types``" -> "``__apple_types``" 1333 * "``.apple_namespaces``" -> "``__apple_namespac``" (16 character limit) 1334 * "``.apple_objc``" -> "``__apple_objc``" 1335 1336
