1================================ 2Source Level Debugging with LLVM 3================================ 4 5.. contents:: 6 :local: 7 8Introduction 9============ 10 11This document is the central repository for all information pertaining to debug 12information in LLVM. It describes the :ref:`actual format that the LLVM debug 13information takes <format>`, which is useful for those interested in creating 14front-ends or dealing directly with the information. Further, this document 15provides specific examples of what debug information for C/C++ looks like. 16 17Philosophy behind LLVM debugging information 18-------------------------------------------- 19 20The idea of the LLVM debugging information is to capture how the important 21pieces of the source-language's Abstract Syntax Tree map onto LLVM code. 22Several design aspects have shaped the solution that appears here. The 23important 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 45The approach used by the LLVM implementation is to use a small set of 46:ref:`intrinsic functions <format_common_intrinsics>` to define a mapping 47between LLVM program objects and the source-level objects. The description of 48the source-level program is maintained in LLVM metadata in an 49:ref:`implementation-defined format <ccxx_frontend>` (the C/C++ front-end 50currently uses working draft 7 of the `DWARF 3 standard 51<http://www.eagercon.com/dwarf/dwarf3std.htm>`_). 52 53When a program is being debugged, a debugger interacts with the user and turns 54the stored debug information into source-language specific information. As 55such, a debugger must be aware of the source-language, and is thus tied to a 56specific language or family of languages. 57 58Debug information consumers 59--------------------------- 60 61The role of debug information is to provide meta information normally stripped 62away during the compilation process. This meta information provides an LLVM 63user a relationship between generated code and the original program source 64code. 65 66Currently, there are two backend consumers of debug info: DwarfDebug and 67CodeViewDebug. DwarfDebug produces DWARF suitable for use with GDB, LLDB, and 68other DWARF-based debuggers. :ref:`CodeViewDebug <codeview>` produces CodeView, 69the Microsoft debug info format, which is usable with Microsoft debuggers such 70as Visual Studio and WinDBG. LLVM's debug information format is mostly derived 71from and inspired by DWARF, but it is feasible to translate into other target 72debug info formats such as STABS. 73 74It would also be reasonable to use debug information to feed profiling tools 75for analysis of generated code, or, tools for reconstructing the original 76source from generated code. 77 78.. _intro_debugopt: 79 80Debug information and optimizations 81----------------------------------- 82 83An extremely high priority of LLVM debugging information is to make it interact 84well with optimizations and analysis. In particular, the LLVM debug 85information provides the following guarantees: 86 87* LLVM debug information **always provides information to accurately read 88 the source-level state of the program**, regardless of which LLVM 89 optimizations have been run. :doc:`HowToUpdateDebugInfo` specifies how debug 90 info should be updated in various kinds of code transformations to avoid 91 breaking this guarantee, and how to preserve as much useful debug info as 92 possible. Note that some optimizations may impact the ability to modify the 93 current state of the program with a debugger, such as setting program 94 variables, or calling functions that have been deleted. 95 96* As desired, LLVM optimizations can be upgraded to be aware of debugging 97 information, allowing them to update the debugging information as they 98 perform aggressive optimizations. This means that, with effort, the LLVM 99 optimizers could optimize debug code just as well as non-debug code. 100 101* LLVM debug information does not prevent optimizations from 102 happening (for example inlining, basic block reordering/merging/cleanup, 103 tail duplication, etc). 104 105* LLVM debug information is automatically optimized along with the rest of 106 the program, using existing facilities. For example, duplicate 107 information is automatically merged by the linker, and unused information 108 is automatically removed. 109 110Basically, the debug information allows you to compile a program with 111"``-O0 -g``" and get full debug information, allowing you to arbitrarily modify 112the program as it executes from a debugger. Compiling a program with 113"``-O3 -g``" gives you full debug information that is always available and 114accurate for reading (e.g., you get accurate stack traces despite tail call 115elimination and inlining), but you might lose the ability to modify the program 116and call functions which were optimized out of the program, or inlined away 117completely. 118 119The :doc:`LLVM test-suite <TestSuiteMakefileGuide>` provides a framework to 120test the optimizer's handling of debugging information. It can be run like 121this: 122 123.. code-block:: bash 124 125 % cd llvm/projects/test-suite/MultiSource/Benchmarks # or some other level 126 % make TEST=dbgopt 127 128This will test impact of debugging information on optimization passes. If 129debugging information influences optimization passes then it will be reported 130as a failure. See :doc:`TestingGuide` for more information on LLVM test 131infrastructure and how to run various tests. 132 133.. _format: 134 135Debugging information format 136============================ 137 138LLVM debugging information has been carefully designed to make it possible for 139the optimizer to optimize the program and debugging information without 140necessarily having to know anything about debugging information. In 141particular, the use of metadata avoids duplicated debugging information from 142the beginning, and the global dead code elimination pass automatically deletes 143debugging information for a function if it decides to delete the function. 144 145To do this, most of the debugging information (descriptors for types, 146variables, functions, source files, etc) is inserted by the language front-end 147in the form of LLVM metadata. 148 149Debug information is designed to be agnostic about the target debugger and 150debugging information representation (e.g. DWARF/Stabs/etc). It uses a generic 151pass to decode the information that represents variables, types, functions, 152namespaces, etc: this allows for arbitrary source-language semantics and 153type-systems to be used, as long as there is a module written for the target 154debugger to interpret the information. 155 156To provide basic functionality, the LLVM debugger does have to make some 157assumptions about the source-level language being debugged, though it keeps 158these to a minimum. The only common features that the LLVM debugger assumes 159exist are `source files <LangRef.html#difile>`_, and `program objects 160<LangRef.html#diglobalvariable>`_. These abstract objects are used by a 161debugger to form stack traces, show information about local variables, etc. 162 163This section of the documentation first describes the representation aspects 164common to any source-language. :ref:`ccxx_frontend` describes the data layout 165conventions used by the C and C++ front-ends. 166 167Debug information descriptors are `specialized metadata nodes 168<LangRef.html#specialized-metadata>`_, first-class subclasses of ``Metadata``. 169 170.. _format_common_intrinsics: 171 172Debugger intrinsic functions 173---------------------------- 174 175LLVM uses several intrinsic functions (name prefixed with "``llvm.dbg``") to 176track source local variables through optimization and code generation. 177 178``llvm.dbg.addr`` 179^^^^^^^^^^^^^^^^^^^^ 180 181.. code-block:: llvm 182 183 void @llvm.dbg.addr(metadata, metadata, metadata) 184 185This intrinsic provides information about a local element (e.g., variable). 186The first argument is metadata holding the address of variable, typically a 187static alloca in the function entry block. The second argument is a 188`local variable <LangRef.html#dilocalvariable>`_ containing a description of 189the variable. The third argument is a `complex expression 190<LangRef.html#diexpression>`_. An `llvm.dbg.addr` intrinsic describes the 191*address* of a source variable. 192 193.. code-block:: text 194 195 %i.addr = alloca i32, align 4 196 call void @llvm.dbg.addr(metadata i32* %i.addr, metadata !1, 197 metadata !DIExpression()), !dbg !2 198 !1 = !DILocalVariable(name: "i", ...) ; int i 199 !2 = !DILocation(...) 200 ... 201 %buffer = alloca [256 x i8], align 8 202 ; The address of i is buffer+64. 203 call void @llvm.dbg.addr(metadata [256 x i8]* %buffer, metadata !3, 204 metadata !DIExpression(DW_OP_plus, 64)), !dbg !4 205 !3 = !DILocalVariable(name: "i", ...) ; int i 206 !4 = !DILocation(...) 207 208A frontend should generate exactly one call to ``llvm.dbg.addr`` at the point 209of declaration of a source variable. Optimization passes that fully promote the 210variable from memory to SSA values will replace this call with possibly 211multiple calls to `llvm.dbg.value`. Passes that delete stores are effectively 212partial promotion, and they will insert a mix of calls to ``llvm.dbg.value`` 213and ``llvm.dbg.addr`` to track the source variable value when it is available. 214After optimization, there may be multiple calls to ``llvm.dbg.addr`` describing 215the program points where the variables lives in memory. All calls for the same 216concrete source variable must agree on the memory location. 217 218 219``llvm.dbg.declare`` 220^^^^^^^^^^^^^^^^^^^^ 221 222.. code-block:: llvm 223 224 void @llvm.dbg.declare(metadata, metadata, metadata) 225 226This intrinsic is identical to `llvm.dbg.addr`, except that there can only be 227one call to `llvm.dbg.declare` for a given concrete `local variable 228<LangRef.html#dilocalvariable>`_. It is not control-dependent, meaning that if 229a call to `llvm.dbg.declare` exists and has a valid location argument, that 230address is considered to be the true home of the variable across its entire 231lifetime. This makes it hard for optimizations to preserve accurate debug info 232in the presence of ``llvm.dbg.declare``, so we are transitioning away from it, 233and we plan to deprecate it in future LLVM releases. 234 235 236``llvm.dbg.value`` 237^^^^^^^^^^^^^^^^^^ 238 239.. code-block:: llvm 240 241 void @llvm.dbg.value(metadata, metadata, metadata) 242 243This intrinsic provides information when a user source variable is set to a new 244value. The first argument is the new value (wrapped as metadata). The second 245argument is a `local variable <LangRef.html#dilocalvariable>`_ containing a 246description of the variable. The third argument is a `complex expression 247<LangRef.html#diexpression>`_. 248 249An `llvm.dbg.value` intrinsic describes the *value* of a source variable 250directly, not its address. Note that the value operand of this intrinsic may 251be indirect (i.e, a pointer to the source variable), provided that interpreting 252the complex expression derives the direct value. 253 254Object lifetimes and scoping 255============================ 256 257In many languages, the local variables in functions can have their lifetimes or 258scopes limited to a subset of a function. In the C family of languages, for 259example, variables are only live (readable and writable) within the source 260block that they are defined in. In functional languages, values are only 261readable after they have been defined. Though this is a very obvious concept, 262it is non-trivial to model in LLVM, because it has no notion of scoping in this 263sense, and does not want to be tied to a language's scoping rules. 264 265In order to handle this, the LLVM debug format uses the metadata attached to 266llvm instructions to encode line number and scoping information. Consider the 267following C fragment, for example: 268 269.. code-block:: c 270 271 1. void foo() { 272 2. int X = 21; 273 3. int Y = 22; 274 4. { 275 5. int Z = 23; 276 6. Z = X; 277 7. } 278 8. X = Y; 279 9. } 280 281.. FIXME: Update the following example to use llvm.dbg.addr once that is the 282 default in clang. 283 284Compiled to LLVM, this function would be represented like this: 285 286.. code-block:: text 287 288 ; Function Attrs: nounwind ssp uwtable 289 define void @foo() #0 !dbg !4 { 290 entry: 291 %X = alloca i32, align 4 292 %Y = alloca i32, align 4 293 %Z = alloca i32, align 4 294 call void @llvm.dbg.declare(metadata i32* %X, metadata !11, metadata !13), !dbg !14 295 store i32 21, i32* %X, align 4, !dbg !14 296 call void @llvm.dbg.declare(metadata i32* %Y, metadata !15, metadata !13), !dbg !16 297 store i32 22, i32* %Y, align 4, !dbg !16 298 call void @llvm.dbg.declare(metadata i32* %Z, metadata !17, metadata !13), !dbg !19 299 store i32 23, i32* %Z, align 4, !dbg !19 300 %0 = load i32, i32* %X, align 4, !dbg !20 301 store i32 %0, i32* %Z, align 4, !dbg !21 302 %1 = load i32, i32* %Y, align 4, !dbg !22 303 store i32 %1, i32* %X, align 4, !dbg !23 304 ret void, !dbg !24 305 } 306 307 ; Function Attrs: nounwind readnone 308 declare void @llvm.dbg.declare(metadata, metadata, metadata) #1 309 310 attributes #0 = { nounwind ssp uwtable "less-precise-fpmad"="false" "frame-pointer"="all" "no-infs-fp-math"="false" "no-nans-fp-math"="false" "stack-protector-buffer-size"="8" "unsafe-fp-math"="false" "use-soft-float"="false" } 311 attributes #1 = { nounwind readnone } 312 313 !llvm.dbg.cu = !{!0} 314 !llvm.module.flags = !{!7, !8, !9} 315 !llvm.ident = !{!10} 316 317 !0 = !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "clang version 3.7.0 (trunk 231150) (llvm/trunk 231154)", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug, enums: !2, retainedTypes: !2, subprograms: !3, globals: !2, imports: !2) 318 !1 = !DIFile(filename: "/dev/stdin", directory: "/Users/dexonsmith/data/llvm/debug-info") 319 !2 = !{} 320 !3 = !{!4} 321 !4 = distinct !DISubprogram(name: "foo", scope: !1, file: !1, line: 1, type: !5, isLocal: false, isDefinition: true, scopeLine: 1, isOptimized: false, retainedNodes: !2) 322 !5 = !DISubroutineType(types: !6) 323 !6 = !{null} 324 !7 = !{i32 2, !"Dwarf Version", i32 2} 325 !8 = !{i32 2, !"Debug Info Version", i32 3} 326 !9 = !{i32 1, !"PIC Level", i32 2} 327 !10 = !{!"clang version 3.7.0 (trunk 231150) (llvm/trunk 231154)"} 328 !11 = !DILocalVariable(name: "X", scope: !4, file: !1, line: 2, type: !12) 329 !12 = !DIBasicType(name: "int", size: 32, align: 32, encoding: DW_ATE_signed) 330 !13 = !DIExpression() 331 !14 = !DILocation(line: 2, column: 9, scope: !4) 332 !15 = !DILocalVariable(name: "Y", scope: !4, file: !1, line: 3, type: !12) 333 !16 = !DILocation(line: 3, column: 9, scope: !4) 334 !17 = !DILocalVariable(name: "Z", scope: !18, file: !1, line: 5, type: !12) 335 !18 = distinct !DILexicalBlock(scope: !4, file: !1, line: 4, column: 5) 336 !19 = !DILocation(line: 5, column: 11, scope: !18) 337 !20 = !DILocation(line: 6, column: 11, scope: !18) 338 !21 = !DILocation(line: 6, column: 9, scope: !18) 339 !22 = !DILocation(line: 8, column: 9, scope: !4) 340 !23 = !DILocation(line: 8, column: 7, scope: !4) 341 !24 = !DILocation(line: 9, column: 3, scope: !4) 342 343 344This example illustrates a few important details about LLVM debugging 345information. In particular, it shows how the ``llvm.dbg.declare`` intrinsic and 346location information, which are attached to an instruction, are applied 347together to allow a debugger to analyze the relationship between statements, 348variable definitions, and the code used to implement the function. 349 350.. code-block:: llvm 351 352 call void @llvm.dbg.declare(metadata i32* %X, metadata !11, metadata !13), !dbg !14 353 ; [debug line = 2:7] [debug variable = X] 354 355The first intrinsic ``%llvm.dbg.declare`` encodes debugging information for the 356variable ``X``. The metadata ``!dbg !14`` attached to the intrinsic provides 357scope information for the variable ``X``. 358 359.. code-block:: text 360 361 !14 = !DILocation(line: 2, column: 9, scope: !4) 362 !4 = distinct !DISubprogram(name: "foo", scope: !1, file: !1, line: 1, type: !5, 363 isLocal: false, isDefinition: true, scopeLine: 1, 364 isOptimized: false, retainedNodes: !2) 365 366Here ``!14`` is metadata providing `location information 367<LangRef.html#dilocation>`_. In this example, scope is encoded by ``!4``, a 368`subprogram descriptor <LangRef.html#disubprogram>`_. This way the location 369information attached to the intrinsics indicates that the variable ``X`` is 370declared at line number 2 at a function level scope in function ``foo``. 371 372Now lets take another example. 373 374.. code-block:: llvm 375 376 call void @llvm.dbg.declare(metadata i32* %Z, metadata !17, metadata !13), !dbg !19 377 ; [debug line = 5:9] [debug variable = Z] 378 379The third intrinsic ``%llvm.dbg.declare`` encodes debugging information for 380variable ``Z``. The metadata ``!dbg !19`` attached to the intrinsic provides 381scope information for the variable ``Z``. 382 383.. code-block:: text 384 385 !18 = distinct !DILexicalBlock(scope: !4, file: !1, line: 4, column: 5) 386 !19 = !DILocation(line: 5, column: 11, scope: !18) 387 388Here ``!19`` indicates that ``Z`` is declared at line number 5 and column 389number 11 inside of lexical scope ``!18``. The lexical scope itself resides 390inside of subprogram ``!4`` described above. 391 392The scope information attached with each instruction provides a straightforward 393way to find instructions covered by a scope. 394 395Object lifetime in optimized code 396================================= 397 398In the example above, every variable assignment uniquely corresponds to a 399memory store to the variable's position on the stack. However in heavily 400optimized code LLVM promotes most variables into SSA values, which can 401eventually be placed in physical registers or memory locations. To track SSA 402values through compilation, when objects are promoted to SSA values an 403``llvm.dbg.value`` intrinsic is created for each assignment, recording the 404variable's new location. Compared with the ``llvm.dbg.declare`` intrinsic: 405 406* A dbg.value terminates the effect of any preceding dbg.values for (any 407 overlapping fragments of) the specified variable. 408* The dbg.value's position in the IR defines where in the instruction stream 409 the variable's value changes. 410* Operands can be constants, indicating the variable is assigned a 411 constant value. 412 413Care must be taken to update ``llvm.dbg.value`` intrinsics when optimization 414passes alter or move instructions and blocks -- the developer could observe such 415changes reflected in the value of variables when debugging the program. For any 416execution of the optimized program, the set of variable values presented to the 417developer by the debugger should not show a state that would never have existed 418in the execution of the unoptimized program, given the same input. Doing so 419risks misleading the developer by reporting a state that does not exist, 420damaging their understanding of the optimized program and undermining their 421trust in the debugger. 422 423Sometimes perfectly preserving variable locations is not possible, often when a 424redundant calculation is optimized out. In such cases, a ``llvm.dbg.value`` 425with operand ``undef`` should be used, to terminate earlier variable locations 426and let the debugger present ``optimized out`` to the developer. Withholding 427these potentially stale variable values from the developer diminishes the 428amount of available debug information, but increases the reliability of the 429remaining information. 430 431To illustrate some potential issues, consider the following example: 432 433.. code-block:: llvm 434 435 define i32 @foo(i32 %bar, i1 %cond) { 436 entry: 437 call @llvm.dbg.value(metadata i32 0, metadata !1, metadata !2) 438 br i1 %cond, label %truebr, label %falsebr 439 truebr: 440 %tval = add i32 %bar, 1 441 call @llvm.dbg.value(metadata i32 %tval, metadata !1, metadata !2) 442 %g1 = call i32 @gazonk() 443 br label %exit 444 falsebr: 445 %fval = add i32 %bar, 2 446 call @llvm.dbg.value(metadata i32 %fval, metadata !1, metadata !2) 447 %g2 = call i32 @gazonk() 448 br label %exit 449 exit: 450 %merge = phi [ %tval, %truebr ], [ %fval, %falsebr ] 451 %g = phi [ %g1, %truebr ], [ %g2, %falsebr ] 452 call @llvm.dbg.value(metadata i32 %merge, metadata !1, metadata !2) 453 call @llvm.dbg.value(metadata i32 %g, metadata !3, metadata !2) 454 %plusten = add i32 %merge, 10 455 %toret = add i32 %plusten, %g 456 call @llvm.dbg.value(metadata i32 %toret, metadata !1, metadata !2) 457 ret i32 %toret 458 } 459 460Containing two source-level variables in ``!1`` and ``!3``. The function could, 461perhaps, be optimized into the following code: 462 463.. code-block:: llvm 464 465 define i32 @foo(i32 %bar, i1 %cond) { 466 entry: 467 %g = call i32 @gazonk() 468 %addoper = select i1 %cond, i32 11, i32 12 469 %plusten = add i32 %bar, %addoper 470 %toret = add i32 %plusten, %g 471 ret i32 %toret 472 } 473 474What ``llvm.dbg.value`` intrinsics should be placed to represent the original variable 475locations in this code? Unfortunately the second, third and fourth 476dbg.values for ``!1`` in the source function have had their operands 477(%tval, %fval, %merge) optimized out. Assuming we cannot recover them, we 478might consider this placement of dbg.values: 479 480.. code-block:: llvm 481 482 define i32 @foo(i32 %bar, i1 %cond) { 483 entry: 484 call @llvm.dbg.value(metadata i32 0, metadata !1, metadata !2) 485 %g = call i32 @gazonk() 486 call @llvm.dbg.value(metadata i32 %g, metadata !3, metadata !2) 487 %addoper = select i1 %cond, i32 11, i32 12 488 %plusten = add i32 %bar, %addoper 489 %toret = add i32 %plusten, %g 490 call @llvm.dbg.value(metadata i32 %toret, metadata !1, metadata !2) 491 ret i32 %toret 492 } 493 494However, this will cause ``!3`` to have the return value of ``@gazonk()`` at 495the same time as ``!1`` has the constant value zero -- a pair of assignments 496that never occurred in the unoptimized program. To avoid this, we must terminate 497the range that ``!1`` has the constant value assignment by inserting an undef 498dbg.value before the dbg.value for ``!3``: 499 500.. code-block:: llvm 501 502 define i32 @foo(i32 %bar, i1 %cond) { 503 entry: 504 call @llvm.dbg.value(metadata i32 0, metadata !1, metadata !2) 505 %g = call i32 @gazonk() 506 call @llvm.dbg.value(metadata i32 undef, metadata !1, metadata !2) 507 call @llvm.dbg.value(metadata i32 %g, metadata !3, metadata !2) 508 %addoper = select i1 %cond, i32 11, i32 12 509 %plusten = add i32 %bar, %addoper 510 %toret = add i32 %plusten, %g 511 call @llvm.dbg.value(metadata i32 %toret, metadata !1, metadata !2) 512 ret i32 %toret 513 } 514 515In general, if any dbg.value has its operand optimized out and cannot be 516recovered, then an undef dbg.value is necessary to terminate earlier variable 517locations. Additional undef dbg.values may be necessary when the debugger can 518observe re-ordering of assignments. 519 520How variable location metadata is transformed during CodeGen 521============================================================ 522 523LLVM preserves debug information throughout mid-level and backend passes, 524ultimately producing a mapping between source-level information and 525instruction ranges. This 526is relatively straightforwards for line number information, as mapping 527instructions to line numbers is a simple association. For variable locations 528however the story is more complex. As each ``llvm.dbg.value`` intrinsic 529represents a source-level assignment of a value to a source variable, the 530variable location intrinsics effectively embed a small imperative program 531within the LLVM IR. By the end of CodeGen, this becomes a mapping from each 532variable to their machine locations over ranges of instructions. 533From IR to object emission, the major transformations which affect variable 534location fidelity are: 535 5361. Instruction Selection 5372. Register allocation 5383. Block layout 539 540each of which are discussed below. In addition, instruction scheduling can 541significantly change the ordering of the program, and occurs in a number of 542different passes. 543 544Some variable locations are not transformed during CodeGen. Stack locations 545specified by ``llvm.dbg.declare`` are valid and unchanging for the entire 546duration of the function, and are recorded in a simple MachineFunction table. 547Location changes in the prologue and epilogue of a function are also ignored: 548frame setup and destruction may take several instructions, require a 549disproportionate amount of debugging information in the output binary to 550describe, and should be stepped over by debuggers anyway. 551 552Variable locations in Instruction Selection and MIR 553--------------------------------------------------- 554 555Instruction selection creates a MIR function from an IR function, and just as 556it transforms ``intermediate`` instructions into machine instructions, so must 557``intermediate`` variable locations become machine variable locations. 558Within IR, variable locations are always identified by a Value, but in MIR 559there can be different types of variable locations. In addition, some IR 560locations become unavailable, for example if the operation of multiple IR 561instructions are combined into one machine instruction (such as 562multiply-and-accumulate) then intermediate Values are lost. To track variable 563locations through instruction selection, they are first separated into 564locations that do not depend on code generation (constants, stack locations, 565allocated virtual registers) and those that do. For those that do, debug 566metadata is attached to SDNodes in SelectionDAGs. After instruction selection 567has occurred and a MIR function is created, if the SDNode associated with debug 568metadata is allocated a virtual register, that virtual register is used as the 569variable location. If the SDNode is folded into a machine instruction or 570otherwise transformed into a non-register, the variable location becomes 571unavailable. 572 573Locations that are unavailable are treated as if they have been optimized out: 574in IR the location would be assigned ``undef`` by a debug intrinsic, and in MIR 575the equivalent location is used. 576 577After MIR locations are assigned to each variable, machine pseudo-instructions 578corresponding to each ``llvm.dbg.value`` and ``llvm.dbg.addr`` intrinsic are 579inserted. There are two forms of this type of instruction. 580 581The first form, ``DBG_VALUE``, appears thus: 582 583.. code-block:: text 584 585 DBG_VALUE %1, $noreg, !123, !DIExpression() 586 587And has the following operands: 588 * The first operand can record the variable location as a register, 589 a frame index, an immediate, or the base address register if the original 590 debug intrinsic referred to memory. ``$noreg`` indicates the variable 591 location is undefined, equivalent to an ``undef`` dbg.value operand. 592 * The type of the second operand indicates whether the variable location is 593 directly referred to by the DBG_VALUE, or whether it is indirect. The 594 ``$noreg`` register signifies the former, an immediate operand (0) the 595 latter. 596 * Operand 3 is the Variable field of the original debug intrinsic. 597 * Operand 4 is the Expression field of the original debug intrinsic. 598 599The second form, ``DBG_VALUE_LIST``, appears thus: 600 601.. code-block:: text 602 603 DBG_VALUE_LIST !123, !DIExpression(DW_OP_LLVM_arg, 0, DW_OP_LLVM_arg, 1, DW_OP_plus), %1, %2 604 605And has the following operands: 606 * The first operand is the Variable field of the original debug intrinsic. 607 * The second operand is the Expression field of the original debug intrinsic. 608 * Any number of operands, from the 3rd onwards, record a sequence of variable 609 location operands, which may take any of the same values as the first 610 operand of the ``DBG_VALUE`` instruction above. These variable location 611 operands are inserted into the final DWARF Expression in positions indicated 612 by the DW_OP_LLVM_arg operator in the `DIExpression 613 <LangRef.html#diexpression>`. 614 615The position at which the DBG_VALUEs are inserted should correspond to the 616positions of their matching ``llvm.dbg.value`` intrinsics in the IR block. As 617with optimization, LLVM aims to preserve the order in which variable 618assignments occurred in the source program. However SelectionDAG performs some 619instruction scheduling, which can reorder assignments (discussed below). 620Function parameter locations are moved to the beginning of the function if 621they're not already, to ensure they're immediately available on function entry. 622 623To demonstrate variable locations during instruction selection, consider 624the following example: 625 626.. code-block:: llvm 627 628 define i32 @foo(i32* %addr) { 629 entry: 630 call void @llvm.dbg.value(metadata i32 0, metadata !3, metadata !DIExpression()), !dbg !5 631 br label %bb1, !dbg !5 632 633 bb1: ; preds = %bb1, %entry 634 %bar.0 = phi i32 [ 0, %entry ], [ %add, %bb1 ] 635 call void @llvm.dbg.value(metadata i32 %bar.0, metadata !3, metadata !DIExpression()), !dbg !5 636 %addr1 = getelementptr i32, i32 *%addr, i32 1, !dbg !5 637 call void @llvm.dbg.value(metadata i32 *%addr1, metadata !3, metadata !DIExpression()), !dbg !5 638 %loaded1 = load i32, i32* %addr1, !dbg !5 639 %addr2 = getelementptr i32, i32 *%addr, i32 %bar.0, !dbg !5 640 call void @llvm.dbg.value(metadata i32 *%addr2, metadata !3, metadata !DIExpression()), !dbg !5 641 %loaded2 = load i32, i32* %addr2, !dbg !5 642 %add = add i32 %bar.0, 1, !dbg !5 643 call void @llvm.dbg.value(metadata i32 %add, metadata !3, metadata !DIExpression()), !dbg !5 644 %added = add i32 %loaded1, %loaded2 645 %cond = icmp ult i32 %added, %bar.0, !dbg !5 646 br i1 %cond, label %bb1, label %bb2, !dbg !5 647 648 bb2: ; preds = %bb1 649 ret i32 0, !dbg !5 650 } 651 652If one compiles this IR with ``llc -o - -start-after=codegen-prepare -stop-after=expand-isel-pseudos -mtriple=x86_64--``, the following MIR is produced: 653 654.. code-block:: text 655 656 bb.0.entry: 657 successors: %bb.1(0x80000000) 658 liveins: $rdi 659 660 %2:gr64 = COPY $rdi 661 %3:gr32 = MOV32r0 implicit-def dead $eflags 662 DBG_VALUE 0, $noreg, !3, !DIExpression(), debug-location !5 663 664 bb.1.bb1: 665 successors: %bb.1(0x7c000000), %bb.2(0x04000000) 666 667 %0:gr32 = PHI %3, %bb.0, %1, %bb.1 668 DBG_VALUE %0, $noreg, !3, !DIExpression(), debug-location !5 669 DBG_VALUE %2, $noreg, !3, !DIExpression(DW_OP_plus_uconst, 4, DW_OP_stack_value), debug-location !5 670 %4:gr32 = MOV32rm %2, 1, $noreg, 4, $noreg, debug-location !5 :: (load 4 from %ir.addr1) 671 %5:gr64_nosp = MOVSX64rr32 %0, debug-location !5 672 DBG_VALUE $noreg, $noreg, !3, !DIExpression(), debug-location !5 673 %1:gr32 = INC32r %0, implicit-def dead $eflags, debug-location !5 674 DBG_VALUE %1, $noreg, !3, !DIExpression(), debug-location !5 675 %6:gr32 = ADD32rm %4, %2, 4, killed %5, 0, $noreg, implicit-def dead $eflags :: (load 4 from %ir.addr2) 676 %7:gr32 = SUB32rr %6, %0, implicit-def $eflags, debug-location !5 677 JB_1 %bb.1, implicit $eflags, debug-location !5 678 JMP_1 %bb.2, debug-location !5 679 680 bb.2.bb2: 681 %8:gr32 = MOV32r0 implicit-def dead $eflags 682 $eax = COPY %8, debug-location !5 683 RET 0, $eax, debug-location !5 684 685Observe first that there is a DBG_VALUE instruction for every ``llvm.dbg.value`` 686intrinsic in the source IR, ensuring no source level assignments go missing. 687Then consider the different ways in which variable locations have been recorded: 688 689* For the first dbg.value an immediate operand is used to record a zero value. 690* The dbg.value of the PHI instruction leads to a DBG_VALUE of virtual register 691 ``%0``. 692* The first GEP has its effect folded into the first load instruction 693 (as a 4-byte offset), but the variable location is salvaged by folding 694 the GEPs effect into the DIExpression. 695* The second GEP is also folded into the corresponding load. However, it is 696 insufficiently simple to be salvaged, and is emitted as a ``$noreg`` 697 DBG_VALUE, indicating that the variable takes on an undefined location. 698* The final dbg.value has its Value placed in virtual register ``%1``. 699 700Instruction Scheduling 701---------------------- 702 703A number of passes can reschedule instructions, notably instruction selection 704and the pre-and-post RA machine schedulers. Instruction scheduling can 705significantly change the nature of the program -- in the (very unlikely) worst 706case the instruction sequence could be completely reversed. In such 707circumstances LLVM follows the principle applied to optimizations, that it is 708better for the debugger not to display any state than a misleading state. 709Thus, whenever instructions are advanced in order of execution, any 710corresponding DBG_VALUE is kept in its original position, and if an instruction 711is delayed then the variable is given an undefined location for the duration 712of the delay. To illustrate, consider this pseudo-MIR: 713 714.. code-block:: text 715 716 %1:gr32 = MOV32rm %0, 1, $noreg, 4, $noreg, debug-location !5 :: (load 4 from %ir.addr1) 717 DBG_VALUE %1, $noreg, !1, !2 718 %4:gr32 = ADD32rr %3, %2, implicit-def dead $eflags 719 DBG_VALUE %4, $noreg, !3, !4 720 %7:gr32 = SUB32rr %6, %5, implicit-def dead $eflags 721 DBG_VALUE %7, $noreg, !5, !6 722 723Imagine that the SUB32rr were moved forward to give us the following MIR: 724 725.. code-block:: text 726 727 %7:gr32 = SUB32rr %6, %5, implicit-def dead $eflags 728 %1:gr32 = MOV32rm %0, 1, $noreg, 4, $noreg, debug-location !5 :: (load 4 from %ir.addr1) 729 DBG_VALUE %1, $noreg, !1, !2 730 %4:gr32 = ADD32rr %3, %2, implicit-def dead $eflags 731 DBG_VALUE %4, $noreg, !3, !4 732 DBG_VALUE %7, $noreg, !5, !6 733 734In this circumstance LLVM would leave the MIR as shown above. Were we to move 735the DBG_VALUE of virtual register %7 upwards with the SUB32rr, we would re-order 736assignments and introduce a new state of the program. Whereas with the solution 737above, the debugger will see one fewer combination of variable values, because 738``!3`` and ``!5`` will change value at the same time. This is preferred over 739misrepresenting the original program. 740 741In comparison, if one sunk the MOV32rm, LLVM would produce the following: 742 743.. code-block:: text 744 745 DBG_VALUE $noreg, $noreg, !1, !2 746 %4:gr32 = ADD32rr %3, %2, implicit-def dead $eflags 747 DBG_VALUE %4, $noreg, !3, !4 748 %7:gr32 = SUB32rr %6, %5, implicit-def dead $eflags 749 DBG_VALUE %7, $noreg, !5, !6 750 %1:gr32 = MOV32rm %0, 1, $noreg, 4, $noreg, debug-location !5 :: (load 4 from %ir.addr1) 751 DBG_VALUE %1, $noreg, !1, !2 752 753Here, to avoid presenting a state in which the first assignment to ``!1`` 754disappears, the DBG_VALUE at the top of the block assigns the variable the 755undefined location, until its value is available at the end of the block where 756an additional DBG_VALUE is added. Were any other DBG_VALUE for ``!1`` to occur 757in the instructions that the MOV32rm was sunk past, the DBG_VALUE for ``%1`` 758would be dropped and the debugger would never observe it in the variable. This 759accurately reflects that the value is not available during the corresponding 760portion of the original program. 761 762Variable locations during Register Allocation 763--------------------------------------------- 764 765To avoid debug instructions interfering with the register allocator, the 766LiveDebugVariables pass extracts variable locations from a MIR function and 767deletes the corresponding DBG_VALUE instructions. Some localized copy 768propagation is performed within blocks. After register allocation, the 769VirtRegRewriter pass re-inserts DBG_VALUE instructions in their original 770positions, translating virtual register references into their physical 771machine locations. To avoid encoding incorrect variable locations, in this 772pass any DBG_VALUE of a virtual register that is not live, is replaced by 773the undefined location. The LiveDebugVariables may insert redundant DBG_VALUEs 774because of virtual register rewriting. These will be subsequently removed by 775the RemoveRedundantDebugValues pass. 776 777LiveDebugValues expansion of variable locations 778----------------------------------------------- 779 780After all optimizations have run and shortly before emission, the 781LiveDebugValues pass runs to achieve two aims: 782 783* To propagate the location of variables through copies and register spills, 784* For every block, to record every valid variable location in that block. 785 786After this pass the DBG_VALUE instruction changes meaning: rather than 787corresponding to a source-level assignment where the variable may change value, 788it asserts the location of a variable in a block, and loses effect outside the 789block. Propagating variable locations through copies and spills is 790straightforwards: determining the variable location in every basic block 791requires the consideration of control flow. Consider the following IR, which 792presents several difficulties: 793 794.. code-block:: text 795 796 define dso_local i32 @foo(i1 %cond, i32 %input) !dbg !12 { 797 entry: 798 br i1 %cond, label %truebr, label %falsebr 799 800 bb1: 801 %value = phi i32 [ %value1, %truebr ], [ %value2, %falsebr ] 802 br label %exit, !dbg !26 803 804 truebr: 805 call void @llvm.dbg.value(metadata i32 %input, metadata !30, metadata !DIExpression()), !dbg !24 806 call void @llvm.dbg.value(metadata i32 1, metadata !23, metadata !DIExpression()), !dbg !24 807 %value1 = add i32 %input, 1 808 br label %bb1 809 810 falsebr: 811 call void @llvm.dbg.value(metadata i32 %input, metadata !30, metadata !DIExpression()), !dbg !24 812 call void @llvm.dbg.value(metadata i32 2, metadata !23, metadata !DIExpression()), !dbg !24 813 %value = add i32 %input, 2 814 br label %bb1 815 816 exit: 817 ret i32 %value, !dbg !30 818 } 819 820Here the difficulties are: 821 822* The control flow is roughly the opposite of basic block order 823* The value of the ``!23`` variable merges into ``%bb1``, but there is no PHI 824 node 825 826As mentioned above, the ``llvm.dbg.value`` intrinsics essentially form an 827imperative program embedded in the IR, with each intrinsic defining a variable 828location. This *could* be converted to an SSA form by mem2reg, in the same way 829that it uses use-def chains to identify control flow merges and insert phi 830nodes for IR Values. However, because debug variable locations are defined for 831every machine instruction, in effect every IR instruction uses every variable 832location, which would lead to a large number of debugging intrinsics being 833generated. 834 835Examining the example above, variable ``!30`` is assigned ``%input`` on both 836conditional paths through the function, while ``!23`` is assigned differing 837constant values on either path. Where control flow merges in ``%bb1`` we would 838want ``!30`` to keep its location (``%input``), but ``!23`` to become undefined 839as we cannot determine at runtime what value it should have in %bb1 without 840inserting a PHI node. mem2reg does not insert the PHI node to avoid changing 841codegen when debugging is enabled, and does not insert the other dbg.values 842to avoid adding very large numbers of intrinsics. 843 844Instead, LiveDebugValues determines variable locations when control 845flow merges. A dataflow analysis is used to propagate locations between blocks: 846when control flow merges, if a variable has the same location in all 847predecessors then that location is propagated into the successor. If the 848predecessor locations disagree, the location becomes undefined. 849 850Once LiveDebugValues has run, every block should have all valid variable 851locations described by DBG_VALUE instructions within the block. Very little 852effort is then required by supporting classes (such as 853DbgEntityHistoryCalculator) to build a map of each instruction to every 854valid variable location, without the need to consider control flow. From 855the example above, it is otherwise difficult to determine that the location 856of variable ``!30`` should flow "up" into block ``%bb1``, but that the location 857of variable ``!23`` should not flow "down" into the ``%exit`` block. 858 859.. _ccxx_frontend: 860 861C/C++ front-end specific debug information 862========================================== 863 864The C and C++ front-ends represent information about the program in a 865format that is effectively identical to `DWARF <http://www.dwarfstd.org/>`_ 866in terms of information content. This allows code generators to 867trivially support native debuggers by generating standard dwarf 868information, and contains enough information for non-dwarf targets to 869translate it as needed. 870 871This section describes the forms used to represent C and C++ programs. Other 872languages could pattern themselves after this (which itself is tuned to 873representing programs in the same way that DWARF does), or they could choose 874to provide completely different forms if they don't fit into the DWARF model. 875As support for debugging information gets added to the various LLVM 876source-language front-ends, the information used should be documented here. 877 878The following sections provide examples of a few C/C++ constructs and 879the debug information that would best describe those constructs. The 880canonical references are the ``DINode`` classes defined in 881``include/llvm/IR/DebugInfoMetadata.h`` and the implementations of the 882helper functions in ``lib/IR/DIBuilder.cpp``. 883 884C/C++ source file information 885----------------------------- 886 887``llvm::Instruction`` provides easy access to metadata attached with an 888instruction. One can extract line number information encoded in LLVM IR using 889``Instruction::getDebugLoc()`` and ``DILocation::getLine()``. 890 891.. code-block:: c++ 892 893 if (DILocation *Loc = I->getDebugLoc()) { // Here I is an LLVM instruction 894 unsigned Line = Loc->getLine(); 895 StringRef File = Loc->getFilename(); 896 StringRef Dir = Loc->getDirectory(); 897 bool ImplicitCode = Loc->isImplicitCode(); 898 } 899 900When the flag ImplicitCode is true then it means that the Instruction has been 901added by the front-end but doesn't correspond to source code written by the user. For example 902 903.. code-block:: c++ 904 905 if (MyBoolean) { 906 MyObject MO; 907 ... 908 } 909 910At the end of the scope the MyObject's destructor is called but it isn't written 911explicitly. This information is useful to avoid to have counters on brackets when 912making code coverage. 913 914C/C++ global variable information 915--------------------------------- 916 917Given an integer global variable declared as follows: 918 919.. code-block:: c 920 921 _Alignas(8) int MyGlobal = 100; 922 923a C/C++ front-end would generate the following descriptors: 924 925.. code-block:: text 926 927 ;; 928 ;; Define the global itself. 929 ;; 930 @MyGlobal = global i32 100, align 8, !dbg !0 931 932 ;; 933 ;; List of debug info of globals 934 ;; 935 !llvm.dbg.cu = !{!1} 936 937 ;; Some unrelated metadata. 938 !llvm.module.flags = !{!6, !7} 939 !llvm.ident = !{!8} 940 941 ;; Define the global variable itself 942 !0 = distinct !DIGlobalVariable(name: "MyGlobal", scope: !1, file: !2, line: 1, type: !5, isLocal: false, isDefinition: true, align: 64) 943 944 ;; Define the compile unit. 945 !1 = distinct !DICompileUnit(language: DW_LANG_C99, file: !2, 946 producer: "clang version 4.0.0", 947 isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug, 948 enums: !3, globals: !4) 949 950 ;; 951 ;; Define the file 952 ;; 953 !2 = !DIFile(filename: "/dev/stdin", 954 directory: "/Users/dexonsmith/data/llvm/debug-info") 955 956 ;; An empty array. 957 !3 = !{} 958 959 ;; The Array of Global Variables 960 !4 = !{!0} 961 962 ;; 963 ;; Define the type 964 ;; 965 !5 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed) 966 967 ;; Dwarf version to output. 968 !6 = !{i32 2, !"Dwarf Version", i32 4} 969 970 ;; Debug info schema version. 971 !7 = !{i32 2, !"Debug Info Version", i32 3} 972 973 ;; Compiler identification 974 !8 = !{!"clang version 4.0.0"} 975 976 977The align value in DIGlobalVariable description specifies variable alignment in 978case it was forced by C11 _Alignas(), C++11 alignas() keywords or compiler 979attribute __attribute__((aligned ())). In other case (when this field is missing) 980alignment is considered default. This is used when producing DWARF output 981for DW_AT_alignment value. 982 983C/C++ function information 984-------------------------- 985 986Given a function declared as follows: 987 988.. code-block:: c 989 990 int main(int argc, char *argv[]) { 991 return 0; 992 } 993 994a C/C++ front-end would generate the following descriptors: 995 996.. code-block:: text 997 998 ;; 999 ;; Define the anchor for subprograms. 1000 ;; 1001 !4 = !DISubprogram(name: "main", scope: !1, file: !1, line: 1, type: !5, 1002 isLocal: false, isDefinition: true, scopeLine: 1, 1003 flags: DIFlagPrototyped, isOptimized: false, 1004 retainedNodes: !2) 1005 1006 ;; 1007 ;; Define the subprogram itself. 1008 ;; 1009 define i32 @main(i32 %argc, i8** %argv) !dbg !4 { 1010 ... 1011 } 1012 1013C++ specific debug information 1014============================== 1015 1016C++ special member functions information 1017---------------------------------------- 1018 1019DWARF v5 introduces attributes defined to enhance debugging information of C++ programs. LLVM can generate (or omit) these appropriate DWARF attributes. In C++ a special member function Ctors, Dtors, Copy/Move Ctors, assignment operators can be declared with C++11 keyword deleted. This is represented in LLVM using spFlags value DISPFlagDeleted. 1020 1021Given a class declaration with copy constructor declared as deleted: 1022 1023.. code-block:: c 1024 1025 class foo { 1026 public: 1027 foo(const foo&) = deleted; 1028 }; 1029 1030A C++ frontend would generate following: 1031 1032.. code-block:: text 1033 1034 !17 = !DISubprogram(name: "foo", scope: !11, file: !1, line: 5, type: !18, scopeLine: 5, flags: DIFlagPublic | DIFlagPrototyped, spFlags: DISPFlagDeleted) 1035 1036and this will produce an additional DWARF attribute as: 1037 1038.. code-block:: text 1039 1040 DW_TAG_subprogram [7] * 1041 DW_AT_name [DW_FORM_strx1] (indexed (00000006) string = "foo") 1042 DW_AT_decl_line [DW_FORM_data1] (5) 1043 ... 1044 DW_AT_deleted [DW_FORM_flag_present] (true) 1045 1046Fortran specific debug information 1047================================== 1048 1049Fortran function information 1050---------------------------- 1051 1052There are a few DWARF attributes defined to support client debugging of Fortran programs. LLVM can generate (or omit) the appropriate DWARF attributes for the prefix-specs of ELEMENTAL, PURE, IMPURE, RECURSIVE, and NON_RECURSIVE. This is done by using the spFlags values: DISPFlagElemental, DISPFlagPure, and DISPFlagRecursive. 1053 1054.. code-block:: fortran 1055 1056 elemental function elem_func(a) 1057 1058a Fortran front-end would generate the following descriptors: 1059 1060.. code-block:: text 1061 1062 !11 = distinct !DISubprogram(name: "subroutine2", scope: !1, file: !1, 1063 line: 5, type: !8, scopeLine: 6, 1064 spFlags: DISPFlagDefinition | DISPFlagElemental, unit: !0, 1065 retainedNodes: !2) 1066 1067and this will materialize an additional DWARF attribute as: 1068 1069.. code-block:: text 1070 1071 DW_TAG_subprogram [3] 1072 DW_AT_low_pc [DW_FORM_addr] (0x0000000000000010 ".text") 1073 DW_AT_high_pc [DW_FORM_data4] (0x00000001) 1074 ... 1075 DW_AT_elemental [DW_FORM_flag_present] (true) 1076 1077There are a few DWARF tags defined to represent Fortran specific constructs i.e DW_TAG_string_type for representing Fortran character(n). In LLVM this is represented as DIStringType. 1078 1079.. code-block:: fortran 1080 1081 character(len=*), intent(in) :: string 1082 1083a Fortran front-end would generate the following descriptors: 1084 1085.. code-block:: text 1086 1087 !DILocalVariable(name: "string", arg: 1, scope: !10, file: !3, line: 4, type: !15) 1088 !DIStringType(name: "character(*)!2", stringLength: !16, stringLengthExpression: !DIExpression(), size: 32) 1089 1090A fortran deferred-length character can also contain the information of raw storage of the characters in addition to the length of the string. This information is encoded in the stringLocationExpression field. Based on this information, DW_AT_data_location attribute is emitted in a DW_TAG_string_type debug info. 1091 1092 !DIStringType(name: "character(*)!2", stringLengthExpression: !DIExpression(), stringLocationExpression: !DIExpression(DW_OP_push_object_address, DW_OP_deref), size: 32) 1093 1094and this will materialize in DWARF tags as: 1095 1096.. code-block:: text 1097 1098 DW_TAG_string_type 1099 DW_AT_name ("character(*)!2") 1100 DW_AT_string_length (0x00000064) 1101 0x00000064: DW_TAG_variable 1102 DW_AT_location (DW_OP_fbreg +16) 1103 DW_AT_type (0x00000083 "integer*8") 1104 DW_AT_data_location (DW_OP_push_object_address, DW_OP_deref) 1105 ... 1106 DW_AT_artificial (true) 1107 1108A Fortran front-end may need to generate a *trampoline* function to call a 1109function defined in a different compilation unit. In this case, the front-end 1110can emit the following descriptor for the trampoline function: 1111 1112.. code-block:: text 1113 1114 !DISubprogram(name: "sub1_.t0p", linkageName: "sub1_.t0p", scope: !4, file: !4, type: !5, spFlags: DISPFlagLocalToUnit | DISPFlagDefinition, unit: !7, retainedNodes: !24, targetFuncName: "sub1_") 1115 1116The targetFuncName field is the name of the function that the trampoline 1117calls. This descriptor results in the following DWARF tag: 1118 1119.. code-block:: text 1120 1121 DW_TAG_subprogram 1122 ... 1123 DW_AT_linkage_name ("sub1_.t0p") 1124 DW_AT_name ("sub1_.t0p") 1125 DW_AT_trampoline ("sub1_") 1126 1127Debugging information format 1128============================ 1129 1130Debugging Information Extension for Objective C Properties 1131---------------------------------------------------------- 1132 1133Introduction 1134^^^^^^^^^^^^ 1135 1136Objective C provides a simpler way to declare and define accessor methods using 1137declared properties. The language provides features to declare a property and 1138to let compiler synthesize accessor methods. 1139 1140The debugger lets developer inspect Objective C interfaces and their instance 1141variables and class variables. However, the debugger does not know anything 1142about the properties defined in Objective C interfaces. The debugger consumes 1143information generated by compiler in DWARF format. The format does not support 1144encoding of Objective C properties. This proposal describes DWARF extensions to 1145encode Objective C properties, which the debugger can use to let developers 1146inspect Objective C properties. 1147 1148Proposal 1149^^^^^^^^ 1150 1151Objective C properties exist separately from class members. A property can be 1152defined only by "setter" and "getter" selectors, and be calculated anew on each 1153access. Or a property can just be a direct access to some declared ivar. 1154Finally it can have an ivar "automatically synthesized" for it by the compiler, 1155in which case the property can be referred to in user code directly using the 1156standard C dereference syntax as well as through the property "dot" syntax, but 1157there is no entry in the ``@interface`` declaration corresponding to this ivar. 1158 1159To facilitate debugging, these properties we will add a new DWARF TAG into the 1160``DW_TAG_structure_type`` definition for the class to hold the description of a 1161given property, and a set of DWARF attributes that provide said description. 1162The property tag will also contain the name and declared type of the property. 1163 1164If there is a related ivar, there will also be a DWARF property attribute placed 1165in the ``DW_TAG_member`` DIE for that ivar referring back to the property TAG 1166for that property. And in the case where the compiler synthesizes the ivar 1167directly, the compiler is expected to generate a ``DW_TAG_member`` for that 1168ivar (with the ``DW_AT_artificial`` set to 1), whose name will be the name used 1169to access this ivar directly in code, and with the property attribute pointing 1170back to the property it is backing. 1171 1172The following examples will serve as illustration for our discussion: 1173 1174.. code-block:: objc 1175 1176 @interface I1 { 1177 int n2; 1178 } 1179 1180 @property int p1; 1181 @property int p2; 1182 @end 1183 1184 @implementation I1 1185 @synthesize p1; 1186 @synthesize p2 = n2; 1187 @end 1188 1189This produces the following DWARF (this is a "pseudo dwarfdump" output): 1190 1191.. code-block:: none 1192 1193 0x00000100: TAG_structure_type [7] * 1194 AT_APPLE_runtime_class( 0x10 ) 1195 AT_name( "I1" ) 1196 AT_decl_file( "Objc_Property.m" ) 1197 AT_decl_line( 3 ) 1198 1199 0x00000110 TAG_APPLE_property 1200 AT_name ( "p1" ) 1201 AT_type ( {0x00000150} ( int ) ) 1202 1203 0x00000120: TAG_APPLE_property 1204 AT_name ( "p2" ) 1205 AT_type ( {0x00000150} ( int ) ) 1206 1207 0x00000130: TAG_member [8] 1208 AT_name( "_p1" ) 1209 AT_APPLE_property ( {0x00000110} "p1" ) 1210 AT_type( {0x00000150} ( int ) ) 1211 AT_artificial ( 0x1 ) 1212 1213 0x00000140: TAG_member [8] 1214 AT_name( "n2" ) 1215 AT_APPLE_property ( {0x00000120} "p2" ) 1216 AT_type( {0x00000150} ( int ) ) 1217 1218 0x00000150: AT_type( ( int ) ) 1219 1220Note, the current convention is that the name of the ivar for an 1221auto-synthesized property is the name of the property from which it derives 1222with an underscore prepended, as is shown in the example. But we actually 1223don't need to know this convention, since we are given the name of the ivar 1224directly. 1225 1226Also, it is common practice in ObjC to have different property declarations in 1227the @interface and @implementation - e.g. to provide a read-only property in 1228the interface, and a read-write interface in the implementation. In that case, 1229the compiler should emit whichever property declaration will be in force in the 1230current translation unit. 1231 1232Developers can decorate a property with attributes which are encoded using 1233``DW_AT_APPLE_property_attribute``. 1234 1235.. code-block:: objc 1236 1237 @property (readonly, nonatomic) int pr; 1238 1239.. code-block:: none 1240 1241 TAG_APPLE_property [8] 1242 AT_name( "pr" ) 1243 AT_type ( {0x00000147} (int) ) 1244 AT_APPLE_property_attribute (DW_APPLE_PROPERTY_readonly, DW_APPLE_PROPERTY_nonatomic) 1245 1246The setter and getter method names are attached to the property using 1247``DW_AT_APPLE_property_setter`` and ``DW_AT_APPLE_property_getter`` attributes. 1248 1249.. code-block:: objc 1250 1251 @interface I1 1252 @property (setter=myOwnP3Setter:) int p3; 1253 -(void)myOwnP3Setter:(int)a; 1254 @end 1255 1256 @implementation I1 1257 @synthesize p3; 1258 -(void)myOwnP3Setter:(int)a{ } 1259 @end 1260 1261The DWARF for this would be: 1262 1263.. code-block:: none 1264 1265 0x000003bd: TAG_structure_type [7] * 1266 AT_APPLE_runtime_class( 0x10 ) 1267 AT_name( "I1" ) 1268 AT_decl_file( "Objc_Property.m" ) 1269 AT_decl_line( 3 ) 1270 1271 0x000003cd TAG_APPLE_property 1272 AT_name ( "p3" ) 1273 AT_APPLE_property_setter ( "myOwnP3Setter:" ) 1274 AT_type( {0x00000147} ( int ) ) 1275 1276 0x000003f3: TAG_member [8] 1277 AT_name( "_p3" ) 1278 AT_type ( {0x00000147} ( int ) ) 1279 AT_APPLE_property ( {0x000003cd} ) 1280 AT_artificial ( 0x1 ) 1281 1282New DWARF Tags 1283^^^^^^^^^^^^^^ 1284 1285+-----------------------+--------+ 1286| TAG | Value | 1287+=======================+========+ 1288| DW_TAG_APPLE_property | 0x4200 | 1289+-----------------------+--------+ 1290 1291New DWARF Attributes 1292^^^^^^^^^^^^^^^^^^^^ 1293 1294+--------------------------------+--------+-----------+ 1295| Attribute | Value | Classes | 1296+================================+========+===========+ 1297| DW_AT_APPLE_property | 0x3fed | Reference | 1298+--------------------------------+--------+-----------+ 1299| DW_AT_APPLE_property_getter | 0x3fe9 | String | 1300+--------------------------------+--------+-----------+ 1301| DW_AT_APPLE_property_setter | 0x3fea | String | 1302+--------------------------------+--------+-----------+ 1303| DW_AT_APPLE_property_attribute | 0x3feb | Constant | 1304+--------------------------------+--------+-----------+ 1305 1306New DWARF Constants 1307^^^^^^^^^^^^^^^^^^^ 1308 1309+--------------------------------------+-------+ 1310| Name | Value | 1311+======================================+=======+ 1312| DW_APPLE_PROPERTY_readonly | 0x01 | 1313+--------------------------------------+-------+ 1314| DW_APPLE_PROPERTY_getter | 0x02 | 1315+--------------------------------------+-------+ 1316| DW_APPLE_PROPERTY_assign | 0x04 | 1317+--------------------------------------+-------+ 1318| DW_APPLE_PROPERTY_readwrite | 0x08 | 1319+--------------------------------------+-------+ 1320| DW_APPLE_PROPERTY_retain | 0x10 | 1321+--------------------------------------+-------+ 1322| DW_APPLE_PROPERTY_copy | 0x20 | 1323+--------------------------------------+-------+ 1324| DW_APPLE_PROPERTY_nonatomic | 0x40 | 1325+--------------------------------------+-------+ 1326| DW_APPLE_PROPERTY_setter | 0x80 | 1327+--------------------------------------+-------+ 1328| DW_APPLE_PROPERTY_atomic | 0x100 | 1329+--------------------------------------+-------+ 1330| DW_APPLE_PROPERTY_weak | 0x200 | 1331+--------------------------------------+-------+ 1332| DW_APPLE_PROPERTY_strong | 0x400 | 1333+--------------------------------------+-------+ 1334| DW_APPLE_PROPERTY_unsafe_unretained | 0x800 | 1335+--------------------------------------+-------+ 1336| DW_APPLE_PROPERTY_nullability | 0x1000| 1337+--------------------------------------+-------+ 1338| DW_APPLE_PROPERTY_null_resettable | 0x2000| 1339+--------------------------------------+-------+ 1340| DW_APPLE_PROPERTY_class | 0x4000| 1341+--------------------------------------+-------+ 1342 1343Name Accelerator Tables 1344----------------------- 1345 1346Introduction 1347^^^^^^^^^^^^ 1348 1349The "``.debug_pubnames``" and "``.debug_pubtypes``" formats are not what a 1350debugger needs. The "``pub``" in the section name indicates that the entries 1351in the table are publicly visible names only. This means no static or hidden 1352functions show up in the "``.debug_pubnames``". No static variables or private 1353class variables are in the "``.debug_pubtypes``". Many compilers add different 1354things to these tables, so we can't rely upon the contents between gcc, icc, or 1355clang. 1356 1357The typical query given by users tends not to match up with the contents of 1358these tables. For example, the DWARF spec states that "In the case of the name 1359of a function member or static data member of a C++ structure, class or union, 1360the name presented in the "``.debug_pubnames``" section is not the simple name 1361given by the ``DW_AT_name attribute`` of the referenced debugging information 1362entry, but rather the fully qualified name of the data or function member." 1363So the only names in these tables for complex C++ entries is a fully 1364qualified name. Debugger users tend not to enter their search strings as 1365"``a::b::c(int,const Foo&) const``", but rather as "``c``", "``b::c``" , or 1366"``a::b::c``". So the name entered in the name table must be demangled in 1367order to chop it up appropriately and additional names must be manually entered 1368into the table to make it effective as a name lookup table for debuggers to 1369use. 1370 1371All debuggers currently ignore the "``.debug_pubnames``" table as a result of 1372its inconsistent and useless public-only name content making it a waste of 1373space in the object file. These tables, when they are written to disk, are not 1374sorted in any way, leaving every debugger to do its own parsing and sorting. 1375These tables also include an inlined copy of the string values in the table 1376itself making the tables much larger than they need to be on disk, especially 1377for large C++ programs. 1378 1379Can't we just fix the sections by adding all of the names we need to this 1380table? No, because that is not what the tables are defined to contain and we 1381won't know the difference between the old bad tables and the new good tables. 1382At best we could make our own renamed sections that contain all of the data we 1383need. 1384 1385These tables are also insufficient for what a debugger like LLDB needs. LLDB 1386uses clang for its expression parsing where LLDB acts as a PCH. LLDB is then 1387often asked to look for type "``foo``" or namespace "``bar``", or list items in 1388namespace "``baz``". Namespaces are not included in the pubnames or pubtypes 1389tables. Since clang asks a lot of questions when it is parsing an expression, 1390we need to be very fast when looking up names, as it happens a lot. Having new 1391accelerator tables that are optimized for very quick lookups will benefit this 1392type of debugging experience greatly. 1393 1394We would like to generate name lookup tables that can be mapped into memory 1395from disk, and used as is, with little or no up-front parsing. We would also 1396be able to control the exact content of these different tables so they contain 1397exactly what we need. The Name Accelerator Tables were designed to fix these 1398issues. In order to solve these issues we need to: 1399 1400* Have a format that can be mapped into memory from disk and used as is 1401* Lookups should be very fast 1402* Extensible table format so these tables can be made by many producers 1403* Contain all of the names needed for typical lookups out of the box 1404* Strict rules for the contents of tables 1405 1406Table size is important and the accelerator table format should allow the reuse 1407of strings from common string tables so the strings for the names are not 1408duplicated. We also want to make sure the table is ready to be used as-is by 1409simply mapping the table into memory with minimal header parsing. 1410 1411The name lookups need to be fast and optimized for the kinds of lookups that 1412debuggers tend to do. Optimally we would like to touch as few parts of the 1413mapped table as possible when doing a name lookup and be able to quickly find 1414the name entry we are looking for, or discover there are no matches. In the 1415case of debuggers we optimized for lookups that fail most of the time. 1416 1417Each table that is defined should have strict rules on exactly what is in the 1418accelerator tables and documented so clients can rely on the content. 1419 1420Hash Tables 1421^^^^^^^^^^^ 1422 1423Standard Hash Tables 1424"""""""""""""""""""" 1425 1426Typical hash tables have a header, buckets, and each bucket points to the 1427bucket contents: 1428 1429.. code-block:: none 1430 1431 .------------. 1432 | HEADER | 1433 |------------| 1434 | BUCKETS | 1435 |------------| 1436 | DATA | 1437 `------------' 1438 1439The BUCKETS are an array of offsets to DATA for each hash: 1440 1441.. code-block:: none 1442 1443 .------------. 1444 | 0x00001000 | BUCKETS[0] 1445 | 0x00002000 | BUCKETS[1] 1446 | 0x00002200 | BUCKETS[2] 1447 | 0x000034f0 | BUCKETS[3] 1448 | | ... 1449 | 0xXXXXXXXX | BUCKETS[n_buckets] 1450 '------------' 1451 1452So for ``bucket[3]`` in the example above, we have an offset into the table 14530x000034f0 which points to a chain of entries for the bucket. Each bucket must 1454contain a next pointer, full 32 bit hash value, the string itself, and the data 1455for the current string value. 1456 1457.. code-block:: none 1458 1459 .------------. 1460 0x000034f0: | 0x00003500 | next pointer 1461 | 0x12345678 | 32 bit hash 1462 | "erase" | string value 1463 | data[n] | HashData for this bucket 1464 |------------| 1465 0x00003500: | 0x00003550 | next pointer 1466 | 0x29273623 | 32 bit hash 1467 | "dump" | string value 1468 | data[n] | HashData for this bucket 1469 |------------| 1470 0x00003550: | 0x00000000 | next pointer 1471 | 0x82638293 | 32 bit hash 1472 | "main" | string value 1473 | data[n] | HashData for this bucket 1474 `------------' 1475 1476The problem with this layout for debuggers is that we need to optimize for the 1477negative lookup case where the symbol we're searching for is not present. So 1478if we were to lookup "``printf``" in the table above, we would make a 32-bit 1479hash for "``printf``", it might match ``bucket[3]``. We would need to go to 1480the offset 0x000034f0 and start looking to see if our 32 bit hash matches. To 1481do so, we need to read the next pointer, then read the hash, compare it, and 1482skip to the next bucket. Each time we are skipping many bytes in memory and 1483touching new pages just to do the compare on the full 32 bit hash. All of 1484these accesses then tell us that we didn't have a match. 1485 1486Name Hash Tables 1487"""""""""""""""" 1488 1489To solve the issues mentioned above we have structured the hash tables a bit 1490differently: a header, buckets, an array of all unique 32 bit hash values, 1491followed by an array of hash value data offsets, one for each hash value, then 1492the data for all hash values: 1493 1494.. code-block:: none 1495 1496 .-------------. 1497 | HEADER | 1498 |-------------| 1499 | BUCKETS | 1500 |-------------| 1501 | HASHES | 1502 |-------------| 1503 | OFFSETS | 1504 |-------------| 1505 | DATA | 1506 `-------------' 1507 1508The ``BUCKETS`` in the name tables are an index into the ``HASHES`` array. By 1509making all of the full 32 bit hash values contiguous in memory, we allow 1510ourselves to efficiently check for a match while touching as little memory as 1511possible. Most often checking the 32 bit hash values is as far as the lookup 1512goes. If it does match, it usually is a match with no collisions. So for a 1513table with "``n_buckets``" buckets, and "``n_hashes``" unique 32 bit hash 1514values, we can clarify the contents of the ``BUCKETS``, ``HASHES`` and 1515``OFFSETS`` as: 1516 1517.. code-block:: none 1518 1519 .-------------------------. 1520 | HEADER.magic | uint32_t 1521 | HEADER.version | uint16_t 1522 | HEADER.hash_function | uint16_t 1523 | HEADER.bucket_count | uint32_t 1524 | HEADER.hashes_count | uint32_t 1525 | HEADER.header_data_len | uint32_t 1526 | HEADER_DATA | HeaderData 1527 |-------------------------| 1528 | BUCKETS | uint32_t[n_buckets] // 32 bit hash indexes 1529 |-------------------------| 1530 | HASHES | uint32_t[n_hashes] // 32 bit hash values 1531 |-------------------------| 1532 | OFFSETS | uint32_t[n_hashes] // 32 bit offsets to hash value data 1533 |-------------------------| 1534 | ALL HASH DATA | 1535 `-------------------------' 1536 1537So taking the exact same data from the standard hash example above we end up 1538with: 1539 1540.. code-block:: none 1541 1542 .------------. 1543 | HEADER | 1544 |------------| 1545 | 0 | BUCKETS[0] 1546 | 2 | BUCKETS[1] 1547 | 5 | BUCKETS[2] 1548 | 6 | BUCKETS[3] 1549 | | ... 1550 | ... | BUCKETS[n_buckets] 1551 |------------| 1552 | 0x........ | HASHES[0] 1553 | 0x........ | HASHES[1] 1554 | 0x........ | HASHES[2] 1555 | 0x........ | HASHES[3] 1556 | 0x........ | HASHES[4] 1557 | 0x........ | HASHES[5] 1558 | 0x12345678 | HASHES[6] hash for BUCKETS[3] 1559 | 0x29273623 | HASHES[7] hash for BUCKETS[3] 1560 | 0x82638293 | HASHES[8] hash for BUCKETS[3] 1561 | 0x........ | HASHES[9] 1562 | 0x........ | HASHES[10] 1563 | 0x........ | HASHES[11] 1564 | 0x........ | HASHES[12] 1565 | 0x........ | HASHES[13] 1566 | 0x........ | HASHES[n_hashes] 1567 |------------| 1568 | 0x........ | OFFSETS[0] 1569 | 0x........ | OFFSETS[1] 1570 | 0x........ | OFFSETS[2] 1571 | 0x........ | OFFSETS[3] 1572 | 0x........ | OFFSETS[4] 1573 | 0x........ | OFFSETS[5] 1574 | 0x000034f0 | OFFSETS[6] offset for BUCKETS[3] 1575 | 0x00003500 | OFFSETS[7] offset for BUCKETS[3] 1576 | 0x00003550 | OFFSETS[8] offset for BUCKETS[3] 1577 | 0x........ | OFFSETS[9] 1578 | 0x........ | OFFSETS[10] 1579 | 0x........ | OFFSETS[11] 1580 | 0x........ | OFFSETS[12] 1581 | 0x........ | OFFSETS[13] 1582 | 0x........ | OFFSETS[n_hashes] 1583 |------------| 1584 | | 1585 | | 1586 | | 1587 | | 1588 | | 1589 |------------| 1590 0x000034f0: | 0x00001203 | .debug_str ("erase") 1591 | 0x00000004 | A 32 bit array count - number of HashData with name "erase" 1592 | 0x........ | HashData[0] 1593 | 0x........ | HashData[1] 1594 | 0x........ | HashData[2] 1595 | 0x........ | HashData[3] 1596 | 0x00000000 | String offset into .debug_str (terminate data for hash) 1597 |------------| 1598 0x00003500: | 0x00001203 | String offset into .debug_str ("collision") 1599 | 0x00000002 | A 32 bit array count - number of HashData with name "collision" 1600 | 0x........ | HashData[0] 1601 | 0x........ | HashData[1] 1602 | 0x00001203 | String offset into .debug_str ("dump") 1603 | 0x00000003 | A 32 bit array count - number of HashData with name "dump" 1604 | 0x........ | HashData[0] 1605 | 0x........ | HashData[1] 1606 | 0x........ | HashData[2] 1607 | 0x00000000 | String offset into .debug_str (terminate data for hash) 1608 |------------| 1609 0x00003550: | 0x00001203 | String offset into .debug_str ("main") 1610 | 0x00000009 | A 32 bit array count - number of HashData with name "main" 1611 | 0x........ | HashData[0] 1612 | 0x........ | HashData[1] 1613 | 0x........ | HashData[2] 1614 | 0x........ | HashData[3] 1615 | 0x........ | HashData[4] 1616 | 0x........ | HashData[5] 1617 | 0x........ | HashData[6] 1618 | 0x........ | HashData[7] 1619 | 0x........ | HashData[8] 1620 | 0x00000000 | String offset into .debug_str (terminate data for hash) 1621 `------------' 1622 1623So we still have all of the same data, we just organize it more efficiently for 1624debugger lookup. If we repeat the same "``printf``" lookup from above, we 1625would hash "``printf``" and find it matches ``BUCKETS[3]`` by taking the 32 bit 1626hash value and modulo it by ``n_buckets``. ``BUCKETS[3]`` contains "6" which 1627is the index into the ``HASHES`` table. We would then compare any consecutive 162832 bit hashes values in the ``HASHES`` array as long as the hashes would be in 1629``BUCKETS[3]``. We do this by verifying that each subsequent hash value modulo 1630``n_buckets`` is still 3. In the case of a failed lookup we would access the 1631memory for ``BUCKETS[3]``, and then compare a few consecutive 32 bit hashes 1632before we know that we have no match. We don't end up marching through 1633multiple words of memory and we really keep the number of processor data cache 1634lines being accessed as small as possible. 1635 1636The string hash that is used for these lookup tables is the Daniel J. 1637Bernstein hash which is also used in the ELF ``GNU_HASH`` sections. It is a 1638very good hash for all kinds of names in programs with very few hash 1639collisions. 1640 1641Empty buckets are designated by using an invalid hash index of ``UINT32_MAX``. 1642 1643Details 1644^^^^^^^ 1645 1646These name hash tables are designed to be generic where specializations of the 1647table get to define additional data that goes into the header ("``HeaderData``"), 1648how the string value is stored ("``KeyType``") and the content of the data for each 1649hash value. 1650 1651Header Layout 1652""""""""""""" 1653 1654The header has a fixed part, and the specialized part. The exact format of the 1655header is: 1656 1657.. code-block:: c 1658 1659 struct Header 1660 { 1661 uint32_t magic; // 'HASH' magic value to allow endian detection 1662 uint16_t version; // Version number 1663 uint16_t hash_function; // The hash function enumeration that was used 1664 uint32_t bucket_count; // The number of buckets in this hash table 1665 uint32_t hashes_count; // The total number of unique hash values and hash data offsets in this table 1666 uint32_t header_data_len; // The bytes to skip to get to the hash indexes (buckets) for correct alignment 1667 // Specifically the length of the following HeaderData field - this does not 1668 // include the size of the preceding fields 1669 HeaderData header_data; // Implementation specific header data 1670 }; 1671 1672The header starts with a 32 bit "``magic``" value which must be ``'HASH'`` 1673encoded as an ASCII integer. This allows the detection of the start of the 1674hash table and also allows the table's byte order to be determined so the table 1675can be correctly extracted. The "``magic``" value is followed by a 16 bit 1676``version`` number which allows the table to be revised and modified in the 1677future. The current version number is 1. ``hash_function`` is a ``uint16_t`` 1678enumeration that specifies which hash function was used to produce this table. 1679The current values for the hash function enumerations include: 1680 1681.. code-block:: c 1682 1683 enum HashFunctionType 1684 { 1685 eHashFunctionDJB = 0u, // Daniel J Bernstein hash function 1686 }; 1687 1688``bucket_count`` is a 32 bit unsigned integer that represents how many buckets 1689are in the ``BUCKETS`` array. ``hashes_count`` is the number of unique 32 bit 1690hash values that are in the ``HASHES`` array, and is the same number of offsets 1691are contained in the ``OFFSETS`` array. ``header_data_len`` specifies the size 1692in bytes of the ``HeaderData`` that is filled in by specialized versions of 1693this table. 1694 1695Fixed Lookup 1696"""""""""""" 1697 1698The header is followed by the buckets, hashes, offsets, and hash value data. 1699 1700.. code-block:: c 1701 1702 struct FixedTable 1703 { 1704 uint32_t buckets[Header.bucket_count]; // An array of hash indexes into the "hashes[]" array below 1705 uint32_t hashes [Header.hashes_count]; // Every unique 32 bit hash for the entire table is in this table 1706 uint32_t offsets[Header.hashes_count]; // An offset that corresponds to each item in the "hashes[]" array above 1707 }; 1708 1709``buckets`` is an array of 32 bit indexes into the ``hashes`` array. The 1710``hashes`` array contains all of the 32 bit hash values for all names in the 1711hash table. Each hash in the ``hashes`` table has an offset in the ``offsets`` 1712array that points to the data for the hash value. 1713 1714This table setup makes it very easy to repurpose these tables to contain 1715different data, while keeping the lookup mechanism the same for all tables. 1716This layout also makes it possible to save the table to disk and map it in 1717later and do very efficient name lookups with little or no parsing. 1718 1719DWARF lookup tables can be implemented in a variety of ways and can store a lot 1720of information for each name. We want to make the DWARF tables extensible and 1721able to store the data efficiently so we have used some of the DWARF features 1722that enable efficient data storage to define exactly what kind of data we store 1723for each name. 1724 1725The ``HeaderData`` contains a definition of the contents of each HashData chunk. 1726We might want to store an offset to all of the debug information entries (DIEs) 1727for each name. To keep things extensible, we create a list of items, or 1728Atoms, that are contained in the data for each name. First comes the type of 1729the data in each atom: 1730 1731.. code-block:: c 1732 1733 enum AtomType 1734 { 1735 eAtomTypeNULL = 0u, 1736 eAtomTypeDIEOffset = 1u, // DIE offset, check form for encoding 1737 eAtomTypeCUOffset = 2u, // DIE offset of the compiler unit header that contains the item in question 1738 eAtomTypeTag = 3u, // DW_TAG_xxx value, should be encoded as DW_FORM_data1 (if no tags exceed 255) or DW_FORM_data2 1739 eAtomTypeNameFlags = 4u, // Flags from enum NameFlags 1740 eAtomTypeTypeFlags = 5u, // Flags from enum TypeFlags 1741 }; 1742 1743The enumeration values and their meanings are: 1744 1745.. code-block:: none 1746 1747 eAtomTypeNULL - a termination atom that specifies the end of the atom list 1748 eAtomTypeDIEOffset - an offset into the .debug_info section for the DWARF DIE for this name 1749 eAtomTypeCUOffset - an offset into the .debug_info section for the CU that contains the DIE 1750 eAtomTypeDIETag - The DW_TAG_XXX enumeration value so you don't have to parse the DWARF to see what it is 1751 eAtomTypeNameFlags - Flags for functions and global variables (isFunction, isInlined, isExternal...) 1752 eAtomTypeTypeFlags - Flags for types (isCXXClass, isObjCClass, ...) 1753 1754Then we allow each atom type to define the atom type and how the data for each 1755atom type data is encoded: 1756 1757.. code-block:: c 1758 1759 struct Atom 1760 { 1761 uint16_t type; // AtomType enum value 1762 uint16_t form; // DWARF DW_FORM_XXX defines 1763 }; 1764 1765The ``form`` type above is from the DWARF specification and defines the exact 1766encoding of the data for the Atom type. See the DWARF specification for the 1767``DW_FORM_`` definitions. 1768 1769.. code-block:: c 1770 1771 struct HeaderData 1772 { 1773 uint32_t die_offset_base; 1774 uint32_t atom_count; 1775 Atoms atoms[atom_count0]; 1776 }; 1777 1778``HeaderData`` defines the base DIE offset that should be added to any atoms 1779that are encoded using the ``DW_FORM_ref1``, ``DW_FORM_ref2``, 1780``DW_FORM_ref4``, ``DW_FORM_ref8`` or ``DW_FORM_ref_udata``. It also defines 1781what is contained in each ``HashData`` object -- ``Atom.form`` tells us how large 1782each field will be in the ``HashData`` and the ``Atom.type`` tells us how this data 1783should be interpreted. 1784 1785For the current implementations of the "``.apple_names``" (all functions + 1786globals), the "``.apple_types``" (names of all types that are defined), and 1787the "``.apple_namespaces``" (all namespaces), we currently set the ``Atom`` 1788array to be: 1789 1790.. code-block:: c 1791 1792 HeaderData.atom_count = 1; 1793 HeaderData.atoms[0].type = eAtomTypeDIEOffset; 1794 HeaderData.atoms[0].form = DW_FORM_data4; 1795 1796This defines the contents to be the DIE offset (eAtomTypeDIEOffset) that is 1797encoded as a 32 bit value (DW_FORM_data4). This allows a single name to have 1798multiple matching DIEs in a single file, which could come up with an inlined 1799function for instance. Future tables could include more information about the 1800DIE such as flags indicating if the DIE is a function, method, block, 1801or inlined. 1802 1803The KeyType for the DWARF table is a 32 bit string table offset into the 1804".debug_str" table. The ".debug_str" is the string table for the DWARF which 1805may already contain copies of all of the strings. This helps make sure, with 1806help from the compiler, that we reuse the strings between all of the DWARF 1807sections and keeps the hash table size down. Another benefit to having the 1808compiler generate all strings as DW_FORM_strp in the debug info, is that 1809DWARF parsing can be made much faster. 1810 1811After a lookup is made, we get an offset into the hash data. The hash data 1812needs to be able to deal with 32 bit hash collisions, so the chunk of data 1813at the offset in the hash data consists of a triple: 1814 1815.. code-block:: c 1816 1817 uint32_t str_offset 1818 uint32_t hash_data_count 1819 HashData[hash_data_count] 1820 1821If "str_offset" is zero, then the bucket contents are done. 99.9% of the 1822hash data chunks contain a single item (no 32 bit hash collision): 1823 1824.. code-block:: none 1825 1826 .------------. 1827 | 0x00001023 | uint32_t KeyType (.debug_str[0x0001023] => "main") 1828 | 0x00000004 | uint32_t HashData count 1829 | 0x........ | uint32_t HashData[0] DIE offset 1830 | 0x........ | uint32_t HashData[1] DIE offset 1831 | 0x........ | uint32_t HashData[2] DIE offset 1832 | 0x........ | uint32_t HashData[3] DIE offset 1833 | 0x00000000 | uint32_t KeyType (end of hash chain) 1834 `------------' 1835 1836If there are collisions, you will have multiple valid string offsets: 1837 1838.. code-block:: none 1839 1840 .------------. 1841 | 0x00001023 | uint32_t KeyType (.debug_str[0x0001023] => "main") 1842 | 0x00000004 | uint32_t HashData count 1843 | 0x........ | uint32_t HashData[0] DIE offset 1844 | 0x........ | uint32_t HashData[1] DIE offset 1845 | 0x........ | uint32_t HashData[2] DIE offset 1846 | 0x........ | uint32_t HashData[3] DIE offset 1847 | 0x00002023 | uint32_t KeyType (.debug_str[0x0002023] => "print") 1848 | 0x00000002 | uint32_t HashData count 1849 | 0x........ | uint32_t HashData[0] DIE offset 1850 | 0x........ | uint32_t HashData[1] DIE offset 1851 | 0x00000000 | uint32_t KeyType (end of hash chain) 1852 `------------' 1853 1854Current testing with real world C++ binaries has shown that there is around 1 185532 bit hash collision per 100,000 name entries. 1856 1857Contents 1858^^^^^^^^ 1859 1860As we said, we want to strictly define exactly what is included in the 1861different tables. For DWARF, we have 3 tables: "``.apple_names``", 1862"``.apple_types``", and "``.apple_namespaces``". 1863 1864"``.apple_names``" sections should contain an entry for each DWARF DIE whose 1865``DW_TAG`` is a ``DW_TAG_label``, ``DW_TAG_inlined_subroutine``, or 1866``DW_TAG_subprogram`` that has address attributes: ``DW_AT_low_pc``, 1867``DW_AT_high_pc``, ``DW_AT_ranges`` or ``DW_AT_entry_pc``. It also contains 1868``DW_TAG_variable`` DIEs that have a ``DW_OP_addr`` in the location (global and 1869static variables). All global and static variables should be included, 1870including those scoped within functions and classes. For example using the 1871following code: 1872 1873.. code-block:: c 1874 1875 static int var = 0; 1876 1877 void f () 1878 { 1879 static int var = 0; 1880 } 1881 1882Both of the static ``var`` variables would be included in the table. All 1883functions should emit both their full names and their basenames. For C or C++, 1884the full name is the mangled name (if available) which is usually in the 1885``DW_AT_MIPS_linkage_name`` attribute, and the ``DW_AT_name`` contains the 1886function basename. If global or static variables have a mangled name in a 1887``DW_AT_MIPS_linkage_name`` attribute, this should be emitted along with the 1888simple name found in the ``DW_AT_name`` attribute. 1889 1890"``.apple_types``" sections should contain an entry for each DWARF DIE whose 1891tag is one of: 1892 1893* DW_TAG_array_type 1894* DW_TAG_class_type 1895* DW_TAG_enumeration_type 1896* DW_TAG_pointer_type 1897* DW_TAG_reference_type 1898* DW_TAG_string_type 1899* DW_TAG_structure_type 1900* DW_TAG_subroutine_type 1901* DW_TAG_typedef 1902* DW_TAG_union_type 1903* DW_TAG_ptr_to_member_type 1904* DW_TAG_set_type 1905* DW_TAG_subrange_type 1906* DW_TAG_base_type 1907* DW_TAG_const_type 1908* DW_TAG_immutable_type 1909* DW_TAG_file_type 1910* DW_TAG_namelist 1911* DW_TAG_packed_type 1912* DW_TAG_volatile_type 1913* DW_TAG_restrict_type 1914* DW_TAG_atomic_type 1915* DW_TAG_interface_type 1916* DW_TAG_unspecified_type 1917* DW_TAG_shared_type 1918 1919Only entries with a ``DW_AT_name`` attribute are included, and the entry must 1920not be a forward declaration (``DW_AT_declaration`` attribute with a non-zero 1921value). For example, using the following code: 1922 1923.. code-block:: c 1924 1925 int main () 1926 { 1927 int *b = 0; 1928 return *b; 1929 } 1930 1931We get a few type DIEs: 1932 1933.. code-block:: none 1934 1935 0x00000067: TAG_base_type [5] 1936 AT_encoding( DW_ATE_signed ) 1937 AT_name( "int" ) 1938 AT_byte_size( 0x04 ) 1939 1940 0x0000006e: TAG_pointer_type [6] 1941 AT_type( {0x00000067} ( int ) ) 1942 AT_byte_size( 0x08 ) 1943 1944The DW_TAG_pointer_type is not included because it does not have a ``DW_AT_name``. 1945 1946"``.apple_namespaces``" section should contain all ``DW_TAG_namespace`` DIEs. 1947If we run into a namespace that has no name this is an anonymous namespace, and 1948the name should be output as "``(anonymous namespace)``" (without the quotes). 1949Why? This matches the output of the ``abi::cxa_demangle()`` that is in the 1950standard C++ library that demangles mangled names. 1951 1952 1953Language Extensions and File Format Changes 1954^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 1955 1956Objective-C Extensions 1957"""""""""""""""""""""" 1958 1959"``.apple_objc``" section should contain all ``DW_TAG_subprogram`` DIEs for an 1960Objective-C class. The name used in the hash table is the name of the 1961Objective-C class itself. If the Objective-C class has a category, then an 1962entry is made for both the class name without the category, and for the class 1963name with the category. So if we have a DIE at offset 0x1234 with a name of 1964method "``-[NSString(my_additions) stringWithSpecialString:]``", we would add 1965an entry for "``NSString``" that points to DIE 0x1234, and an entry for 1966"``NSString(my_additions)``" that points to 0x1234. This allows us to quickly 1967track down all Objective-C methods for an Objective-C class when doing 1968expressions. It is needed because of the dynamic nature of Objective-C where 1969anyone can add methods to a class. The DWARF for Objective-C methods is also 1970emitted differently from C++ classes where the methods are not usually 1971contained in the class definition, they are scattered about across one or more 1972compile units. Categories can also be defined in different shared libraries. 1973So we need to be able to quickly find all of the methods and class functions 1974given the Objective-C class name, or quickly find all methods and class 1975functions for a class + category name. This table does not contain any 1976selector names, it just maps Objective-C class names (or class names + 1977category) to all of the methods and class functions. The selectors are added 1978as function basenames in the "``.debug_names``" section. 1979 1980In the "``.apple_names``" section for Objective-C functions, the full name is 1981the entire function name with the brackets ("``-[NSString 1982stringWithCString:]``") and the basename is the selector only 1983("``stringWithCString:``"). 1984 1985Mach-O Changes 1986"""""""""""""" 1987 1988The sections names for the apple hash tables are for non-mach-o files. For 1989mach-o files, the sections should be contained in the ``__DWARF`` segment with 1990names as follows: 1991 1992* "``.apple_names``" -> "``__apple_names``" 1993* "``.apple_types``" -> "``__apple_types``" 1994* "``.apple_namespaces``" -> "``__apple_namespac``" (16 character limit) 1995* "``.apple_objc``" -> "``__apple_objc``" 1996 1997.. _codeview: 1998 1999CodeView Debug Info Format 2000========================== 2001 2002LLVM supports emitting CodeView, the Microsoft debug info format, and this 2003section describes the design and implementation of that support. 2004 2005Format Background 2006----------------- 2007 2008CodeView as a format is clearly oriented around C++ debugging, and in C++, the 2009majority of debug information tends to be type information. Therefore, the 2010overriding design constraint of CodeView is the separation of type information 2011from other "symbol" information so that type information can be efficiently 2012merged across translation units. Both type information and symbol information is 2013generally stored as a sequence of records, where each record begins with a 201416-bit record size and a 16-bit record kind. 2015 2016Type information is usually stored in the ``.debug$T`` section of the object 2017file. All other debug info, such as line info, string table, symbol info, and 2018inlinee info, is stored in one or more ``.debug$S`` sections. There may only be 2019one ``.debug$T`` section per object file, since all other debug info refers to 2020it. If a PDB (enabled by the ``/Zi`` MSVC option) was used during compilation, 2021the ``.debug$T`` section will contain only an ``LF_TYPESERVER2`` record pointing 2022to the PDB. When using PDBs, symbol information appears to remain in the object 2023file ``.debug$S`` sections. 2024 2025Type records are referred to by their index, which is the number of records in 2026the stream before a given record plus ``0x1000``. Many common basic types, such 2027as the basic integral types and unqualified pointers to them, are represented 2028using type indices less than ``0x1000``. Such basic types are built in to 2029CodeView consumers and do not require type records. 2030 2031Each type record may only contain type indices that are less than its own type 2032index. This ensures that the graph of type stream references is acyclic. While 2033the source-level type graph may contain cycles through pointer types (consider a 2034linked list struct), these cycles are removed from the type stream by always 2035referring to the forward declaration record of user-defined record types. Only 2036"symbol" records in the ``.debug$S`` streams may refer to complete, 2037non-forward-declaration type records. 2038 2039Working with CodeView 2040--------------------- 2041 2042These are instructions for some common tasks for developers working to improve 2043LLVM's CodeView support. Most of them revolve around using the CodeView dumper 2044embedded in ``llvm-readobj``. 2045 2046* Testing MSVC's output:: 2047 2048 $ cl -c -Z7 foo.cpp # Use /Z7 to keep types in the object file 2049 $ llvm-readobj --codeview foo.obj 2050 2051* Getting LLVM IR debug info out of Clang:: 2052 2053 $ clang -g -gcodeview --target=x86_64-windows-msvc foo.cpp -S -emit-llvm 2054 2055 Use this to generate LLVM IR for LLVM test cases. 2056 2057* Generate and dump CodeView from LLVM IR metadata:: 2058 2059 $ llc foo.ll -filetype=obj -o foo.obj 2060 $ llvm-readobj --codeview foo.obj > foo.txt 2061 2062 Use this pattern in lit test cases and FileCheck the output of llvm-readobj 2063 2064Improving LLVM's CodeView support is a process of finding interesting type 2065records, constructing a C++ test case that makes MSVC emit those records, 2066dumping the records, understanding them, and then generating equivalent records 2067in LLVM's backend. 2068