[llvm-dev] RFC: Supporting macros in LLVM debug info
Adrian Prantl via llvm-dev
llvm-dev at lists.llvm.org
Wed Oct 28 10:36:53 PDT 2015
> On Oct 28, 2015, at 7:56 AM, Aboud, Amjad via llvm-dev <llvm-dev at lists.llvm.org> wrote:
>
> Hi,
> I would like to implement macro debug info support in LLVM.
> Below you will find 4 parts:
> 1. Background on what does it mean to debug macros.
> 2. A brief explanation on how to represent macro debug info in DWARF 4.0.
Thanks for looking into that!
Without having read the remainder of proposal yet — are you aware that the DWARF4 macro support is being replaced with a more efficient representation in DWARF5?
http://dwarfstd.org/ShowIssue.php?issue=110722.1
It may make sense to either only implement the new version or design it in a way to support both formats.
-- adrian
> 3. The suggested design.
> 4. A full example: Source -> AST -> LLVM IR -> DWARF.
>
> Feel free to skip first two parts if you think you know the background.
> Please, let me know if you have any comment or feedback on this approach.
>
> Thanks,
> Amjad
>
> [Background]
> There are two kind of macro definition:
> 1. Simple macro definition, e.g. #define M1 Value1
> 2. Function macro definition, e.g. #define M2(x, y) (x) + (y)
> Macro scope starts with the "#define" directive and ends with "#undef" directive.
>
> GDB supports debugging macros. This means, it can evaluate the macro expression for all macros, which have a scope that interleaves with the current breakpoint.
> For example:
> GDB command: print M2(3, 5)
> GDB Result: 8
>
> GDB can evaluate the macro expression based on the ".debug_macroinfo" section (DWARF 4.0).
>
> [DWARF 4.0 ".debug_macroinfo" section]
> In this section there are 4 kinds of entries
> 1. DW_MACROINFO_define
> 2. DW_MACROINFO_undef
> 3. DW_MACROINFO_start_file
> 4. DW_MACROINFO_end_file
>
> Note: There is a 5th kind of entry for vendor specific macro information, that we do not need to support.
>
> The first two entries contain information about the line number where the macro is defined/undefined, and a null terminated string, which contain the macro name (followed by the replacement value in case of a definition, or a list of parameters then the replacement value in case of function macro definition).
> The third entry contains information about the line where the file was included followed by the file id (an offset into the files table in the debug line section).
> The fourth entry contains nothing, and it just close the previous entry of third kind (start_file) .
>
> Macro definition and file including entries must appear at the same order as they appear in the source file. Where all macro entries between "start_file" and "end_file" entries represent macros appears directly/indirectly in the included file.
>
> Special cases:
> 1. The main source file should be the first "start_file" entry in the sequence, and should have line number "0".
> 2. Command line/Compiler definitions must also have line number "0" but must appear before the first "start_file" entry.
> 3. Command line include files, must also have line number "0" but will appear straight after the "start_file" of the main source.
>
> [Design]
> To support macros the following components need to be modified: Clang, LLVM IR, Dwarf Debug emitter.
>
> In clang, we need to handle these source directives:
> 1. #define
> 2. #undef
> 3. #include
> The idea is to make a use of "PPCallbacks" class, which allows preprocessor to notify the parser each time one of the above directives occurs.
> These are the callbacks that should be implemented:
> "MacroDefined", "MacroUndefined", "FileChanged", and "InclusionDirective".
>
> AST will be extended to support two new DECL types: "MacroDecl" and "FileIncludeDecl".
> Where "FileIncludeDecl" AST might contain other "FileIncludeDecl"/"MacroDecl" ASTs.
> These two new AST DECLs are not part of TranslationUnitDecl and are handled separately (see AST example below).
>
> In the LLVM IR, metadata debug info will be extended to support new DIs as well:
> "DIMacro", "DIFileInclude", and "MacroNode".
> The last, is needed as we cannot use DINode as a base class of "DIMacro" and DIFileInclude" nodes.
>
> DIMacro will contain:
> · type (definition/undefinition).
> · line number (interger).
> · name (null terminated string).
> · replacement value (null terminated string - optional).
>
> DIFileMacro will contain:
> · line number (interger).
> · file (DIFile).
> · macro list (MacroNodeArray) - optional.
>
> In addition, the DICompileUnit will contain a new optional field of macro list of type (MacroNodeArray).
>
> Finally, I assume that macro support should be disabled by default, and there should be a flag to enable this feature. I would say that we should introduce a new specific flag, e.g. "-gmacro", that could be used with "-g".
>
> [Example]
> Here is an example that demonstrate the macro support from Source->AST->LLVM IR->DWARF.
>
> Source
> =========================================================
> mainfile.c:
> --------------------------------------------------------------------------------------
> 1. #define M1 Value1
> 2. #include "myfile.h"
> 3. #define M2( x , y) ( (x) + (y) * Value2)
> --------------------------------------------------------------------------------------
>
> myfile.h:
> --------------------------------------------------------------------------------------
> 1.
> 2.
> 3.
> 4. #undef M1
> 5. #define M1 NewValue1
> --------------------------------------------------------------------------------------
>
> myfile2.h:
> --------------------------------------------------------------------------------------
> 1. #define M4 Value4
> --------------------------------------------------------------------------------------
> =========================================================
>
> Command line:
> clang -c -g -gmacro -O0 -DM3=Value3 -include myfile2.h mainfile.c
>
>
> AST
> =========================================================
> MacroDecl 0xd6c5c0 <<invalid sloc>> <invalid sloc> __llvm__ defined
> MacroDecl 0xd6c618 <<invalid sloc>> <invalid sloc> __clang__ defined
>
> … <More compiler macros> …
>
> MacroDecl 0x11c01b0 <<invalid sloc>> <invalid sloc> M3 defined
> FileIncludeDecl 0x11c0208 <mainfile.c:1:1> col:1
> |-FileIncludeDecl 0x11c0238 <myfile2.h:1:1> col:1
> | `-MacroDecl 0x11c0268 <<invalid sloc>> <invalid sloc> M4 defined
> |-MacroDecl 0x11c02c0 <mainfile.c:1:9> col:9 M1 defined
> |-FileIncludeDecl 0x11c0318 <myfile.h:1:1> col:1
> | |-MacroDecl 0x11c0348 <line:4:8> col:8 M1 undefined
> | `-MacroDecl 0x11c03a0 <line:5:9> col:9 M1 defined
> `-MacroDecl 0x11c03f8 <mainfile.c:3:9> col:9 M2 defined
> TranslationUnitDecl 0xd6c078 <<invalid sloc>> <invalid sloc>
> |-TypedefDecl 0xd6c330 <<invalid sloc>> <invalid sloc> implicit __int128_t '__int128'
> |-TypedefDecl 0xd6c370 <<invalid sloc>> <invalid sloc> implicit __uint128_t 'unsigned __int128'
> |-TypedefDecl 0xd6c3c8 <<invalid sloc>> <invalid sloc> implicit __builtin_ms_va_list 'char *'
> `-TypedefDecl 0xd6c590 <<invalid sloc>> <invalid sloc> implicit __builtin_va_list 'struct __va_list_tag [1]'
> =========================================================
>
>
> LLVM IR
> =========================================================
> target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
> target triple = "x86_64-pc-linux"
>
> !llvm.dbg.cu = !{!0}
> !llvm.module.flags = !{!327}
> !llvm.ident = !{!328}
>
> !0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "clang version 3.8.0 (trunk 251321)", isOptimized: false, runtimeVersion: 0, emissionKind: 1, enums: !2, macros: !3)
> !1 = !DIFile(filename: "mainfile.c", directory: "/")
> !2 = !{}
> !3 = !{!4, !5, … <More compiler macros> … , !312, !313}
> !4 = !DIMacro(macro type: DW_MACINFO_define, name: "__llvm__", value: !"1")
> !5 = !DIMacro(macro type: DW_MACINFO_define, name: "__clang__", value: !"1")
>
> … <More compiler macros> …
>
> !312 = !DIMacro(macro type: DW_MACINFO_define, name: "M3", value: !"Value3")
> !313 = !DIFileInclude(file: !314, nodes: !315)
> !314 = !DIFile(filename: "mainfile.c", directory: "/")
> !315 = !{!316, !320, !321, !326}
> !316 = !DIFileInclude(file: !317, nodes: !318)
> !317 = !DIFile(filename: "myfile2.h", directory: "/")
> !318 = !{!319}
> !319 = !DIMacro(macro type: DW_MACINFO_define, name: "M4", value: !"Value4")
> !320 = !DIMacro(macro type: DW_MACINFO_define, name: "M1", line: 1, value: !"Value1")
> !321 = !DIFileInclude(line: 2, file: !322, nodes: !323)
> !322 = !DIFile(filename: "myfile.h", directory: "/")
> !323 = !{!324, !325}
> !324 = !DIMacro(macro type: DW_MACINFO_undef, name: "M1", line: 4)
> !325 = !DIMacro(macro type: DW_MACINFO_define, name: "M1", line: 5, value: !"NewValue1")
> !326 = !DIMacro(macro type: DW_MACINFO_define, name: "M2(x,y)", line: 3, value: !"( (x) + (y) * Value2)")
> !327 = !{i32 2, !"Debug Info Version", i32 3}
> !328 = !{!"clang version 3.8.0 (trunk 251321)"}
> =========================================================
>
>
> DWARF
> =========================================================
> Command line: llvm-dwarfdump.exe -debug-dump=macro mainfile.o
> --------------------------------------------------------------------------------------
> mainfile3.o: file format ELF64-x86-64
>
> .debug_macinfo contents:
> DW_MACINFO_define - lineno: 0 macro: __llvm__ 1
> DW_MACINFO_define - lineno: 0 macro: __clang__ 1
>
> … <More compiler macros> …
>
> DW_MACINFO_define - lineno: 0 macro: M3 Value3
> DW_MACINFO_start_file - lineno: 0 filenum: 1
> DW_MACINFO_start_file - lineno: 0 filenum: 2
> DW_MACINFO_define - lineno: 0 macro: M4 Value4
> DW_MACINFO_end_file
> DW_MACINFO_define - lineno: 1 macro: M1 Value1
> DW_MACINFO_start_file - lineno: 2 filenum: 3
> DW_MACINFO_undef - lineno: 4 macro: M1
> DW_MACINFO_define - lineno: 5 macro: M1 NewValue1
> DW_MACINFO_end_file
> DW_MACINFO_define - lineno: 3 macro: M2(x,y) ( (x) + (y) * Value2)
> DW_MACINFO_end_file
>
> --------------------------------------------------------------------------------------
> Command line: llvm-dwarfdump.exe -debug-dump=line mainfile.o
> --------------------------------------------------------------------------------------
> .debug_line contents:
>
> … <Other line table Info> …
>
> Dir Mod Time File Len File Name
> ---- ---------- ---------- ---------------------------
> file_names[ 1] 1 0x00000000 0x00000000 mainfile.c
> file_names[ 2] 1 0x00000000 0x00000000 myfile2.h
> file_names[ 3] 1 0x00000000 0x00000000 myfile.h
> =========================================================
>
> ---------------------------------------------------------------------
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