[llvm-dev] RFC: Supporting macros in LLVM debug info

Aboud, Amjad via llvm-dev llvm-dev at lists.llvm.org
Wed Oct 28 07:56:46 PDT 2015

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.
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.


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.

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".

Here is an example that demonstrate the macro support from Source->AST->LLVM IR->DWARF.

1. #define M1 Value1
2. #include "myfile.h"
3. #define M2( x , y)   ( (x)    + (y)  * Value2)

4. #undef M1
5. #define M1 NewValue1

1. #define M4 Value4

Command line:
clang -c -g -gmacro -O0 -DM3=Value3 -include myfile2.h mainfile.c

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]'

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)"}

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_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_define - lineno: 3 macro: M2(x,y) ( (x) + (y) * Value2)

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

Intel Israel (74) Limited

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