On Thu, Sep 12, 2024 at 11:08 AM Jakub Jelinek <ja...@redhat.com> wrote:
> On Thu, Sep 12, 2024 at 10:52:33AM -0700, David Blaikie via Dwarf-discuss
> wrote:
> > Looks probably like a bug in GCC and best discussed in their forum?
> (clang
> > seems to produce the macro in the macinfo if you compile with
> > `-fdebug-macro` (& you have to put some emitted entity (like a function
> > definition) in the file before Clang will produce any debug info for the
> > file))
>
> GCC emits that the .debug_macro snippets from headers (in chunks
> uninterrupted by inclusion of other headers) in .debug_macro comdat
> sections
> with wm4.0.6147fde142b63cfbd46fc3f1300479b2 etc. comdat groups and it is
> just the linker which merges stuff back (and gets rid of redundancies).
>
Oooh, fascinating. Certainly llvm-dwarfdump has explicit support for
sections it expects to be fragmented (multiple sections with the same name,
for comdat purposes, etc) - and so llvm-dwarfdump certainly wouldn't dump
this correctly (probably ignores all but the first section named
`.debug_macro`) from an object file (of course in a linked executable, none
of this still exists so it should be fine).
>
> So say for
> /tmp/1.h
> #define MAC4
> #define MAC5 1
> /tmp/1.c
> #define MAC1 2
> #define MAC2 3
> #include "/tmp/1.h"
> #define MAC3 4
> int main () { return 0; }
> gcc -O2 -g3 -c -o /tmp/1.o /tmp/1.c
> gcc -o /tmp/1 /tmp/1.o
> readelf -WS /tmp/1.o | grep debug_macro
> [18] .debug_macro PROGBITS 0000000000000000 0001a4 00002c
> 00 0 0 1
> [19] .rela.debug_macro RELA 0000000000000000 003548 000090
> 18 I 31 18 8
> [20] .debug_macro PROGBITS 0000000000000000 0001d0 000934
> 00 G 0 0 1
> [21] .rela.debug_macro RELA 0000000000000000 0035d8 0024c0
> 18 IG 31 20 8
> [22] .debug_macro PROGBITS 0000000000000000 000b04 000010
> 00 G 0 0 1
> [23] .rela.debug_macro RELA 0000000000000000 005a98 000030
> 18 IG 31 22 8
> there are 2 extra .debug_macro sections in comdat groups, and then in the
> resulting binary one has:
> readelf -wm /tmp/1
> Contents of the .debug_macro section:
>
> Offset: 0x0
> Version: 5
> Offset size: 4
> Offset into .debug_line: 0x0
>
> DW_MACRO_import - offset : 0x2c
> DW_MACRO_start_file - lineno: 0 filenum: 1
> DW_MACRO_define_strp - lineno : 1 macro : MAC1 2
> DW_MACRO_define_strp - lineno : 2 macro : MAC2 3
> DW_MACRO_start_file - lineno: 3 filenum: 2
> DW_MACRO_import - offset : 0x960
> DW_MACRO_end_file
> DW_MACRO_define_strp - lineno : 4 macro : MAC3 4
> DW_MACRO_end_file
> ...
> Offset: 0x960
> Version: 5
> Offset size: 4
>
> DW_MACRO_define_strp - lineno : 1 macro : MAC4
> DW_MACRO_define_strp - lineno : 2 macro : MAC5 1
>
> Left out the chunk at 0x2c, that is for predefined macros, for
> showing how it works that is just a clutter.
>
> Jakub
>
>
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