[Bug macros/29725] Dwarf 5 C macros not handled

["simark at simark dot ca" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=29725

--- Comment #4 from Simon Marchi <simark at simark dot ca> ---
I don't think GDB has enough debug info to figure this out.

With a simple case, it works as expected:

$ cat test.c
#define FOO 2
int main() { return FOO; }
$ gcc test.c -g3 -O0
$ ./gdb --data-directory=data-directory -nx -q -iex "set debuginfod enabled
off" ./a.out -ex start -ex "info macro FOO" -batch
...
Defined at /home/simark/build/binutils-gdb/gdb/test.c:1
#define FOO 2

But then if you add a line control directive, like you have for a .c file
generated from a .y file:
$ cat test.c
#define FOO 2
#line 17 "potato.c"
int main() { return FOO; }
$ gcc test.c -g3 -O0
$ ./gdb --data-directory=data-directory -nx -q -iex "set debuginfod enabled
off" ./a.out -ex start -ex "info macro FOO" -batch
...
The symbol `FOO' has no definition as a C/C++ preprocessor macro
at /home/simark/build/binutils-gdb/gdb/test.c:-1

Looking at the macro debug info (my annotations after the #s):

$ readelf --debug-dump=macro a.out

 DW_MACRO_import - offset : 0x20 # built-in macros
 DW_MACRO_start_file - lineno: 0 filenum: 2 # test.c
 DW_MACRO_start_file - lineno: 0 filenum: 3 # stdc-predef.h
 DW_MACRO_import - offset : 0x900 # STDC macros
 DW_MACRO_end_file # end of stdc-predef.h
 DW_MACRO_define_strp - lineno : 1 macro : FOO 2
    <--- here
 DW_MACRO_end_file # end of test.c

The line table tells GDB that the current PC is in potato.c, line 17:

CU: ./test.c:
File name                            Line number    Starting address    View   
Stmt
potato.c                                      17              0x1119           
   x
potato.c                                      17              0x111d           
   x
potato.c                                      17              0x1122           
   x
potato.c                                       -              0x1124

But GDB has no way to figure out where this is in the macro inclusion tree, as
potato.c is never represented in there.  Ideally, GDB would figure out that we
are where I maked `here`, above.

Looking at the DWARF opcodes for macro, I don't really see a way to describe
the #line stuff.  There's only DW_MACRO_start_file, which represents the
inclusion of a complete file.

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