Re: GDB and debug fission

[Alexander Yermolovich via Gdb <gdb@sourceware.org>]

Thanks for clarification.
Yep I saw LLD builds gdb_index using gnu-pubnames. It just wasn't clear to me if gdb needs it with split-dwarf. I saw a comment on one of llvm reviews from couple years ago about it, but things might have changed since then. So thought I would ask. 🙂

Without gdb_index and gnu_pubnames what will be the behavior of gdb?
I tried a toy example locally with split-dwarf without gdb_index and pubnames and it seemed to work with binary compiled with -O2 and -g2.
By work I mean I was able to step through code and print same variables as when I compiled it with monolithic debug information.

Thank You
Alex

________________________________
From: Sterling Augustine <saugustine@google.com>
Sent: Thursday, January 7, 2021 5:48 PM
To: Alexander Yermolovich <ayermolo@fb.com>
Cc: gdb@sourceware.org <gdb@sourceware.org>
Subject: Re: GDB and debug fission

On Thu, Jan 7, 2021 at 5:25 PM Alexander Yermolovich via Gdb
<gdb@sourceware.org> wrote:
>
> Hello.
>
> For latest gdb to work with -gsplit-dwarf debug information generated by clang, either in split or single mode does it need gdb_index or pubnames?
> In normal case where debug information is part of executable gdb_index is nice to speed up startup time, and I think I read pubnames is not used, but with debug fission are either gdb_index or pubnames necessary?

For gdb to work with split-dwarf debug info, it needs a gdb_index.
That can be generated in several ways. Gdb can build one itself--there
is a script somewhere to add a gdb index.

But it is somewhat easier to have the linker you use generate
gdb_index for you (both gnu-ld and llvm's lld can do this). They do
this by reading the .gnu_pubnames section, so in some way, you need
both pubnames *and* an index.

So you would ordinarily use the following commands:

$(CC) $(normal_arguments) -ggnu-pubnames
$(LD)  $(normal_arguments) -Wl,--gdb-index