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From: "sergiodj at sergiodj dot net" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug debug/109805] New: LTO affecting -fdebug-prefix-map
Date: Thu, 11 May 2023 00:48:11 +0000
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X-Bugzilla-Version: 14.0
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D109805

            Bug ID: 109805
           Summary: LTO affecting -fdebug-prefix-map
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: debug
          Assignee: unassigned at gcc dot gnu.org
          Reporter: sergiodj at sergiodj dot net
  Target Milestone: ---

Hi,

In Ubuntu we use -fdebug-prefix-map to remap a package's build directory (w=
hich
contains random stuff) into a predictable path under /usr/src.  This is don=
e in
order to help debuginfod index the source code for our packages.

Things work very well, but I found a weird corner case involving LTO.  The
affected package is vim.  You can see the build logs for it here:

https://launchpadlibrarian.net/665520301/buildlog_ubuntu-mantic-amd64.vim_2=
%3A9.0.1378-2ubuntu1_BUILDING.txt.gz

As you can notice, we're using -fdebug-prefix-map and LTO for the build.

The problem is that the resulting debuginfo doesn't have the remaped direct=
ory.
 I can replicate the issue locally, and at first I thought this was either =
bug
#108464 or #87726, but after some digging I'm convinced it's something else=
.=20
I've compiled gcc (GCC) 14.0.0 20230510 from the master branch
(608e7f3ab47fe746279c552c3574147aa3d8ee76), and I still can reproduce the
problem.

A simple reproducer for the problem follows:

$ echo 'int main(){}' > foo.c $ ~/gcc/install/bin/gcc -c foo.c -O2 -g
-flto=3Dauto -ffat-lto-objects -fdebug-prefix-map=3D`pwd`=3D/aaaaaaa -o foo=
 $
~/gcc/install/bin/gcc foo -flto=3Dauto -ffat-lto-objects -o bar

A workaround for this bug is to either stop using LTO or explicitly set
-fdebug-prefix-map when linking the object.=