Re: Help with non-reproducible .debug_loclists

On 2026-07-15 08:11, Richard Biener wrote:
> Is this happening in a build without -flto?  If so then I'd suggest to
> isolate the TU having the issue.  You can also try to see whether
> -fcompare-debug catches any issue (though in principle that's
> somewhat an unrelated class of problems).
> 
> You can see whether disabling address-space randomization helps
> (it would still be a GCC bug).
> 
> Once you have a reproducer (preprocessed source) please file
> a bugreport.

Hello Richard,

I've managed to get a reproducer, it's not minimal though :)
I didn't manage to create an account on bugzilla (got no response
to the email asking for an account), so I'll post reproduction
steps here. Sorry in advance for a lot of text.

I've tested this on:
* 16-core x86_64 machine with 32G of RAM
   kernel: Linux 6.12.95+deb13-amd64 #1 SMP PREEMPT_DYNAMIC Debian 
6.12.95-1 (2026-07-04) x86_64 GNU/Linux
* 8-core x86_64 machine with 16G of RAM
   kernel: Linux 7.1.5 #1-NixOS SMP PREEMPT_DYNAMIC Fri Jul 24 14:21:27 
UTC 2026 x86_64 GNU/Linux

I've run the following commands:
podman run -it --rm --network host --workdir /src -v .:/src 
debian:sid-20260803
apt update && apt install g++ git #installs gcc (Debian 16.1.0-3) 16.1.0
git clone https://github.com/KhronosGroup/glslang.git
cd glslang && git checkout vulkan-sdk-1.4.350.1

cat <<EOF >/src/glslang/glslang/build_info.h
#ifndef GLSLANG_BUILD_INFO
#define GLSLANG_BUILD_INFO

#define GLSLANG_VERSION_MAJOR 16
#define GLSLANG_VERSION_MINOR 3
#define GLSLANG_VERSION_PATCH 0
#define GLSLANG_VERSION_FLAVOR ""

#define GLSLANG_VERSION_GREATER_THAN(major, minor, patch) \
     ((GLSLANG_VERSION_MAJOR) > (major) || ((major) == 
GLSLANG_VERSION_MAJOR && \
     ((GLSLANG_VERSION_MINOR) > (minor) || ((minor) == 
GLSLANG_VERSION_MINOR && \
      (GLSLANG_VERSION_PATCH) > (patch)))))

#define GLSLANG_VERSION_GREATER_OR_EQUAL_TO(major, minor, patch) \
     ((GLSLANG_VERSION_MAJOR) > (major) || ((major) == 
GLSLANG_VERSION_MAJOR && \
     ((GLSLANG_VERSION_MINOR) > (minor) || ((minor) == 
GLSLANG_VERSION_MINOR && \
      (GLSLANG_VERSION_PATCH >= (patch))))))

#define GLSLANG_VERSION_LESS_THAN(major, minor, patch) \
     ((GLSLANG_VERSION_MAJOR) < (major) || ((major) == 
GLSLANG_VERSION_MAJOR && \
     ((GLSLANG_VERSION_MINOR) < (minor) || ((minor) == 
GLSLANG_VERSION_MINOR && \
      (GLSLANG_VERSION_PATCH) < (patch)))))

#define GLSLANG_VERSION_LESS_OR_EQUAL_TO(major, minor, patch) \
     ((GLSLANG_VERSION_MAJOR) < (major) || ((major) == 
GLSLANG_VERSION_MAJOR && \
     ((GLSLANG_VERSION_MINOR) < (minor) || ((minor) == 
GLSLANG_VERSION_MINOR && \
      (GLSLANG_VERSION_PATCH <= (patch))))))

#endif // GLSLANG_BUILD_INFO
EOF
mkdir -p /src/glslang/_builddir/glslang/CMakeFiles/glslang.dir
cat <<EOF 
 >/src/glslang/_builddir/glslang/CMakeFiles/glslang.dir/cmake_pch.hxx
/* generated by CMake */

#pragma GCC system_header
#ifdef __cplusplus
#include "/src/glslang/glslang/MachineIndependent/pch.h"
#endif // __cplusplus
EOF
cat <<EOF 
 >/src/glslang/_builddir/glslang/CMakeFiles/glslang.dir/cmake_pch.hxx.cxx
/* generated by CMake */
EOF
cd _builddir
cat <<EOF >compile.sh
#!/usr/bin/env bash
set -eux
c++ -DENABLE_HLSL -DENABLE_OPT=0 -DENABLE_SPIRV 
-DGLSLANG_IS_SHARED_LIBRARY=1 -DGLSLANG_OSINCLUDE_UNIX 
-DGLSLANG_TEST_BUILD -Dglslang_EXPORTS -I/src/glslang/glslang/.. 
-I/src/glslang/_builddir/include -O2 -pipe -g -Wp,-D_FORTIFY_SOURCE=3 
-Wp,-D_GLIBCXX_ASSERTIONS -fexceptions -fstack-protector-strong 
-grecord-gcc-switches -fasynchronous-unwind-tables 
-fstack-clash-protection -fcf-protection -fno-omit-frame-pointer 
-mno-omit-leaf-frame-pointer -fdump-rtl-vartrack-details -DNDEBUG 
-std=c++17 -fPIC -fvisibility=hidden -Wall -Wmaybe-uninitialized 
-Wuninitialized -Wunused -Wunused-local-typedefs -Wimplicit-fallthrough 
-Wunused-parameter -Wunused-value -Wunused-variable 
-Wunused-but-set-parameter -Wunused-but-set-variable -fno-rtti 
-fno-exceptions -Werror=deprecated-copy -Winvalid-pch -x c++-header 
-include 
/src/glslang/_builddir/glslang/CMakeFiles/glslang.dir/cmake_pch.hxx -o 
glslang/CMakeFiles/glslang.dir/cmake_pch.hxx.gch -c 
/src/glslang/_builddir/glslang/CMakeFiles/glslang.dir/cmake_pch.hxx.cxx
c++ -DENABLE_HLSL -DENABLE_OPT=0 -DENABLE_SPIRV 
-DGLSLANG_IS_SHARED_LIBRARY=1 -DGLSLANG_OSINCLUDE_UNIX 
-DGLSLANG_TEST_BUILD -Dglslang_EXPORTS -I/src/glslang -O2 -pipe -g 
-Wp,-D_FORTIFY_SOURCE=3 -Wp,-D_GLIBCXX_ASSERTIONS -fexceptions 
-fstack-protector-strong -grecord-gcc-switches 
-fasynchronous-unwind-tables -fstack-clash-protection -fcf-protection 
-fno-omit-frame-pointer -mno-omit-leaf-frame-pointer 
-fdump-rtl-vartrack-details -DNDEBUG -std=c++17 -fPIC 
-fvisibility=hidden -Wall -Wmaybe-uninitialized -Wuninitialized -Wunused 
-Wunused-local-typedefs -Wimplicit-fallthrough -Wunused-parameter 
-Wunused-value -Wunused-variable -Wunused-but-set-parameter 
-Wunused-but-set-variable -fno-rtti -fno-exceptions 
-Werror=deprecated-copy -Winvalid-pch -include 
/src/glslang/_builddir/glslang/CMakeFiles/glslang.dir/cmake_pch.hxx -o 
/src/glslang/glslang/MachineIndependent/ShaderLang.cpp.o -c 
/src/glslang/glslang/MachineIndependent/ShaderLang.cpp

EOF
chmod +x compile.sh
cat <<EOF >clean.sh
#!/usr/bin/env bash
set -eux
rm -f glslang/CMakeFiles/glslang.dir/cmake_pch.hxx.gch
rm -f /src/glslang/glslang/MachineIndependent/ShaderLang.cpp.*
EOF
chmod +x clean.sh

./clean.sh
./compile.sh
mkdir build_reference
sha256sum /src/glslang/glslang/MachineIndependent/ShaderLang.cpp.o > 
build_reference/sha256.txt
mv glslang/CMakeFiles/glslang.dir/cmake_pch.hxx.gch 
/src/glslang/glslang/MachineIndependent/ShaderLang.cpp.* 
build_reference/

for x in $(seq 1 1000); do ./clean.sh; ./compile.sh; sha256sum 
/src/glslang/glslang/MachineIndependent/ShaderLang.cpp.o > 
build_result_$x.txt; if [ "$(cat build_result_$x.txt)" != "$(cat 
build_reference/sha256.txt)" ]; then mkdir checkout_$x; mv 
glslang/CMakeFiles/glslang.dir/cmake_pch.hxx.gch 
/src/glslang/glslang/MachineIndependent/ShaderLang.cpp.* checkout_$x/; 
fi; done

After about 15-20 minutes I reliably get a checkout_* directory with a 
different
build result.

My guess is that it's related to the interaction between pre-compiled 
headers and
variable tracking pass. I've worked around that by disabling 
pre-compiled headers
in glslang build for now, but the bug is still there in gcc I think.

Thanks,
Nazar Kazakov