https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121115
Bug ID: 121115
Summary: GCC 15+ miscompiles Julia programming language
Product: gcc
Version: 15.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: keno at juliacomputing dot com
Target Milestone: ---
As mentioned in https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115236#c7 (and
originally reported in https://github.com/JuliaLang/julia/issues/58466),
building the Julia programming language with GCC 15 results in a crashing
build. This was bisected on the Julia issue tracker to
https://gcc.gnu.org/cgit/gcc/commit/?id=c08b0d3f7b3539b26031de31d88dea6b94474577.
I have attempted to come up with a minimal reproducer, but the bug is somewhat
fragile. As a result, the reduced reproducer I have still depends on the
julia/llvm libraries, but it should be standalone enough to not require a lot
of setup. For completeness, I am also providing reproduction instructions for
the original issue below.
# Instructions for reduced reproducer
The attached reproducer.expanded.cpp is a preprecossor-expanded and manually
reduced version of Julia's aotcompile.cpp. However, running it depends on the
julia/llvm external libraries, but not particularly closely, so the easiest way
is to grab these from any recent nightly build of julia:
```
$ curl -LO
https://julialangnightlies-s3.julialang.org/bin/linux/x86_64/julia-latest-linux-x86_64.tar.gz
$ tar --strip-components=1 --wildcards -xf julia-latest-linux-x86_64.tar.gz
'*LLVM.so*' '*libjulia*.so*' '*libunwind.so*' '*libopenlibm.so*'
$ g++-15 -mprefer-vector-width=256 -std=c++17 -fno-exceptions -funwind-tables
-fno-rtti -Llib -Llib/julia -lLLVM -ljulia-internal -ljulia -Wl,-rpath lib
-Wl,-rpath lib/julia -O3 reproducer.expanded.cpp
[snip warnings]
$ ./a.out
Unreachable reached at 0x4022b9
[3424449] signal 4 (2): Illegal instruction
in expression starting at none:0
jl_emit_native_impl.cold at ./a.out (unknown line)
main at ./a.out (unknown line)
unknown function (ip: 0x746dbbe2a1c9) at /lib/x86_64-linux-gnu/libc.so.6
__libc_start_main at /lib/x86_64-linux-gnu/libc.so.6 (unknown line)
_start at ./a.out (unknown line)
Allocations: 1 (Pool: 1; Big: 0); GC: 0
Illegal instruction
$ g++-15 -mprefer-vector-width=256 -std=c++17 -fno-exceptions -funwind-tables
-fno-rtti -Llib -Llib/julia -lLLVM -ljulia-internal -ljulia -Wl,-rpath lib
-Wl,-rpath lib/julia -O0 reproducer.expanded.cpp
[snip warnings]
$ ./a.out
[no crash]
```
The `Illegal instruction` here is a `__builtin_trap` that marks the bug - the
original reproducer segfaults on the next line instead.
The `-mprefer-vector-width=256` is not required on current GCC, but I've
included it to facilitate bisect, since the regression commit range includes a
commit that switches the default for this flag (and it affects the presence of
the bug).
# Instructions for upstream reproducer
To test upstream,
```
$ git clone https://github.com/JuliaLang/julia.git
$ cd julia
$ cat > Make.user <<END
CC=gcc-15
CXX=g++-15
END
$ make -j40
```
The symptoms of the bug are that julia will fail to compile some of it's
stdlibs, crashing in aotcompile.cpp. After a build like the above failed, a
good quick test is `./julia -e 'using Unicode'`, which will attempt to compile
the Julia unicode standard library and will crash if the bug is present.
