[Bug tree-optimization/115395] New: [15 regression] libarchive miscompiled with -O2 -march=znver2 -fno-vect-cost-model

["sjames at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115395

            Bug ID: 115395
           Summary: [15 regression] libarchive miscompiled with -O2
                    -march=znver2 -fno-vect-cost-model
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: sjames at gcc dot gnu.org
  Target Milestone: ---

Created attachment 58382
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=58382&action=edit
bad.c

libarchive fails several tests with -O3 -march=znver2 -fno-vect-cost-model. I
picked 'libarchive_test_read_format_rar_multivolume_seek_data' to reduce.

```
$ gcc-15 test.c -o /tmp/test -O2 -march=znver2 && /tmp/test ; echo $?
0

$ gcc-15 test.c -o /tmp/test -O2 -fno-vect-cost-model -march=znver2 &&
/tmp/test && echo $?
aborting on wrong offset=214
Aborted (core dumped)
134
```

--

Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/libexec/gcc/x86_64-pc-linux-gnu/15/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with:
/var/tmp/portage/sys-devel/gcc-15.0.9999/work/gcc-15.0.9999/configure
--host=x86_64-pc-linux-gnu --build=x86_64-pc-linux-gnu --prefix=/usr
--bindir=/usr/x86_64-pc-linux-gnu/gcc-bin/15
--includedir=/usr/lib/gcc/x86_64-pc-linux-gnu/15/include
--datadir=/usr/share/gcc-data/x86_64-pc-linux-gnu/15
--mandir=/usr/share/gcc-data/x86_64-pc-linux-gnu/15/man
--infodir=/usr/share/gcc-data/x86_64-pc-linux-gnu/15/info
--with-gxx-include-dir=/usr/lib/gcc/x86_64-pc-linux-gnu/15/include/g++-v15
--disable-silent-rules --disable-dependency-tracking
--with-python-dir=/share/gcc-data/x86_64-pc-linux-gnu/15/python
--enable-languages=c,c++,fortran,rust --enable-obsolete --enable-secureplt
--disable-werror --with-system-zlib --enable-nls --without-included-gettext
--disable-libunwind-exceptions --enable-checking=yes,extra,rtl
--with-bugurl=https://bugs.gentoo.org/ --with-pkgversion='Gentoo Hardened
15.0.9999 p, commit 9a866462097fe24696c924a3874fd307c775e860'
--with-gcc-major-version-only --enable-libstdcxx-time --enable-lto
--disable-libstdcxx-pch --enable-shared --enable-threads=posix
--enable-__cxa_atexit --enable-clocale=gnu --enable-multilib
--with-multilib-list=m32,m64 --disable-fixed-point --enable-targets=all
--enable-libgomp --disable-libssp --disable-libada --disable-cet
--disable-systemtap --enable-valgrind-annotations --disable-vtable-verify
--disable-libvtv --with-zstd --with-isl --disable-isl-version-check
--enable-default-pie --enable-host-pie --enable-host-bind-now
--enable-default-ssp --disable-fixincludes --with-build-config='bootstrap-O3
bootstrap-lto'
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 15.0.0 20240607 (experimental)
a3d68b5155018817dd7eef5abbaeadf3959b8e5e (Gentoo Hardened 15.0.9999 p, commit
9a866462097fe24696c924a3874fd307c775e860)