https://gcc.gnu.org/bugzilla/show_bug.cgi?id=118662
Bug ID: 118662
Summary: -ftree-slp-vectorize causes incorrect math
Product: gcc
Version: 14.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: joakim.rosqvist at gmail dot com
Target Milestone: ---
Hi, here is the preprocessed .i file for a simple testprogram using no
includefiles that adds up 4 numbers and maybe one more:
# 0 "test.c"
# 0 "<built-in>"
# 0 "<command-line>"
# 1 "/usr/include/stdc-predef.h" 1 3 4
# 0 "<command-line>" 2
# 1 "test.c"
int addup(char *num)
{
int val = num[0]+num[1]+num[2]+num[3];
if (num[3]>=0) val++;
return val;
}
int main(int , char *[])
{
char num[4] = {1,1,1,-1};
return addup(num);
}
When compiled like this it returns the expected value 2:
>gcc -O1 -Wall -Wextra -march=native -o test test.c
>./test; echo $?
2
But with extra optimization, the result is different:
>gcc -O1 -Wall -Wextra -march=native -ftree-slp-vectorize -o test test.c
>./test; echo $?
3
This was run on an AMD Ryzen 3700X in Ubuntu 24.10
>gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/libexec/gcc/x86_64-linux-gnu/14/lto-wrapper
OFFLOAD_TARGET_NAMES=nvptx-none:amdgcn-amdhsa
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu 14.2.0-4ubuntu2'
--with-bugurl=file:///usr/share/doc/gcc-14/README.Bugs
--enable-languages=c,ada,c++,go,d,fortran,objc,obj-c++,m2,rust --prefix=/usr
--with-gcc-major-version-only --program-suffix=-14
--program-prefix=x86_64-linux-gnu- --enable-shared --enable-linker-build-id
--libexecdir=/usr/libexec --without-included-gettext --enable-threads=posix
--libdir=/usr/lib --enable-nls --enable-bootstrap --enable-clocale=gnu
--enable-libstdcxx-debug --enable-libstdcxx-time=yes
--with-default-libstdcxx-abi=new --enable-libstdcxx-backtrace
--enable-gnu-unique-object --disable-vtable-verify --enable-plugin
--enable-default-pie --with-system-zlib --enable-libphobos-checking=release
--with-target-system-zlib=auto --enable-objc-gc=auto --enable-multiarch
--disable-werror --enable-cet --with-arch-32=i686 --with-abi=m64
--with-multilib-list=m32,m64,mx32 --enable-multilib --with-tune=generic
--enable-offload-targets=nvptx-none=/build/gcc-14-zdkDXv/gcc-14-14.2.0/debian/tmp-nvptx/usr,amdgcn-amdhsa=/build/gcc-14-zdkDXv/gcc-14-14.2.0/debian/tmp-gcn/usr
--enable-offload-defaulted --without-cuda-driver --enable-checking=release
--build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu
--with-build-config=bootstrap-lto-lean --enable-link-serialization=2
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 14.2.0 (Ubuntu 14.2.0-4ubuntu2)
