https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120162
Bug ID: 120162
Summary: Prefetch causes vectorization to fail
Product: gcc
Version: 15.1.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: viktor.horsmanheimo at gmail dot com
Target Milestone: ---
Hi,
It seems that prefetching causes vectorization to fail in some cases, I spoke
on the dev IRC where using the builtin method was suggested, this also fails to
vectorize the code properly.
This behavior seems to be consistent with any GCC version I've tried (including
older ones, 14.1 that I have on my computer)
Incidentally this behavior can be seen on other major compiler vendors (Clang,
MSVC) as well (if this is relevant).
Command line: gcc -O3 -march=skylake
Compiler output: N/a
Preprocessed file: attached
I've created a small PoC where this can be seen here:
C++: https://godbolt.org/z/961hGaGvG.
C: https://godbolt.org/z/Th5n3MceW
Compiler information from godbolt:
COLLECT_GCC=/opt/compiler-explorer/gcc-15.1.0/bin/gcc
Target: x86_64-linux-gnu
Configured with: ../gcc-15.1.0/configure
--prefix=/opt/compiler-explorer/gcc-build/staging
--enable-libstdcxx-backtrace=yes --build=x86_64-linux-gnu
--host=x86_64-linux-gnu --target=x86_64-linux-gnu --disable-bootstrap
--enable-multiarch --with-abi=m64 --with-multilib-list=m32,m64,mx32
--enable-multilib --enable-clocale=gnu
--enable-languages=c,c++,fortran,ada,objc,obj-c++,go,d,m2,rust,cobol
--enable-ld=yes --enable-gold=yes --enable-libstdcxx-debug
--enable-libstdcxx-time=yes --enable-linker-build-id --enable-lto
--enable-plugins --enable-threads=posix
--with-pkgversion=Compiler-Explorer-Build-gcc--binutils-2.42
Thread model: posix
Supported LTO compression algorithms: zlib
gcc version 15.1.0 (Compiler-Explorer-Build-gcc--binutils-2.42)
COLLECT_GCC_OPTIONS='-fdiagnostics-color=always' '-g' '-o' '/app/output.s'
'-masm=intel' '-fno-verbose-asm' '-S' '-O3' '-march=skylake' '-v' '-dumpdir'
'/app/'
/opt/compiler-explorer/gcc-15.1.0/bin/../libexec/gcc/x86_64-linux-gnu/15.1.0/cc1
-quiet -v -imultiarch x86_64-linux-gnu -iprefix
/opt/compiler-explorer/gcc-15.1.0/bin/../lib/gcc/x86_64-linux-gnu/15.1.0/
<source> -quiet -dumpdir /app/ -dumpbase output.c -dumpbase-ext .c -masm=intel
-march=skylake -g -O3 -version -fdiagnostics-color=always -fno-verbose-asm -o
/app/output.s
GNU C23 (Compiler-Explorer-Build-gcc--binutils-2.42) version 15.1.0
(x86_64-linux-gnu)
compiled by GNU C version 11.4.0, GMP version 6.2.1, MPFR version
4.1.0, MPC version 1.2.1, isl version isl-0.24-GMP
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
ignoring nonexistent directory
"/opt/compiler-explorer/gcc-15.1.0/bin/../lib/gcc/x86_64-linux-gnu/15.1.0/../../../../x86_64-linux-gnu/include"
ignoring duplicate directory
"/opt/compiler-explorer/gcc-15.1.0/bin/../lib/gcc/../../lib/gcc/x86_64-linux-gnu/15.1.0/include"
ignoring nonexistent directory "/usr/local/include/x86_64-linux-gnu"
ignoring duplicate directory
"/opt/compiler-explorer/gcc-15.1.0/bin/../lib/gcc/../../lib/gcc/x86_64-linux-gnu/15.1.0/include-fixed/x86_64-linux-gnu"
ignoring duplicate directory
"/opt/compiler-explorer/gcc-15.1.0/bin/../lib/gcc/../../lib/gcc/x86_64-linux-gnu/15.1.0/include-fixed"
ignoring nonexistent directory
"/opt/compiler-explorer/gcc-15.1.0/bin/../lib/gcc/../../lib/gcc/x86_64-linux-gnu/15.1.0/../../../../x86_64-linux-gnu/include"
#include "..." search starts here:
#include <...> search starts here:
/opt/compiler-explorer/gcc-15.1.0/bin/../lib/gcc/x86_64-linux-gnu/15.1.0/include
/opt/compiler-explorer/gcc-15.1.0/bin/../lib/gcc/x86_64-linux-gnu/15.1.0/include-fixed/x86_64-linux-gnu
/opt/compiler-explorer/gcc-15.1.0/bin/../lib/gcc/x86_64-linux-gnu/15.1.0/include-fixed
/usr/local/include
/opt/compiler-explorer/gcc-15.1.0/bin/../lib/gcc/../../include
/usr/include/x86_64-linux-gnu
/usr/include
End of search list.
Compiler executable checksum: 8cc01216ec82be553d57fcd4d8cd204d
COMPILER_PATH=/opt/compiler-explorer/gcc-15.1.0/bin/../libexec/gcc/x86_64-linux-gnu/15.1.0/:/opt/compiler-explorer/gcc-15.1.0/bin/../libexec/gcc/x86_64-linux-gnu/:/opt/compiler-explorer/gcc-15.1.0/bin/../libexec/gcc/:/opt/compiler-explorer/gcc-15.1.0/bin/../lib/gcc/x86_64-linux-gnu/15.1.0/../../../../x86_64-linux-gnu/bin/
LIBRARY_PATH=/opt/compiler-explorer/gcc-15.1.0/bin/../lib/gcc/x86_64-linux-gnu/15.1.0/:/opt/compiler-explorer/gcc-15.1.0/bin/../lib/gcc/x86_64-linux-gnu/:/opt/compiler-explorer/gcc-15.1.0/bin/../lib/gcc/:/opt/compiler-explorer/gcc-15.1.0/bin/../lib/gcc/x86_64-linux-gnu/15.1.0/../../../../lib64/:/lib/x86_64-linux-gnu/:/lib/../lib64/:/usr/lib/x86_64-linux-gnu/:/usr/lib/../lib64/:/opt/compiler-explorer/gcc-15.1.0/bin/../lib/gcc/x86_64-linux-gnu/15.1.0/../../../../x86_64-linux-gnu/lib/:/opt/compiler-explorer/gcc-15.1.0/bin/../lib/gcc/x86_64-linux-gnu/15.1.0/../../../:/lib/:/usr/lib/
COLLECT_GCC_OPTIONS='-fdiagnostics-color=always' '-g' '-o' '/app/output.s'
'-masm=intel' '-fno-verbose-asm' '-S' '-O3' '-march=skylake' '-v' '-dumpdir'
'/app/output.'
ASM generation compiler returned: 0
Using built-in specs.
COLLECT_GCC=/opt/compiler-explorer/gcc-15.1.0/bin/gcc
COLLECT_LTO_WRAPPER=/opt/compiler-explorer/gcc-15.1.0/bin/../libexec/gcc/x86_64-linux-gnu/15.1.0/lto-wrapper
Target: x86_64-linux-gnu
Configured with: ../gcc-15.1.0/configure
--prefix=/opt/compiler-explorer/gcc-build/staging
--enable-libstdcxx-backtrace=yes --build=x86_64-linux-gnu
--host=x86_64-linux-gnu --target=x86_64-linux-gnu --disable-bootstrap
--enable-multiarch --with-abi=m64 --with-multilib-list=m32,m64,mx32
--enable-multilib --enable-clocale=gnu
--enable-languages=c,c++,fortran,ada,objc,obj-c++,go,d,m2,rust,cobol
--enable-ld=yes --enable-gold=yes --enable-libstdcxx-debug
--enable-libstdcxx-time=yes --enable-linker-build-id --enable-lto
--enable-plugins --enable-threads=posix
--with-pkgversion=Compiler-Explorer-Build-gcc--binutils-2.42
Thread model: posix
Supported LTO compression algorithms: zlib
gcc version 15.1.0 (Compiler-Explorer-Build-gcc--binutils-2.42)
