[Bug c/91934] New: Performance regression on 8.3.0 with -O3 and avx

[tochansky at tochlab dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91934

            Bug ID: 91934
           Summary: Performance regression on 8.3.0 with -O3 and avx
           Product: gcc
           Version: 8.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: tochansky at tochlab dot net
  Target Milestone: ---

Created attachment 46980
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=46980&action=edit
Function where problem occurs

GCC version is 8.3.0 building on Gentoo.
Configured with:
/var/tmp/portage/sys-devel/gcc-8.3.0-r1/work/gcc-8.3.0/configure
--host=x86_64-pc-linux-gnu --build=x86_64-pc-linux-gnu --prefix=/usr
--bindir=/usr/x86_64-pc-linux-gnu/gcc-bin/8.3.0
--includedir=/usr/lib/gcc/x86_64-pc-linux-gnu/8.3.0/include
--datadir=/usr/share/gcc-data/x86_64-pc-linux-gnu/8.3.0
--mandir=/usr/share/gcc-data/x86_64-pc-linux-gnu/8.3.0/man
--infodir=/usr/share/gcc-data/x86_64-pc-linux-gnu/8.3.0/info
--with-gxx-include-dir=/usr/lib/gcc/x86_64-pc-linux-gnu/8.3.0/include/g++-v8
--with-python-dir=/share/gcc-data/x86_64-pc-linux-gnu/8.3.0/python
--enable-languages=c,c++,go,fortran --enable-obsolete --enable-secureplt
--disable-werror --with-system-zlib --enable-nls --without-included-gettext
--enable-checking=release --with-bugurl=https://bugs.gentoo.org/
--with-pkgversion='Gentoo 8.3.0-r1 p1.1' --disable-esp --enable-libstdcxx-time
--enable-shared --enable-threads=posix --enable-__cxa_atexit
--enable-clocale=gnu --enable-multilib --with-multilib-list=m32,m64
--disable-altivec --disable-fixed-point --enable-targets=all --enable-libgomp
--disable-libmudflap --disable-libssp --disable-libmpx --disable-systemtap
--enable-vtable-verify --enable-lto --without-isl --enable-default-pie
--enable-default-ssp
Thread model: posix
gcc version 8.3.0 (Gentoo 8.3.0-r1 p1.1) 


Building my source with:
-Wall -Wextra -Wshadow -pedantic -march=native -mavx -mavx2 -O3 -g -save-temps 
 -std=gnu99

resulted .i file is attached. I got similar results on Debian 10 and
godbolt.org (https://godbolt.org/z/-0cGgu)

The problem is low performance when building with 8.3.0. I tested this code on
7.4.0, 8.3.0, 9.1.0 and generated assembler in 8.3.0 contains some strange SIMD
instructions before main work. Problem occurs only with -O3 flag.