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From: "jhllawrence963 at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/106902] New: Program compiled with -O3 -fmfa
 produces different result
Date: Sat, 10 Sep 2022 18:58:12 +0000
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X-Bugzilla-Type: new
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: tree-optimization
X-Bugzilla-Version: 12.2.0
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X-Bugzilla-Severity: normal
X-Bugzilla-Who: jhllawrence963 at gmail dot com
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X-Bugzilla-Priority: P3
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D106902

            Bug ID: 106902
           Summary: Program compiled with -O3 -fmfa produces different
                    result
           Product: gcc
           Version: 12.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jhllawrence963 at gmail dot com
  Target Milestone: ---

Created attachment 53560
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=3D53560&action=3Dedit
Sample C++ program

Compiling the attached sample program with g++ -mfma -O3 and executing it l=
eads
to the wrong output starting with GCC version 11.1. The expected output is
approximately 0.905017, but the actual output is -415762. GCC 10.4 and lower
works as expected. Compiling with other optimization flags and -mno-fma wor=
ks
as expected too.

About the program:
It starts with an array of 1s, performs a local average for each element, t=
hen
prints one result from the middle of the array. The algorithm has been redu=
ced
to remove code that is not needed to reproduce the bug, which is why the
expected output is not exactly 1. The sample contains extra code which is n=
ot
relevant to the bug, but removing them causes the bug to be not reproducibl=
e.
The relevant parts have been commented with "FIXME". I'm not 100% certain, =
but
there appears to be some loss of precision which gets compounded because the
result of one loop iteration is used as an input to the next iterations. The
program output becomes more incorrect as the input array size increases.

GCC Version:
$ gcc -v
Using built-in specs.
COLLECT_GCC=3Dgcc
COLLECT_LTO_WRAPPER=3D/usr/lib/gcc/x86_64-pc-linux-gnu/12.2.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: /build/gcc/src/gcc/configure
--enable-languages=3Dc,c++,ada,fortran,go,lto,objc,obj-c++,d --enable-boots=
trap
--prefix=3D/usr --libdir=3D/usr/lib --libexecdir=3D/usr/lib --mandir=3D/usr=
/share/man
--infodir=3D/usr/share/info --with-bugurl=3Dhttps://bugs.archlinux.org/
--with-build-config=3Dbootstrap-lto --with-linker-hash-style=3Dgnu
--with-system-zlib --enable-__cxa_atexit --enable-cet=3Dauto
--enable-checking=3Drelease --enable-clocale=3Dgnu --enable-default-pie
--enable-default-ssp --enable-gnu-indirect-function --enable-gnu-unique-obj=
ect
--enable-libstdcxx-backtrace --enable-link-serialization=3D1
--enable-linker-build-id --enable-lto --enable-multilib --enable-plugin
--enable-shared --enable-threads=3Dposix --disable-libssp --disable-libstdc=
xx-pch
--disable-werror
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 12.2.0 (GCC)=