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From: "pinskia at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/94587] Intrinsics optimization bug with -O2
 -march=skylake-avx512
Date: Tue, 14 Apr 2020 18:51:45 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D94587

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |WONTFIX
             Status|REOPENED                    |RESOLVED

--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Patrick J. LoPresti from comment #3)
> That works; thank you. However...
>=20
> I realize there is no formal spec for intrinsics. But when I use them, I
> expect deterministic behavior by default. This has been true on every
> compiler with every set of optimization and architecture flags I have ever
> used (GCC before AVX, many versions of Clang, many versions of the Intel
> compiler).
>=20
> Also, the "-DHEISENBUG" example shows that simply adding a side-effect-fr=
ee
> assert() changes the behavior. This seems... unfriendly... as a default.

Note this is true even without using intrinsics really.  You can get the sa=
me
behavior you are seeing with using standard C code.=20=20

>=20
> Wouldn't fp-contract be more appropriate as part of "-ffast-math"?

No.  This has been discussed many times and decided no.=20=20

>=20
> To my knowledge, no other compiler behaves this way. Are there any other
> options I need to ensure deterministic behavior for SSE intrinsics on GCC?
> Will there be more in the future? I do apologize if I missed the answer in
> the 1000-page GCC manual.

https://gcc.gnu.org/onlinedocs/gcc-9.3.0/gcc/Floating-point-implementation.=
html#Floating-point-implementation=