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From: "ubizjak at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug middle-end/110832] [14 Regression] 14% capacita -O2 regression
 between g:9fdbd7d6fa5e0a76 (2023-07-26 01:45) and g:ca912a39cccdd990
 (2023-07-27 03:44) on zen3 and core
Date: Fri, 28 Jul 2023 15:46:29 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D110832

--- Comment #8 from Uro=C5=A1 Bizjak <ubizjak at gmail dot com> ---
(In reply to Richard Biener from comment #6)
> Do we know whether we could in theory improve the sanitizing by optimizat=
ion
> without -funsafe-math-optimizations (I think -fno-trapping-math,
> -ffinite-math-only -fno-signalling-nans should be a better guard?)?

Regarding the sanitizing, we can remove all sanitizing MOVQ instructions
between trapping instructions (IOW, the result of ADDPS is guaranteed to ha=
ve
zeros in the high part outside V2SF, so MOVQ is unnecessary in front of a
follow-up MULPS).

I think that some instruction back-walking pass on the RTL insn stream woul=
d be
able to identify these unnecessary instructions and remove them.

Also, as mentioned elsewhere, it is really hard to get non-zero value to the
highpart of XMM register. The compiler takes great care to always load valu=
es
via MOVQ, so one has to craft a special code that works around all these
fences. OTOH, in two years since gcc-11 was released with the V2SF support,=
 not
a single PR involving spurious exceptions was reported. Even capacita bench=
mark
enables:

Note: The following floating-point exceptions are signalling:
IEEE_UNDERFLOW_FLAG IEEE_DENORMAL

without problems.

As an example here, it looks that polyhedron capacita greatly benefits from
V2SF vectors, and I was surprised that sanitizing MOVQ has such an effect h=
ere.=