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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/22326] promotions (from float to double) are not removed when they should be able to
Date: Fri, 11 Dec 2020 14:24:19 +0000	[thread overview]
Message-ID: <bug-22326-4-Lxwp3XMuBn@http.gcc.gnu.org/bugzilla/> (raw)
In-Reply-To: <bug-22326-4@http.gcc.gnu.org/bugzilla/>

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=22326

--- Comment #19 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Segher Boessenkool from comment #18)
> Why is it correct to convert the double x to single precision here?!

For

float foo(float f, float x, float y) {
return (fabs(f)*x+y);
}

it is not unless when using FMA intermediate _overflow_ is also "ignored"
(as opposed to just performing one rounding step and thus requiring one bit
more precision).

Note that the set of inputs where it is errorneous (producing +-Inf rather than
a finite number) is small and thus it might fall into the realms of
-funsafe-math-optimizations (even a 1 ulp change can make a big difference
when in the divisor for example).

These kind of FP "optimizations" are always tricky.

  parent reply	other threads:[~2020-12-11 14:24 UTC|newest]

Thread overview: 22+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <bug-22326-4@http.gcc.gnu.org/bugzilla/>
2020-11-17  7:07 ` luoxhu at gcc dot gnu.org
2020-11-17  7:10 ` luoxhu at gcc dot gnu.org
2020-11-17 10:21 ` pinskia at gcc dot gnu.org
2020-11-17 21:56 ` joseph at codesourcery dot com
2020-11-18 15:22 ` segher at gcc dot gnu.org
2020-11-23  8:39 ` luoxhu at gcc dot gnu.org
2020-11-23  8:44 ` luoxhu at gcc dot gnu.org
2020-11-23 12:09 ` rguenther at suse dot de
2020-11-24 10:00 ` rsandifo at gcc dot gnu.org
2020-11-27  6:20 ` luoxhu at gcc dot gnu.org
2020-11-27  6:27 ` luoxhu at gcc dot gnu.org
2020-11-27  7:12 ` rguenther at suse dot de
2020-11-27  9:24 ` rsandifo at gcc dot gnu.org
2020-11-30  8:31 ` luoxhu at gcc dot gnu.org
2020-12-11 13:49 ` segher at gcc dot gnu.org
2020-12-11 14:24 ` rguenth at gcc dot gnu.org [this message]
2020-12-11 17:51 ` segher at gcc dot gnu.org
2020-12-11 19:24 ` rguenther at suse dot de
2020-12-14  7:21 ` luoxhu at gcc dot gnu.org
     [not found] <bug-22326-6528@http.gcc.gnu.org/bugzilla/>
2006-08-21  6:08 ` pinskia at gcc dot gnu dot org
2007-07-09  6:08 ` pinskia at gcc dot gnu dot org
2005-07-06 15:40 [Bug tree-optimization/22326] New: " pinskia at gcc dot gnu dot org
2005-07-12 20:37 ` [Bug tree-optimization/22326] " pinskia at gcc dot gnu dot org

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