From: Toon Moene <toon@moene.indiv.nluug.nl>
To: Joern Rennecke <amylaar@redhat.com>
Cc: Jan Hubicka <jh@suse.cz>, gcc@gcc.gnu.org
Subject: Re: associative law in combine
Date: Wed, 18 Jul 2001 01:01:00 -0000 [thread overview]
Message-ID: <3B55427F.4790646A@moene.indiv.nluug.nl> (raw)
In-Reply-To: <200107172259.f6HMxYU31134@phal.cambridge.redhat.com>
Joern Rennecke wrote:
> > > Jan Hubicka wrote:
> > OK, so to speak loud - where is the proper place to convert
> > a/b/c to a/(b*c) at tree level. fold-const or some other?
>
> Only if b and c are constants, the operations are floating point, and
> b can be multiplied with c without loss of precision or overflow.
> Or if b and/or c is a power of two, and b can be multiplied with c without
> overflow.
Joern is right, Jan. One can argue about the loss of precision (under
unsafe math optimisations), but not the overflow. I overlooked that
issue in my reply to you. Because overflow can only be determined at
compile time with constants, this conversion cannot be right for
variables.
--
Toon Moene - mailto:toon@moene.indiv.nluug.nl - phoneto: +31 346 214290
Saturnushof 14, 3738 XG Maartensdijk, The Netherlands
Maintainer, GNU Fortran 77: http://gcc.gnu.org/onlinedocs/g77_news.html
Join GNU Fortran 95: http://g95.sourceforge.net/ (under construction)
next prev parent reply other threads:[~2001-07-18 1:01 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <20010716224423.F29145@atrey.karlin.mff.cuni.cz>
[not found] ` <3B547DF4.7F39BF61@moene.indiv.nluug.nl>
[not found] ` <20010717200629.E5166@atrey.karlin.mff.cuni.cz>
[not found] ` <3B5489F1.54DE59CE@moene.indiv.nluug.nl>
[not found] ` <20010717205837.I5166@atrey.karlin.mff.cuni.cz>
[not found] ` <3B548CBF.DDD59F97@moene.indiv.nluug.nl>
[not found] ` <20010717211516.N5166@atrey.karlin.mff.cuni.cz>
[not found] ` <3B548FAA.E174E765@moene.indiv.nluug.nl>
2001-07-17 12:23 ` Jan Hubicka
2001-07-17 15:41 ` Dima Volodin
2001-07-17 15:59 ` Joern Rennecke
2001-07-18 1:01 ` Toon Moene [this message]
2001-07-18 1:47 ` What is acceptable for -ffast-math? (Was: associative law in combine) Jan Hubicka
2001-07-28 23:04 ` Tim Prince
2001-07-29 6:33 ` Jan Hubicka
2001-07-29 10:18 ` Tim Prince
2001-07-29 10:26 ` Jan Hubicka
2001-07-29 12:11 ` Tim Prince
2001-07-29 12:17 ` Jan Hubicka
2001-07-29 10:50 ` Linus Torvalds
2001-07-18 11:30 ` associative law in combine Joern Rennecke
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