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[Bug other/32172] Behavior of -ffast-math
- From: "dominiq at lps dot ens dot fr" <gcc-bugzilla at gcc dot gnu dot org>
- To: gcc-bugs at gcc dot gnu dot org
- Date: 31 May 2007 21:31:05 -0000
- Subject: [Bug other/32172] Behavior of -ffast-math
- References: <bug-32172-12313@http.gcc.gnu.org/bugzilla/>
- Reply-to: gcc-bugzilla at gcc dot gnu dot org
------- Comment #3 from dominiq at lps dot ens dot fr 2007-05-31 21:31 -------
> This behavior with -ffast-math is expected and is hoped for.
Yes indeed: I said I would like to see it at some level of -On.
> That is PR 323.
If I understood correctly the PR, it deals with side effects to do the extra
precision in x87.
I have done the tests on a PPC (sorry for not mentionning it). In the later
case it is due to the fact that a/b may have a different round-off error as
a*(1.0/b).
> *** This bug has been marked as a duplicate of 28684 ***
Related, but a different issue.
My understanding of the Fortran standard (7.1.8.3) is that reordering is
allowed: 'a+b+c' can be computed as '(a+c)+b' (note 7.18), even if it is
usually computed as '(a+b)+c', but "7.1.8.2 Integrity of parentheses" forbids
to compute '(a+f)-f' as 'a'.
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=32172