From: Gabriel Dos_Reis <gdosreis@sophia.inria.fr>
To: Alexandre Oliva <aoliva@redhat.com>
Cc: Gabriel Dos Reis <gdr@codesourcery.com>,
Gabriel Dos_Reis <Gabriel.Dos_Reis@sophia.inria.fr>,
Linus Torvalds <torvalds@transmeta.com>,
Tim Hollebeek <tim@hollebeek.com>,
Theodore Papadopoulo <Theodore.Papadopoulo@sophia.inria.fr>,
<dewar@gnat.com>, <amylaar@redhat.com>, <gcc@gcc.gnu.org>,
<moshier@moshier.ne.mediaone.net>, <tprince@computer.org>
Subject: Re: What is acceptable for -ffast-math? (Was: associative law in combine)
Date: Wed, 01 Aug 2001 13:21:00 -0000 [thread overview]
Message-ID: <15208.25047.127984.580486@perceval.inria.fr> (raw)
In-Reply-To: <or66c7zjj7.fsf@feijoada.lsd.ic.unicamp.br>
| > | > The one obvious case is (b*c) overflows and turns into Inf, at which
| > | > point the end result will be +-0 with the optimization.
| > |
| > | If there is a difference, will you care to enlighten me?
|
| > You're quoting just a tiny part of his rhetorics
|
| Indeed, I failed to quote the other relevant part. Linus wrote:
More accurately he wrote:
The one obvious case is (b*c) overflows and turns into Inf, at which point
the end result will be +-0 with the optimization. Now, in order for the
original to _not_ have been close to zero, we're talking about 'a' being
at the very limits of the number space. And remember that on x86 register
arithmetic - regardless of precision setting - the exponent is 15 bits. So
in order to get the overflow, AND to get a original value that wasn't zero
(ie to get an error), 'a' would have had to be on the order of
2**(16383-64)
which won't even _fit_ in a double anyway (the -64 is to take even
denormals into account, but others have said that flush-to-zero would have
been acceptable anyway by default). Did I mis-do my calculations.
Emphasis is not mine. Please note, what he showed: That the
situation (i.e. a/b/c) couldn't have been well-formed in the first
place. For x86, that may be true. But that is a specific case.
| > Now, in order for the original to _not_ have been close to zero,
| > we're talking about 'a' being at the very limits of the number
| > space.
|
| It appears to me that 0.5 * DBL_MAX is indeed at the very limits of
| the number space. So, I still fail to see the difference.
I don't consider 0.5 * DBL_MAX to be at the "very limits of the number
space".
| > He used the argument above to derive that for a/b/c not to yeild 0.0,
| > a should have been very very big, which can't happen in practice.
|
| But he was explicitly talking about x86's extended precision when he
| made this argument.
Yes, he was talking about x86. But again, when we started discussing
the transformation, there were no limitation to targets. Again, I
exhibit the example to show that there are out there bunch of
processors where the stance "it won't matter " isn't true. Now
if the transformation were to be restricted to x86, then that is a
different issue.
-- Gaby
next prev parent reply other threads:[~2001-08-01 13:21 UTC|newest]
Thread overview: 120+ messages / expand[flat|nested] mbox.gz Atom feed top
2001-07-30 21:13 dewar
2001-07-30 21:34 ` Gabriel Dos Reis
2001-07-30 21:43 ` Joern Rennecke
2001-07-30 21:53 ` Gabriel Dos Reis
2001-08-03 7:12 ` Nick Ing-Simmons
2001-07-31 18:12 ` What is acceptable for -ffast-math? (Was: associative law incombine) Linus Torvalds
2001-08-01 8:55 ` What is acceptable for -ffast-math? (Was: associative law in combine) Theodore Papadopoulo
2001-08-01 9:15 ` Gabriel Dos Reis
2001-08-01 11:21 ` Theodore Papadopoulo
2001-08-01 11:44 ` Gabriel Dos Reis
2001-08-01 11:58 ` What is acceptable for -ffast-math? (Was: associative law incombine) Linus Torvalds
2001-08-01 9:24 ` What is acceptable for -ffast-math? (Was: associative law in combine) Tim Hollebeek
2001-08-01 9:54 ` What is acceptable for -ffast-math? (Was: associative law incombine) Linus Torvalds
2001-08-01 10:26 ` Gabriel Dos_Reis
2001-08-01 10:35 ` Linus Torvalds
2001-08-01 10:45 ` Gabriel Dos_Reis
2001-08-01 11:13 ` What is acceptable for -ffast-math? (Was: associative law in combine) Alexandre Oliva
2001-08-01 11:36 ` Gabriel Dos Reis
2001-08-01 12:07 ` Alexandre Oliva
2001-08-01 13:21 ` Gabriel Dos_Reis [this message]
2001-08-01 14:20 ` Toon Moene
-- strict thread matches above, loose matches on Subject: below --
2001-08-03 14:48 dewar
2001-08-02 3:37 Vincent Penquerc'h
2001-08-01 19:04 Carlo Wood
2001-08-01 12:06 Phil Edwards
2001-08-01 10:39 dewar
2001-08-01 10:38 dewar
2001-08-01 10:13 dewar
2001-08-01 10:05 dewar
2001-08-01 10:04 dewar
2001-08-01 10:28 ` Gabriel Dos_Reis
2001-08-01 9:59 dewar
2001-08-01 9:58 What is acceptable for -ffast-math? (Was: associative law incombine) Gabriel Dos_Reis
2001-08-01 10:08 ` Wolfgang Bangerth
2001-08-01 11:12 ` Gabriel Dos_Reis
2001-08-01 11:27 ` What is acceptable for -ffast-math? (Was: associative law in combine) Theodore Papadopoulo
2001-08-01 11:47 ` Gabriel Dos_Reis
2001-08-03 7:32 ` Nick Ing-Simmons
2001-08-03 6:01 ` Per Abrahamsen
2001-08-01 6:52 dewar
2001-08-01 6:04 dewar
2001-08-01 6:48 ` Vincent Penquerc'h
2001-08-03 0:46 ` Eric W. Biederman
2001-08-01 3:02 Vincent Penquerc'h
2001-07-31 19:10 dewar
2001-07-31 18:23 dewar
2001-07-31 18:20 dewar
2001-07-31 18:50 ` Joern Rennecke
2001-07-31 21:27 ` Tim Prince
2001-07-31 18:15 dewar
2001-07-31 18:12 What is acceptable for -ffast-math? (Was: associative law incombine) Linus Torvalds
2001-07-31 20:55 ` What is acceptable for -ffast-math? (Was: associative law in combine) Gabriel Dos Reis
2001-07-31 17:37 dewar
2001-07-31 16:38 dewar
2001-07-31 9:22 mike stump
2001-07-31 8:37 dewar
2001-07-31 8:36 dewar
2001-07-31 8:36 mike stump
2001-07-31 8:35 dewar
2001-07-31 8:19 mike stump
2001-07-31 7:59 mike stump
2001-07-31 7:26 dewar
2001-07-31 15:57 ` Toon Moene
2001-07-31 21:55 ` Tim Prince
2001-08-03 6:12 ` Per Abrahamsen
2001-07-30 20:54 dewar
2001-07-30 21:11 ` Gabriel Dos Reis
2001-07-30 21:39 ` Joern Rennecke
2001-07-30 19:46 dewar
2001-07-30 20:00 ` Gabriel Dos Reis
2001-07-30 20:20 ` Alexandre Oliva
2001-07-30 20:25 ` Gabriel Dos Reis
2001-07-30 18:39 dewar
2001-07-30 18:38 dewar
2001-07-30 18:08 dewar
2001-07-30 18:02 dewar
2001-07-30 18:00 dewar
2001-07-30 18:25 ` Joe Buck
2001-07-30 16:11 dewar
2001-07-30 16:29 ` Alexandre Oliva
2001-07-31 8:13 ` Kevin Handy
2001-07-30 15:29 dewar
2001-07-30 15:39 ` Toon Moene
2001-07-30 13:10 dewar
2001-07-30 12:26 dewar
2001-07-30 11:52 dewar
2001-07-30 11:37 What is acceptable for -ffast-math? (Was: associative law incombine) Linus Torvalds
2001-07-30 11:53 ` What is acceptable for -ffast-math? (Was: associative law in combine) Gabriel Dos Reis
2001-07-30 18:40 ` Olivier Galibert
2001-07-30 19:06 ` Gabriel Dos Reis
2001-07-31 1:35 ` Linus Torvalds
2001-07-31 2:04 ` Gabriel Dos Reis
2001-07-31 2:35 ` Olivier Galibert
2001-07-31 2:58 ` Gabriel Dos Reis
2001-07-31 18:10 ` Linus Torvalds
2001-07-30 8:59 mike stump
2001-07-30 6:14 dewar
2001-07-30 8:30 ` Kevin Handy
2001-07-30 6:01 dewar
2001-07-30 6:53 ` Tim Hollebeek
2001-07-30 6:00 dewar
2001-07-30 13:08 ` Toon Moene
2001-07-30 5:57 dewar
2001-07-29 21:33 What is acceptable for -ffast-math? (Was: associative law incombine) Linus Torvalds
2001-07-30 14:43 ` What is acceptable for -ffast-math? (Was: associative law in combine) Alexandre Oliva
2001-07-30 15:45 ` Neil Booth
2001-07-30 16:03 ` Alexandre Oliva
2001-07-30 16:11 ` Neil Booth
2001-07-30 16:28 ` Alexandre Oliva
2001-07-30 19:08 ` Joern Rennecke
2001-07-30 19:22 ` Alexandre Oliva
2001-07-30 19:29 ` Gabriel Dos Reis
2001-07-30 19:34 ` Alexandre Oliva
2001-07-30 19:54 ` Gabriel Dos Reis
2001-07-30 19:27 ` Gabriel Dos Reis
2001-07-29 14:22 dewar
2001-07-29 12:52 * Re: What is acceptable for -ffast-math? (Was: associative lawin combine) Linus Torvalds
2001-07-29 14:03 ` What is acceptable for -ffast-math? (Was: associative law incombine) Stephen L Moshier
2001-07-29 21:17 ` What is acceptable for -ffast-math? (Was: associative law in combine) Fergus Henderson
2001-07-30 0:23 ` Gabriel Dos Reis
2001-07-17 15:59 associative law in combine Joern Rennecke
2001-07-18 1:01 ` Toon Moene
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
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