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Re: Patch gcc-4.0 should not apply mathematical associative rulesfor addition or multiplication
- From: Roger Sayle <roger at eyesopen dot com>
- To: Fariborz Jahanian <fjahanian at apple dot com>, GCC Patches <gcc-patches at gcc dot gnu dot org>
- Cc: Dale Johannesen <dalej at apple dot com>, Ziemowit Laski <zlaski at apple dot com>, Geoff Keating <geoffk at geoffk dot org>, Richard Henderson <rth at redhat dot com>
- Date: Thu, 7 Oct 2004 21:41:26 -0600 (MDT)
- Subject: Re: Patch gcc-4.0 should not apply mathematical associative rulesfor addition or multiplication
Hi Fariborz,
Has this issue been filed as a PR in bugzilla yet? I've just tried
some tests on mainline and I'm shocked at the unsafe transformations
being performed by the tree-ssa optimizers.
If you've not already done so, make sure this is targeted at 4.0.0
with critical priority (a potential show stopper), then to keep RTH
and Geoff K happy, also file a low priority enhancement request that
(X * 2.0) * 2.0 gets optimized. The first is a severe regression from
all previous GCC releases, the latter isn't and possibly may not even
be suitable for stage3. If it makes people more comfortable, feel free
to assign the enhancement request to me. I wouldn't want either issue
to get lost.
My concern now is how much the performance benefits of the tree-ssa
merge were exagerated by the fact that we've been performing these
unsafe mathematical transformations. Fixing this problem may hurt
our SPECfp2000 scores :<
Roger
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