From: Marc Glisse <marc.glisse@inria.fr>
To: Richard Biener <rguenther@suse.de>
Cc: gcc-patches@gcc.gnu.org, Jakub Jelinek <jakub@redhat.com>
Subject: Re: [PATCH] FIx endless match.pd recursion on cst1 + cst2 + cst3 (PR tree-optimization/84334)
Date: Wed, 14 Feb 2018 13:00:00 -0000 [thread overview]
Message-ID: <alpine.DEB.2.21.1802140858540.23969@stedding.saclay.inria.fr> (raw)
In-Reply-To: <alpine.LSU.2.20.1802141208470.18265@zhemvz.fhfr.qr>
On Wed, 14 Feb 2018, Richard Biener wrote:
> On Tue, 13 Feb 2018, Marc Glisse wrote:
>
>> On Tue, 13 Feb 2018, Richard Biener wrote:
>>
>>> On February 13, 2018 6:51:29 PM GMT+01:00, Jakub Jelinek <jakub@redhat.com>
>>> wrote:
>>>> Hi!
>>>>
>>>> On the following testcase, we recurse infinitely, because
>>>> we have float re-association enabled, but also rounding-math, so
>>>> we try to optimize (cst1 + cst2) + cst3 as (cst2 + cst3) + cst1
>>>> but (cst2 + cst3) doesn't simplify and we try again and optimize
>>>> it as (cst3 + cst1) + cst2 and then (cst1 + cst2) + cst3 and so on
>>>> forever. If @0 is not a CONSTANT_CLASS_P, there is not a problem,
>>>> if it is, the code just checks if we can actually simplify the
>>>> operation between cst2 and cst3 into a constant.
>>>
>>> Is there a reason to try simplifying at all for constant @0?
>>
>> Yes. cst2+cst3 might simplify (the operation happens to be exact and not
>> require rounding), which leaves us with only one addition instead of 2.
>>
>> On the other hand, mixing -frounding-math with reassociation seems strange to
>> me, and likely not worth optimizing for.
>
> ./cc1 -quiet t.c -O -frounding-math -fassociative-math
> cc1: warning: -fassociative-math disabled; other options take precedence
>
> So _maybe_ we should disable these patterns for !flag_associative_math
> when dealing with FP?
There is
(if ((!FLOAT_TYPE_P (type) || flag_associative_math)
&& !FIXED_POINT_TYPE_P (type))
above, which I think covers this transformation.
--
Marc Glisse
next prev parent reply other threads:[~2018-02-14 13:00 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-02-13 17:51 Jakub Jelinek
2018-02-13 18:04 ` Richard Biener
2018-02-13 19:28 ` Marc Glisse
2018-02-14 3:49 ` Jakub Jelinek
2018-02-14 11:10 ` Richard Biener
2018-02-14 13:00 ` Marc Glisse [this message]
2018-02-14 14:49 ` Jakub Jelinek
2018-02-14 20:52 ` [PATCH] Fix endless match.pd recursion on cst1 + cst2 + cst3 (PR tree-optimization/84334, take 2) Jakub Jelinek
2018-02-15 9:59 ` Richard Biener
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