From: Richard Biener <richard.guenther@gmail.com>
To: Jeff Law <law@redhat.com>
Cc: Bernd Schmidt <bschmidt@redhat.com>,
GCC Patches <gcc-patches@gcc.gnu.org>
Subject: Re: fwprop fix for PR79405
Date: Thu, 23 Feb 2017 09:02:00 -0000 [thread overview]
Message-ID: <CAFiYyc19LUxOmN3KWYwJV5jwG5AF2BU7yp-wvK3=0dQ_J7b9qA@mail.gmail.com> (raw)
In-Reply-To: <cce14bb3-d166-3490-c13c-9b7753ed0ec6@redhat.com>
On Wed, Feb 22, 2017 at 6:19 PM, Jeff Law <law@redhat.com> wrote:
> On 02/16/2017 12:41 PM, Bernd Schmidt wrote:
>>
>> We have two registers being assigned to each other:
>>
>> (set (reg 213) (reg 209))
>> (set (reg 209) (reg 213))
>>
>> These being the only definitions, we are happy to forward propagate reg
>> 209 for reg 213 into a third insn, making a new use for reg 209. We are
>> then happy to forward propagate reg 213 for it in the same insn...
>> ending up in an infinite loop.
>>
>> I don't really see an elegant way to prevent this, so the following just
>> tries to detect the situation (and more general ones) by brute force.
>> Bootstrapped and tested on x86_64-linux, verified that the test passes
>> with a ppc cross, ok?
>>
>>
>> Bernd
>>
>>
>> 79405.diff
>>
>>
>> PR rtl-optimization/79405
>> * fwprop.c (forward_propagate_into): Detect potentially cyclic
>> replacements and bail out for them.
>>
>> PR rtl-optimization/79405
>> * gcc.dg/torture/pr79405.c: New test.
>
> OK.
Err - this looks quite costly done for each fwprop. And placing it before
less costly bailouts even...
See my discussion with Bernd anyway.
Richard.
> jeff
>
next prev parent reply other threads:[~2017-02-23 8:57 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-02-16 19:44 Bernd Schmidt
2017-02-17 9:11 ` Richard Biener
2017-02-17 9:21 ` Richard Biener
2017-02-20 14:02 ` Bernd Schmidt
2017-02-20 17:15 ` Richard Biener
2017-02-22 17:54 ` Jeff Law
2017-02-23 9:02 ` Richard Biener [this message]
2017-02-23 22:21 ` Jeff Law
2017-02-24 10:08 ` Richard Biener
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