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Re: Enabling -frename-registers?
- From: David Edelsohn <dje dot gcc at gmail dot com>
- To: Richard Biener <richard dot guenther at gmail dot com>, Bernd Schmidt <bschmidt at redhat dot com>, Jeffrey Law <law at redhat dot com>
- Cc: GCC Patches <gcc-patches at gcc dot gnu dot org>, Segher Boessenkool <segher at kernel dot crashing dot org>
- Date: Tue, 3 May 2016 17:26:18 -0400
- Subject: Re: Enabling -frename-registers?
- Authentication-results: sourceware.org; auth=none
- References: <CAGWvnykBEjYMgOCaR_ZGLwB3fxPg-pB3WPSwAvmUBXzmXct8gg at mail dot gmail dot com> <57236285 dot 5090908 at redhat dot com> <CAGWvnykKwAtfSin23FP4L0bNgH4bSAdV+vMRTOe+dPk15oAfGg at mail dot gmail dot com> <57236547 dot 2070707 at redhat dot com> <CAGWvnyka9pPjOmFFb8avhBWNAOWGygOeCfUzKCZbY3dxux48LQ at mail dot gmail dot com> <443D7834-6E23-4046-8DD7-F1D8F2593F2B at gmail dot com>
On Fri, Apr 29, 2016 at 10:21 AM, Richard Biener
<richard.guenther@gmail.com> wrote:
> On April 29, 2016 3:48:37 PM GMT+02:00, David Edelsohn <dje.gcc@gmail.com> wrote:
>>On Fri, Apr 29, 2016 at 9:44 AM, Bernd Schmidt <bschmidt@redhat.com>
>>wrote:
>>>
>>>
>>> On 04/29/2016 03:42 PM, David Edelsohn wrote:
>>>>
>>>> On Fri, Apr 29, 2016 at 9:32 AM, Bernd Schmidt <bschmidt@redhat.com>
>>>> wrote:
>>>>>
>>>>> On 04/29/2016 03:02 PM, David Edelsohn wrote:
>>>>>>
>>>>>>
>>>>>> How has this show general benefit for all architectures to deserve
>>>>>> enabling it by default at -O2?
>>>>>
>>>>>
>>>>>
>>>>> It should improve postreload scheduling in general, and it can also
>>help
>>>>> clear up bad code generation left behind by register allocation.
>>>>
>>>>
>>>> Did you test the actual performance benefit on any architectures,
>>>> especially architectures other than x86?
>>>
>>>
>>> No. If that's the standard, I'll back out the change.
>>
>>It seems rather strange to enable an optimization by default across
>>all targets without even knowing the performance impact.
>>
>>I'm eager to learn the opinion of others about this.
>
> It shows overall benefit on Itanic and ups and downs on x86.
>
> It's stage1 and the easiest to get feedback for all archs is to enable it by default.
Bernd and Richard,
The IBM LTC team has tested the benefit of -frename-registers at -O2
and sees no net benefit for PowerPC -- some benchmarks improve
slightly but others degrade slightly (a few percent). You mentioned
no overall benefit for x86. Although you mentioned benefit for
Itanium, it is not a primary nor secondary architecture target for GCC
and continues to have limited adoption. Andreas also reported a
bootstrap comparison failure for Itanium due to the change.
If I understand correctly, the change to enable -frename-registers was
motivated by PR 59173 for SSE. The performance experiments, bootstrap
failure, and testsuite fallout do not provide good justification for
enabling this optimization by default at -O2 for all targets. If this
helps SSE, this optimization can be enabled using target-specific
override mechanisms for that target.
Optimizations enabled by default at -O2 should show an overall net
benefit -- that is the general justification that we have used in the
past. I request that this change be reverted until more compelling
evidence of benefit is presented.
Thanks, David