From: Richard Guenther <richard.guenther@gmail.com>
To: Oleg Smolsky <oleg.smolsky@riverbed.com>
Cc: Xinliang David Li <davidxl@google.com>, gcc@gcc.gnu.org
Subject: Re: Performance degradation on g++ 4.6
Date: Tue, 02 Aug 2011 09:27:00 -0000 [thread overview]
Message-ID: <CAFiYyc2DdBFYC_bkj9hUJ-vso+nk_QxXLYZ8D_eB_KBT52Fhog@mail.gmail.com> (raw)
In-Reply-To: <4E36F3BD.9080804@riverbed.com>
On Mon, Aug 1, 2011 at 8:43 PM, Oleg Smolsky
<oleg.smolsky@riverbed.com> wrote:
> On 2011/7/29 14:07, Xinliang David Li wrote:
>>
>> Profiling tools are your best friend here. If you don't have access to
>> any, the least you can do is to build the program with -pg option and
>> use gprof tool to find out differences.
>
> The test suite has a bunch of very basic C++ tests that are executed an
> enormous number of times. I've built one with the obvious performance
> degradation and attached the source, output and reports.
>
> Here are some highlights:
> v4.1: Total absolute time for int8_t constant folding: 30.42 sec
> v4.6: Total absolute time for int8_t constant folding: 43.32 sec
>
> Every one of the tests in this section had degraded... the first half more
> than the second. I am not sure how much further I can take this - the
> benchmarked code is very short and plain. I can post disassembly for one
> (some?) of them if anyone is willing to take a look...
I have a hard time actually seeing what expressions they try to fold
(argh, templates everywhere ...). One thing that changed between
4.1 and 4.6 is that we can no longer re-associate freely signed integers
because of undefined overflow concerns - which is a correctness issue.
Depending on the way the tests are written the folding in 4.1 was
probably a bug.
Richard.
> Thanks,
> Oleg.
>
next prev parent reply other threads:[~2011-08-02 9:27 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-07-29 18:08 Oleg Smolsky
2011-07-29 18:14 ` Xinliang David Li
2011-07-29 21:08 ` Oleg Smolsky
2011-07-29 21:29 ` Xinliang David Li
2011-08-01 18:44 ` Oleg Smolsky
2011-08-02 5:48 ` Xinliang David Li
2011-08-23 1:09 ` Oleg Smolsky
2011-08-23 1:34 ` Oleg Smolsky
2011-08-23 1:37 ` Andrew Pinski
2011-08-23 17:47 ` Oleg Smolsky
2011-08-23 18:38 ` Xinliang David Li
2011-08-24 19:51 ` Oleg Smolsky
2011-08-24 20:03 ` Xinliang David Li
2011-08-24 21:26 ` Oleg Smolsky
2011-08-24 21:57 ` Xinliang David Li
2011-08-24 22:14 ` Oleg Smolsky
2011-08-02 9:27 ` Richard Guenther [this message]
2011-08-03 19:12 ` Xinliang David Li
2011-07-30 9:24 ` Richard Guenther
2011-07-30 14:57 Benjamin Redelings I
2011-08-01 18:04 ` Oleg Smolsky
2011-08-01 18:14 ` Marc Glisse
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