Performance degradation on g++ 4.6

[Richard Guenther]

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.
>