Re: Better performance on older version of GCC

[Xinliang David Li <davidxl@google.com>]

Briefly looked at it -- the trunk gcc also regresses a lot compared to
the binary you attached. (To match your binary, also added
-mfpmath=387 -m32 options)

Two problems:

1) more register spills in the trunk version -- the old compiler seems
more effective in using fp stack registers;
2) the complex multiplication -- the old version emits inline sequence
while the trunk version emits call to _muld3c intrinsinc.

You can probably file a bug report on this.

Thanks,

David

On Fri, Aug 27, 2010 at 8:39 AM, Corey Kasten
<corey@materialintelligencellc.com> wrote:
> On Fri, 2010-08-27 at 17:09 +0200, Richard Guenther wrote:
>> On Fri, Aug 27, 2010 at 5:02 PM, Corey Kasten
>> <corey@materialintelligencellc.com> wrote:
>> > On Fri, 2010-08-27 at 06:50 -0700, Nathan Froyd wrote:
>> >> On Fri, Aug 27, 2010 at 09:44:25AM -0400, Corey Kasten wrote:
>> >> > I find that the executable compiled on system A runs faster (on both
>> >> > systems) than the executable compiled on system B (on both system), by a
>> >> > factor about approximately 4 times. I have attempted to play with the
>> >> > GCC optimizer flags and have not been able to get System B (with the
>> >> > later GCC version) to compile code with any better performance. Could
>> >> > someone please help figure this out?
>> >>
>> >> It's almost impossible to tell what's going on without an actual
>> >> testcase.  You might not be able to provide the actual code, but you
>> >> could try distilling it down to something you could release.
>> >>
>> >> -Nathan
>> >
>> > Thanks for the reply Nathan.
>> >
>> > I have attached an archive with the test case code. The code is built by
>> > build.sh and outputs the number of microseconds to complete the
>> > processing.
>> >
>> > Compiling with GCC version "4.1.2 20070925 (Red Hat 4.1.2-33)" produces
>> > code that runs in about 66% of the time than does GCC version "4.3.0
>> > 20080428 (Red Hat 4.3.0-8)"
>>
>> -fcx-limited-range or -fcx-fortran-rules.  4.3 now is more conforming than 4.1.
>>
>> Richard.
>>
>> > Thanks
>> >
>> > Corey
>> >
>
> Richard,
>
> -fcx-limited-range worked great on both my real benchmark and my test
> achive. GCC didn't recognize -fcx-fortran-rules, but obviously I don't
> need it.
>
> Thanks so much,
> Corey
>
>
>
>