[gmx-developers] CFLAGS for ppc, etc. with CMake

[hess at sbc.su.se]

> On Wed, Jun 30, 2010 at 5:58 PM, Szilárd Páll
> <szilard.pall at cbr.su.se>wrote:
>
>> Hi,
>>
>> > -Kieee. Does someone know whether this is required for GROMACS? One
>> can
>> also
>> > gain with GCC by adding -ffast-math. Is this safe?
>>
>> When testing/debugging the strange performance hit we experienced a
>> while ago with gcc 4.3.3/4 - which turned out to be a problem with
>> expf - I also tried the -ffast-math flag and as far as I remember I
>> got no relevant performance improvement (tested on x86_64). With some
>> gcc versions it even seemed to produce a small performance hit,
>> overall showing -1-2% "improvement".
>>
> Yes. I did the same test back then and got 5% improvement for only the
> solve_pme part. So this agrees with your result.
> Of course this is not that much, but since it is no work to activate it, I
> think it is still interested.
>
> The question is whether it is safe. Thus is there some code which requires
> strict accordance of the IEEE rules? My understanding is that fast-math
> usually doesn't affect the accuracy. Only if bitwise results are important
> or things are unstable it should be important. I'm not aware of any code
> like this, but I'm not sure at all. If we have any code like this we might
> consider to activate fast-math only for the solve_pme function.
>
> Roland
>

I don't think we need ieee for anything.

But do you still get 5% on solve pme after I added the SSE exp?
I also had an SSE 1/x which gave 1 or 2%.
But solve pme is now so little of the total time that such things
become irrelevant.

Berk

>>
>> Take these metrics with a grain of salt though as my benchmarking was
>> focused on a 10-20% performance hit on PME. Moreover it was done ~1.5
>> months ago so It might be worth to test again.
>>
>> --
>> Szilárd
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>
>
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