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

[Roland Schulz]

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

>
> 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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