RE: [PATCH] Enabling Software Prefetching by Default at -O3

["Fang, Changpeng" <Changpeng.Fang@amd.com>]

Hi, Mark:

>> It also might be worth to investigate if overriding the parameters per
>> -mtune=XXX results in an overall win for -fprefetch-loop-arrays. We did
>> that on s390 since the default values were not ideal

>Yes, that might be a good idea for i7.

>But, in the meantime, I think we should get a version of the patch that
>turns on prefetching on AMD CPUs with -O3.  There's no reason to demand
>consistency for all CPUs and it clearly benefits the AMD CPUs.
>Changpeng, would you please submit a patch that activates this
>optimization only with tuning for AMD CPUs?

It seems I have a problem in setting prefetching default at -O3 only for AMD CPUs.
When we know it is AMD CPUS after the command line options have been parsed,
we don't know whether -fno-prefetch-loop-arrays has been specified in the command line
or not. If we turn on prefetching at -O3, then we could not explicitly turn it off if we want.

What I want is something like:
 if  (!OPTION_SET_P (flag_prefetch_loop_arrays))
   flag_prefetch_loop_arrays = 1;

An alternative way is parsing -m options before other options.

What do you think?

Thanks,

Changpeng