FDO and LTO on ARM

[Richard Guenther]

On Thu, Aug 4, 2011 at 8:42 PM, Jan Hubicka <jh@suse.de> wrote:
>>> Did you try using FDO with -Os?  FDO should make hot code parts
>>> optimized similar to -O3 but leave other pieces optimized for size.
>>> Using FDO with -O3 gives you the opposite, cold portions optimized
>>> for size while the rest is optimized for speed.
>
> FDO with -Os still optimize for size, even in hot parts.

I don't think so.  Or at least that would be a bug.  Shouldn't 'hot'
BBs/functions
be optimized for speed even at -Os?  Hm, I see predict.c indeed returns
always false for optimize_size :(

I thought we had just the neither cold or hot parts optimized according
to optimize_size.

>  So to get resonale
> speedups you need -O3+FDO.  -O3+FDO effectively defaults to -Os in cold
> portions of program.

Well, but unless your training coverage is 100% all parts with no coverage
get optimized with -O3 instead of -Os.  And I bet coverage for mozilla
isn't even close to 100%.  Thus I think recommending -O3 for FDO is
usually a bad idea.

So - did you try FDO with -O2? ;)

> Still -Os+FDO should be somewhat faster than -Os alone, so a slowdown is
> bug.  It is not very thoroughly since it is not really used in practice.
>
>>> Also do you get any warnings on profile mismatches? Perhaps something
>>> is wrong to the degree that the relevant part of profile gets
>>> misapplied.
>>
>> I don't get any warning on profile mismatches. I only get a "few"
>> missing gcda files warning, but that's expected.
>
> Perhaps you could compile one of less trivial files you are sure that are
> covered by train run and send me -fdump-tree-all-blocks -fdump-ipa-all dumps
> of the compilation so I can double check the profile seems sane. This could
> be good start to rule out something stupid.
>
> Honza
>>
>> Cheers,
>>
>> Mike
>>
>
>
>