FDO and LTO on ARM

[Jan Hubicka]

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

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
>