GCC-4.5.0 comparison with previous releases and LLVM-2.7 on SPEC2000 for x86/x86_64

[Xinliang David Li]

On Thu, Apr 29, 2010 at 2:27 PM, Jan Hubicka <hubicka@ucw.cz> wrote:
>> Thanks for the comments.  FDO will probably improve SPEC2000 score.
>> Although it is not obvious for some tests because the train data sets
>> for them are different from the reference data sets and it might
>> actually mislead the  compiler.
>
> There are several studies on the topic and it is not that bad in practice.
> In wast majority of cases even pretty bad training runs gets significant
> portion of improvement you can get from training on the final benchmark
> data.  In SPEC case FDO improves pretty much all benchmarks.

Agree.

>
> I think the FDO is relatively little used because it is relatively hard
> to use (i.e. user has to modify makefiles and learn how the feature works)
> and also because there is very little support for it (i.e. in automake and such)
>> As for vortex FDO improvement, vortex contains a moderate size loop in
>> which most of time is spent.  The loop has if-then-else on the top loop
>> level.  On all SPEC2000 data sets, one if-branch  is  taken practically
>> always  (like 1 to  1,000,000).   So it is not amazing for me that FDO
>> gives such improvement for vortex.
>
> It would be interesting to know if same improvement happens with LTO and if
> not what LIPO does.  I will unbreak vortex on our tester.
>

Vortex needs -fno-strict-aliasing.  It casts between two record types
with one record being a 'prefix' of another.

David



> Honza
>