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

[Xinliang David Li]

>>
>
> 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.
>
> FDO is important for optimizations where all possible data sets do not
> change branch probability distribution much.  IMHO therefore FDO is not
> widely used by most of developers (although I am sure that for Google
> applications it is extremely important) and therefore I don't measure it and
> it is not so interesting for me.  Although bigger reason not use FDO is
> inconvenience to use it for regular compiler user.
>
> 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.

Actually what I was trying to say is that LTO will be more powerful
when combined with FDO. In other words, I expect LTO + FDO improves
over plain FDO more than 1.86%.


>>
>> It would be great if there is number collected comparing LTO + FDO vs
>> plain FDO in the same setup.
>>
>>
>
> Usually after such posting the comparisons,  I am getting a lot of requests.
>  I'd like to do all of them but unfortunately running and the result
> preparation takes a lot of my time.  May be I'll do such comparison next
> year.

Ok. Another comment is that using SPEC2000 for performance testing
won't be indicative of today's real world program size. Even
SPEC2006's largest C++ programs are not that big.

Thanks,

David

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