Re: [google gcc-4_8] alternate hirate for builtin_expert

[Jan Hubicka <hubicka at ucw dot cz>]

> Hi,
> 
> Current default probably for builtin_expect is 0.9996.
> This makes the freq of unlikely bb very low (4), which
> suppresses the inlining of any calls within those bb.
> 
> We used FDO data to measure the branch probably for
> the branch annotated with builtin_expert.
>                                                          For google
> internal benchmarks, the weight average
> (the profile count value as the weight) is 0.9081.
> 
> Linux kernel is another program that is heavily annotated
> with builtin-expert. We measured its weight average as 0.8717,
>                                               using google search as
> the workload.

This is interesting.  I was always unsure if programmers use builtin_expect
more often to mark an impossible paths (as those leading to crash) or just
unlikely paths.  Your data seems to suggest the second.

We probably also ought to get analyze_brprob working again. It was always
useful to get such a data.
> 
> 
>     This patch sets the alternate hirate probability for
> builtin_expert
> to 90%. With the alternate hirate, we measured performance
>                                               improvement for google
> benchmarks and Linux kernel.
> 
> 
>                               -Rong
> 2013-09-26  Rong Xu  <xur@google.com>
> 
> 	* params.def (DEFPARAM): New.
> 	* params.def: New.
> 	* predict.c (tree_predict_by_opcode): Alternate 
>         probablity hirate for builtin_expect.

This also seems resonable for mainline.  Please add a comment
to the parameter explaining how the value was determined.
Please also add invoke.texi documentation.

For patches that seems resonable for mainline in FDO/IPA area,
i would apprechiate if you just added me to CC, so I do not lose
track of them.
Honza