Re: function order optimization

[Ãngel GonzÃlez <keisial at gmail dot com>]

Please keep the mailing list in CC.

On 03/08/13 13:42, alex margolin wrote:
> Did you mean "-fprofile-use=path" ? I read about it and it seems it
> triggers other optimizations (loop unrolling, branch prediction) but
> not the function order. Is this true, or does it optimize function
> order as well?
>
> Below is its excerpt from the manual:
>
> Enable profile feedback directed optimizations, and optimizations generally
> profitable only with profile feedback available.
> The following options are enabled: -fbranch-probabilities, -fvpt,
> -funroll-loops, -fpeel-loops, -ftracer, -ftree-vectorize,
> ftree-loop-distribute-patterns
> By default, GCC emits an error message if the feedback profiles do not
> match the source code. This error can be turned into a warning by using
> â-Wcoverage-mismatchâ. Note this may result in poorly optimized code.
> If path is specified, GCC looks at the path to find the profile feedback data
> files. See â-fprofile-dirâ.
> The following options control compiler behavior regarding
> floating-point arithmetic.
> These options trade off between speed and correctness. All must be
> specifically enabled.

Take a look at -freorder-functions:
> '-freorder-functions'
>      Reorder functions in the object file in order to improve code
>      locality.  This is implemented by using special subsections
>      '.text.hot' for most frequently executed functions and
>      '.text.unlikely' for unlikely executed functions.  Reordering is
>      done by the linker so object file format must support named
>      sections and linker must place them in a reasonable way.
>
>      Also profile feedback must be available to make this option
>      effective.  See '-fprofile-arcs' for details.
>
>      Enabled at levels '-O2', '-O3', '-Os'.

There are many profile-related options, and I know little of them, that's
why I directed you to the manual instead of trying to provide a recipe.
But you should be able to improve the results with them.