Re: [gentoo-dev] -fbranch-probabilities optimisation

[Matt Tucker <tuck@whistlingfish.net>]

-- Sven Vermeulen <swift@gentoo.org> spake thusly:

> On Sat, May 17, 2003 at 03:49:11AM +0100, Chris Davies wrote:
>> I had the idea to build -fbranch-probabilities optimisation into
>> portage. This is where a sample run of data is taken using the
>> program compiled with -fprofile-arcs, and that data then used to
>> reorganise the object code so conditional branches are layed out in
>> a more efficient manner.
> 
> Why implement it in Portage? Isn't it up to the user to define it in
> the CFLAGS or not? 

This isn't a simple matter of turning on a compiler flag.

>From 'man gcc':

	After running a program compiled with -fprofile-arcs, you can
	compile it a second time using -fbranch-probabilities, to
	improve optimizations based on the number of times each branch
	was taken.  When the program compiled with -fprofile-arcs exits
	it saves arc execution counts to a file called sourcename.da for
	each source file The information in this data file is very
	dependent on the structure of the generated code, so you must
	use the same source code and the same optimization options for
	both compilations.

However, the problem with trying to integrate this into portage is that
it must either:

 1) Compile programs on the user's machine, allow the user to run
    them, and then rebuild using the generated data. Quite a
    lengthy process, and fraught with issues.

or

 2) Distribute the profile data along with the source code. The
    trouble with this is that (as the man page says) it requires
    using the same source code and optimization options. So not only
    must the CFLAGS used be the same as with the profiled code, but
    the configure settings (which alter #define's and therefore the
    source) must be the same as well.


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