Gentoo Archives: gentoo-dev

From: Anupam Kapoor <anupamk@×××××××××.net>
To: Chris Davies <c.davies@×××××××.org>
Cc: gentoo-dev@g.o
Subject: Re: [gentoo-dev] -fbranch-probabilities optimisation
Date: Sat, 17 May 2003 07:52:14
Message-Id: 87y91675tu.fsf@speakeasy.net
In Reply to: [gentoo-dev] -fbranch-probabilities optimisation by Chris Davies
hi,

i was under the assumption that most processors already perform branch
prediction. no ? do you think that -fbranch-probabilities provides a
more 'comprehensive' view of the executing program ?

kind regards
anupam

> Hi, > > 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. > > To give an idea about how much time this optimisation actually saves, > I ran a test with bladeenc (an MP3 encoder) encoding an entire album > of wav files. The CFLAGS used only differed by a the > -fbranch-probabilities, and the test was run 5 times to get an > average. The result was that the version with no branch data took > 8.35.106s on average to complete the encoding, whereas the the > optimised version took only 8.11.502s. The branch data was 64KBs in > total. > > Does this sound like something we could work into portage? I initially > had the idea of building patches for every package likely to be > improved by this optimisation (mainly CPU bound ones) and just > applying them to the package before compilation. The user could then > choose wether or not to actually use the data by including or > excluding -fbranch-probabilities in make.conf. > > Obviously I'd have to work up some kind of test rig to automatically > generate arc data for packages, but that shouldn't be too much of a > problem as long as we keep the number of packages under control. The > only problem I can see is that -fbranch-probabilities spits out ugly > warnings if no arc data is present, although in this situation gcc > defaults back to it's standard behaviour (I think). > > Thoughts? Opinions? Thanks, C.Davies
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Replies

Subject Author
Re: [gentoo-dev] -fbranch-probabilities optimisation Evan Powers <powers.161@×××.edu>