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
In Reply to: [gentoo-dev] -fbranch-probabilities optimisation by Chris Davies
1 hi,
3 i was under the assumption that most processors already perform branch
4 prediction. no ? do you think that -fbranch-probabilities provides a
5 more 'comprehensive' view of the executing program ?
7 kind regards
8 anupam
10 > Hi,
11 >
12 > I had the idea to build -fbranch-probabilities optimisation into
13 > portage. This is where a sample run of data is taken using the program
14 > compiled with -fprofile-arcs, and that data then used to reorganise
15 > the object code so conditional branches are layed out in a more
16 > efficient manner.
17 >
18 > To give an idea about how much time this optimisation actually saves,
19 > I ran a test with bladeenc (an MP3 encoder) encoding an entire album
20 > of wav files. The CFLAGS used only differed by a the
21 > -fbranch-probabilities, and the test was run 5 times to get an
22 > average. The result was that the version with no branch data took
23 > 8.35.106s on average to complete the encoding, whereas the the
24 > optimised version took only 8.11.502s. The branch data was 64KBs in
25 > total.
26 >
27 > Does this sound like something we could work into portage? I initially
28 > had the idea of building patches for every package likely to be
29 > improved by this optimisation (mainly CPU bound ones) and just
30 > applying them to the package before compilation. The user could then
31 > choose wether or not to actually use the data by including or
32 > excluding -fbranch-probabilities in make.conf.
33 >
34 > Obviously I'd have to work up some kind of test rig to automatically
35 > generate arc data for packages, but that shouldn't be too much of a
36 > problem as long as we keep the number of packages under control. The
37 > only problem I can see is that -fbranch-probabilities spits out ugly
38 > warnings if no arc data is present, although in this situation gcc
39 > defaults back to it's standard behaviour (I think).
40 >
41 > Thoughts? Opinions? Thanks, C.Davies
44 --
45 gentoo-dev@g.o mailing list


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