Gentoo Archives: gentoo-dev

From: Matt Tucker <tuck@×××××××××××××.net>
To: Sven Vermeulen <swift@g.o>, gentoo-dev@g.o
Subject: Re: [gentoo-dev] -fbranch-probabilities optimisation
Date: Sat, 17 May 2003 12:30:57
In Reply to: Re: [gentoo-dev] -fbranch-probabilities optimisation by Sven Vermeulen
1 -- Sven Vermeulen <swift@g.o> spake thusly:
3 > On Sat, May 17, 2003 at 03:49:11AM +0100, Chris Davies wrote:
4 >> I had the idea to build -fbranch-probabilities optimisation into
5 >> portage. This is where a sample run of data is taken using the
6 >> program compiled with -fprofile-arcs, and that data then used to
7 >> reorganise the object code so conditional branches are layed out in
8 >> a more efficient manner.
9 >
10 > Why implement it in Portage? Isn't it up to the user to define it in
11 > the CFLAGS or not?
13 This isn't a simple matter of turning on a compiler flag.
15 >From 'man gcc':
17 After running a program compiled with -fprofile-arcs, you can
18 compile it a second time using -fbranch-probabilities, to
19 improve optimizations based on the number of times each branch
20 was taken. When the program compiled with -fprofile-arcs exits
21 it saves arc execution counts to a file called sourcename.da for
22 each source file The information in this data file is very
23 dependent on the structure of the generated code, so you must
24 use the same source code and the same optimization options for
25 both compilations.
27 However, the problem with trying to integrate this into portage is that
28 it must either:
30 1) Compile programs on the user's machine, allow the user to run
31 them, and then rebuild using the generated data. Quite a
32 lengthy process, and fraught with issues.
34 or
36 2) Distribute the profile data along with the source code. The
37 trouble with this is that (as the man page says) it requires
38 using the same source code and optimization options. So not only
39 must the CFLAGS used be the same as with the profiled code, but
40 the configure settings (which alter #define's and therefore the
41 source) must be the same as well.
44 --
45 gentoo-dev@g.o mailing list