Re: [gentoo-dev] CFLAGS moved to ebuilds.

[Robin H. Johnson]

On Tue, Dec 09, 2003 at 11:17:23PM +0100, John Nilsson wrote:
> >Thats the express purpose that genflags was created for, to provide
> >users with a known good set of high-performance CFLAGS so they didn't
> >need to mess around with it too much.
> Still there is no room for improvement when dealing with system wide
> optimization.
Your wording here is unclear, I think you mean to say that there IS room for
improvement over system wide constant CFLAGS?

> >strip-flags to remove problematic flags on a per ebuild basis is the
> >best solution. I do agree that unstable gcc settings are a big
> >problem, eg in a recent bug it turned out the submitter's system (an
> >older Pentium I) couldn't handle -O3 without flaking out. Reduce it
> >to -O2 and the box went fine (both for compiling and already compiled
> >packages).
> This is a bug in GCC. While a workaround may be a quick solution for
> Gentoo, one shouldn't base the whole system on bugs.
No it isn't a bug in GCC, it's a bug with the user's specific hardware.
I have an older Pentium I system that runs just fine with -O3 and the
user's specified CFLAGS. I didn't force everybody to use -O2, I just got
that user to change his own system down to -O2.

> >again, genflags was created for this. I've considered a sequel to
> >genflags based on the genetic optimization of compiler flags as
> >mentioned on Slashdot a while ago, but for lack of time, i'm not even
> >looking at doing it now.
> You might want to chek:
> http://www.coyotegulch.com/potential/gccga/gccga.html
This is the original item I was referencing, but you still run into the
problem that you need to run things on a system basis to get effective
results.

http://www.coyotegulch.com/acovea/index.html is the rest of the article,

> http://www.rocklinux.net/packages/ccbench.html
This basically brute forces the genetic algorithms, with absolutely no
thought as to the net effects on the results of the given flags, eg, on
my home server (an AthlonXP 2400+), it returns these results:
gcc -O3 -march=athlon -fomit-frame-pointer -funroll-loops
-frerun-loop-opt -funroll-all-loops -fschedule-insns

Of that, '-frerun-loop-opt' and '-fschedule-insns' are redundant as they
are implied by -O3.

-fomit-frame-pointer and I can't debug code properly anymore, and if I
try to use -funroll-all-loops to compile mysql, even with it's
--with-low-memory option, gcc wants 600mb of memory to compile it's
sql_yacc.cc.

> I meant by evolution: the process of users submiting patches to improve
> individual ebuilds.
What improves the performance of a given application on one machine does
NOT nessicary improve it on another machine.

Read the gcc manpage and see:
-fprofile-arcs
-fbranch-probabilities
(also read http://gcc.gnu.org/news/profiledriven.html)

Just adding these to ccbench doubles the amount of time taken to
test (as you must compile with -fprofile-arcs, run, compile with
-fbranch-probabilities, run again). It also provides some extremely
interesting and varying results. The bubblesort test for example,
improves between +15% and +300% depending on the other compiler flags.
Towers of Hanoi goes from -20% to +50%.

If users submitted _good_ non-interactive testcases for every ebuild, it
wouldn't difficult to apply -fprofile-arcs/branch-probabilities and or
acovea to most packages at all, apart from the massive increase in
compile time.

> >Stable and high-performance is an per-system definition, as evidenced
> >by the bug I mentioned with -O3.
> And should as such be fixed... in gcc. If gcc cant optimize correct
> knowing the cache size of the cpu, gcc is broken. Fix gcc.
Again, it isn't a gcc bug, it's an issue with a specific machine (not
even a class of systems or cpus).

Lets take a tangent on this whole issue for a moment. Ignoring the
implementation concerns, the end goal of your GLEP is this:
The basic gain you want, is for the support of per-package CFLAG
modifications (inside the ebuilds), for the purpose of performance
optimization.

Do I have this correct?

-- 
Robin Hugh Johnson
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