Re: [gentoo-dev] Re: ICC Profile

[Ciaran McCreesh]

On Fri, 18 Jul 2008 15:34:53 -0400
Richard Freeman <rich0@g.o> wrote:
> Ciaran McCreesh wrote:
> > The more interesting question, then, is whether users run any
> > non-trivial cpu-bound programs. We know the applied science types
> > do, but they tend to be the ones who're doing clever things with icc
> > anyway. What about normal users?
>
> I'm sure they do on some occasion if they encode compressed
> audio/video, or when compressing data with zip/etc.  That is probably
> the biggest application of cpu-bound software.

How much of that is memory bound? Of the things that aren't, how many
aren't written in assembly anyway? Of those things, what proportion of
the runtime is spent in those areas?

If you double the speed of something that takes up 2% of the overall
execution time, you can't measure the improvement.

Or looking at it the other way -- is there any reason to believe that
using icc (which can end up being a substantial pain in the arse, given
the way it tries to use gcc's c++ headers but doesn't support some of
the extensions or quirks that g++ does) will provide a genuine gain
for people who aren't already doing clever profile-directed trickery
anyway?

> I'd probably benefit from using -O3 on the aforementioned
> CPU-intensive apps.

The problem with -O3 is that function inlining can lead to a
substantial cache hit. Unless you're using profile-directed
optimisations, which Gentoo doesn't support, it's extremely hit and
miss as to whether O3 helps or hurts.

-- 
Ciaran McCreesh