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Re: GCSE store motion
- From: Mark Mitchell <mark at codesourcery dot com>
- To: Robert Dewar <dewar at gnat dot com>, "dberlin at dberlin dot org" <dberlin at dberlin dot org>, "roger at eyesopen dot com" <roger at eyesopen dot com>
- Cc: "aj at suse dot de" <aj at suse dot de>, "davem at redhat dot com" <davem at redhat dot com>, "gcc at gcc dot gnu dot org" <gcc at gcc dot gnu dot org>, "rth at redhat dot com" <rth at redhat dot com>
- Date: Thu, 16 May 2002 07:34:33 -0700
- Subject: Re: GCSE store motion
- References: <20020516114838.949B6F28C9@nile.gnat.com>
--On Thursday, May 16, 2002 07:48:38 AM -0400 Robert Dewar <firstname.lastname@example.org>
>> That means we shouldn't be spending much time trying to do software
>> loop pipelining when compiling GCC, so the optimization shouldn't
>> make compiling the compiler significantly slower.
> I don't see how you conclude this. You have to do the analysis on every
> loop. There will definitely be loops in GCC where the optimization is
> possible, there will be loops where it is not. I would expect the
> compiler to spend quite a bit of time trying to improve code for
> loops in GCC. What I am saying is that I doubt that the overall
> effect will be that benficial for GCC.
I think we're wandering into a good pub conversation, rather than a useful
debate about criteria for accepting changes in the compiler, but...
There aren't very many loops in GCC; certainly nowhere near as many (per
line of code) as in your average scientific application, say. And, a
little heuristicness (most of these loops have lots of internal branches
and even function calls, therefore they're less liikely to be hot spots,
therefore I won't spend a lot of effor trying to wring a few cycles out
of them), doesn't seem unreasonable to me.
I picked a random GCC file (regmove.c) and counted 127 lines with the
word "for", 8 with "while", and 54 with "do" out of 73694 lines. (That's
probably an over-estimate; I'd bet some of those show up in comments.)
How much time can/should the compiler waste compiling those relatively
My point is that I can well imagine the compiler spending 10% of its time
on software pipelining for scientific code, but that would seem highly
out of whack on code like that in GCC.
Mark Mitchell email@example.com
CodeSourcery, LLC http://www.codesourcery.com