linux-kernel - Re: kernel + gcc 4.1 = several problems

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Message-Id: <ea5b255f38ef273251d6af7c8ead65fc@kernel.crashing.org>
Date:	Sat, 6 Jan 2007 09:23:01 +0100
From:	Segher Boessenkool <segher@...nel.crashing.org>
To:	Pavel Machek <pavel@....cz>
Cc:	akpm@...l.org, Albert Cahalan <acahalan@...il.com>,
	linux-kernel@...r.kernel.org, s0348365@....ed.ac.uk,
	bunk@...sta.de, mikpe@...uu.se, torvalds@...l.org
Subject: Re: kernel + gcc 4.1 = several problems

>> For a different mailing list indeed; let me just point
>> out
>> that for certain important quite common cases it's an
>> ~50%
>> overall speedup.
>
> Hmm, what code was that? 'signed int does not wrap around' does not
> seem to provide _that_ much info...

One of the recent huge threads on the GCC dev list has a
post that says *some other* compiler gets a result like
this from this optimisation (I don't have a link to the
exact post and I don't remember the details; perhaps it
was XLC?)

Sorry if I wasn't clear enough and you understood I meant
that GCC exploits this optimisation opportunity well
enough for such nice results already.

  - - -

So I searched for it anyway:

<http://gcc.gnu.org/ml/gcc/2006-12/msg00768.html>

It looks like the result for *integer* code wasn't *all*
that dramatic a difference.  Anyway, it's obvious that
the optimisation can certainly give nice results and it
wouldn't be a good idea for the Linux kernel to dismiss
it without really evaluating the impact first; and anyway,
this is for some future date, GCC-4.2 isn't here yet.


Segher

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