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

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Message-Id: <7171DDC6-A144-4DFD-96F1-B2DEEF09C5B0@adacore.com>
Date:	Thu, 4 Jan 2007 17:02:48 -0500
From:	Geert Bosch <bosch@...core.com>
To:	Segher Boessenkool <segher@...nel.crashing.org>
Cc:	Linus Torvalds <torvalds@...l.org>, akpm@...l.org,
	Albert Cahalan <acahalan@...il.com>,
	linux-kernel@...r.kernel.org, s0348365@....ed.ac.uk,
	bunk@...sta.de, mikpe@...uu.se
Subject: Re: kernel + gcc 4.1 = several problems


On Jan 4, 2007, at 13:34, Segher Boessenkool wrote:

> The "signed wrap is undefined" thing doesn't fit in this category
> though:
>
> -- It is an important optimisation for loops with a signed
>    induction variable;

It certainly isn't that important. Even SpecINT compiled with
-O3 and top-of-tree GCC *improves* 1% by adding -fwrapv.
If the compiler itself can rely on wrap-around semantics and
doesn't have to worry about introducing overflows between
optimization passes, it can reorder simple chains of additions.
This is more important for many real-world applications than
being able to perform some complex loop-interchange.
Compiler developers always make the mistake of overrating
their optimizations.

If GCC does really poorly on a few important loops that matter,
that issue is easily addressed. If GCC generates unreliable
code for millions of boring lines of important real-world C,
the compiler is worthless.

   -Geert
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