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

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Message-Id: <200701070525.45974.vda.linux@googlemail.com>
Date:	Sun, 7 Jan 2007 05:25:45 +0100
From:	Denis Vlasenko <vda.linux@...glemail.com>
To:	Linus Torvalds <torvalds@...l.org>
Cc:	Albert Cahalan <acahalan@...il.com>,
	Segher Boessenkool <segher@...nel.crashing.org>, akpm@...l.org,
	linux-kernel@...r.kernel.org, s0348365@....ed.ac.uk,
	bunk@...sta.de, mikpe@...uu.se
Subject: Re: kernel + gcc 4.1 = several problems

On Thursday 04 January 2007 18:37, Linus Torvalds wrote:
> With 7+ million lines of C code and headers, I'm not interested in 
> compilers that read the letter of the law. We don't want some really 
> clever code generation that gets us .5% on some unrealistic load. We want 
> good _solid_ code generation that does the obvious thing.
> 
> Compiler writers seem to seldom even realize this. A lot of commercial 
> code gets shipped with basically no optimizations at all (or with specific 
> optimizations turned off), because people want to ship what they debug and 
> work with.

I'd say "care about obvious, safe optimizations which we still not do".
I want this:

char v[4];
...
	memcmp(v, "abcd", 4) == 0

compile to single cmpl on i386. This (gcc 4.1.1) is ridiculous:

.LC0:
        .string "abcd"
        .text
...
        pushl   $4
        pushl   $.LC0
        pushl   $v
        call    memcmp
        addl    $12, %esp
        testl   %eax, %eax

There are tons of examples where you can improve code generation.
--
vda
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