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References: <787b0d920701032311l2c37c248s3a97daf111fe88f3@mail.gmail.com>
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Cc: akpm@osdl.org, linux-kernel@vger.kernel.org, s0348365@sms.ed.ac.uk,
       bunk@stusta.de, mikpe@it.uu.se, torvalds@osdl.org
From: Segher Boessenkool <segher@kernel.crashing.org>
Subject: Re: kernel + gcc 4.1 = several problems
Date: Thu, 4 Jan 2007 17:43:36 +0100
To: "Albert Cahalan" <acahalan@gmail.com>
Sender: linux-kernel-owner@vger.kernel.org
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> Adjusting gcc flags to eliminate optimizations is another way to go.
> Adding -fwrapv would be an excellent start. Lack of this flag breaks
> most code which checks for integer wrap-around.

Lack of the flag does not break any valid C code, only code
making unwarranted assumptions (i.e., buggy code).

> The compiler "knows"
> that signed integers don't ever wrap, and thus eliminates any code
> which checks for values going negative after a wrap-around.

You cannot assume it eliminates such code; the compiler is free
to do whatever it wants in such a case.

You should typically write such a computation using unsigned
types, FWIW.

Anyway, with 4.1 you shouldn't see frequent problems due to
"not using -fwrapv while my code is broken WRT signed overflow"
yet; and if/when problems start to happen, to "correct" action
to take is not to add the compiler flag, but to fix the code.


Segher

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