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

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Date:	Thu, 4 Jan 2007 17:43:36 +0100
From:	Segher Boessenkool <segher@...nel.crashing.org>
To:	"Albert Cahalan" <acahalan@...il.com>
Cc:	akpm@...l.org, 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

> 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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