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

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Date:	Thu, 4 Jan 2007 12:04:18 -0500
From:	"Albert Cahalan" <acahalan@...il.com>
To:	"Segher Boessenkool" <segher@...nel.crashing.org>
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

On 1/4/07, Segher Boessenkool <segher@...nel.crashing.org> wrote:
> > 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).

Right, if "C" means "strictly conforming ISO C" to you.
(in which case, nearly all real-world code is broken)

FYI, the kernel also assumes that a "char" is 8 bits.
Maybe you should run away screaming.

> > 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

Right, it gets much worse with the current gcc snapshots.

IMHO you should play such games with "g++ -O9", but that's
a discussion for a different mailing list.

> "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.

Nope, unless we decide that the performance advantages of
a language change are worth the risk and pain.
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