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

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Date:	Thu, 4 Jan 2007 19:53:59 +0100
From:	Segher Boessenkool <segher@...nel.crashing.org>
To:	Linus Torvalds <torvalds@...l.org>
Cc:	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

>>> (in which case, nearly all real-world code is broken)
>>
>> Not "nearly all" -- but lots of code, yes.
>
> I wouldn't say "lots of code". I would say "all real projects".

All projects that tell the compiler they're written in ISO C,
while they're not, can easily break, sure.  You can't say this
is GCC's fault; sure in some cases decisions were made that
resulted in more of those programs breaking than was really
necessary, but it's obviously *impossible* to prevent all
from breaking.

And yes it's true: most people do not program in ISO C at all,
_even if they think they do_, simply because they are not aware
of all the rules.  For some of the areas where most of the
mistakes are made, for example aliasing rules and signed overflow,
GCC provides helpful options to switch behaviour to something
that makes those people's programs work.  You can also use those
options if you have made a conscious decision that you want to
write your code in one of the resulting dialects of C.

Segher

p.s.  If it's decided to not use -fwrapv, a debug option that
sets -ftrapv can be introduced -- it will make it a BUG() if
any (accidental) signed overflow happens after all.

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