linux-kernel - Re: C language lawyers needed

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Message-ID: <48B5CE61.8020809@zytor.com>
Date:	Wed, 27 Aug 2008 15:00:01 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Roland Dreier <rdreier@...co.com>
CC:	linux-kernel@...r.kernel.org
Subject: Re: C language lawyers needed

Roland Dreier wrote:
> 
> A fairly small test case that I don't understand either is:
> 
> unsigned foo(int x)
> {
> 	return (((x & 0xffffff) | (1 << 30)) & 0xff000000) >> 24;
> }
> 
> just running "gcc -c" (ie no extra warnings enabled) on that produces
> the same:
> 
>     b.c: In function 'foo':
>     b.c:3: warning: integer overflow in expression
> 
> I'm sure there's some promotion rule or something that makes sense of
> this, but it's a mystery to me...
> 

Looks like a gcc bug to me.

0xff000000 is unsigned, like any hexadecimal constant.

unsigned foo(int x)
{
	return ((x & 0xffffff) | (1 << 30)) & 0x80000000;
}

... is enough to reproduce the bug -- explicitly casting either side or 
both of the & operator to unsigned doesn't affect the warning, either.

	-hpa
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