Re: C language lawyers needed

[Al Viro <viro@ZenIV.linux.org.uk>]

On Wed, Aug 27, 2008 at 02:43:05PM -0700, Roland Dreier wrote:
> Can anyone explain why building the current kernel gives
> 
>     drivers/net/mlx4/mcg.c: In function 'mlx4_multicast_attach':
>     drivers/net/mlx4/mcg.c:217: warning: integer overflow in expression
> 
> (with gcc (Ubuntu 4.3.1-9ubuntu1) 4.3.1 on x86-64), while the patch
> below gets rid of the warning?  I can't see what is overflowing in the
> expression that is warned out.
> 
> MGM_BLCK_LB_BIT is 30, and 1 << 30 is not negative or close to
> overflowing.  And MGM_QPN_MASK is 0x00FFFFFF, which is also nowhere near
> to integer overflow.
> 
> 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...

There isn't.  It's gcc being buggy and inconsistent.