Messages in this thread | | | Date | Wed, 27 Aug 2008 23:06:52 +0100 | From | Al Viro <> | Subject | Re: C language lawyers needed |
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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.
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