Re: New filesystem for Linux

On Fri, Nov 03, 2006 at 06:36:09PM +0100, Oleg Verych wrote:
On Fri, Nov 03, 2006 at 06:09:39PM +0100, Mikulas Patocka wrote:
In gmane.linux.kernel, you wrote:
[]
From: Andrew Morton <akpm@xxxxxxxx>

As Mikulas points out, (1 << anything) won't be evaluating to zero.

How about integer overflow ?

C standard defines that shifts by more bits than size of a type are
undefined (in fact 1<<32 produces 1 on i386, because processor uses only 5
bits of a count).
,--
|#include <stdio.h>
|int main(void) {
| unsigned int b = 1;
|
| printf("%u\n", (1 << 33));
| printf("%u\n", (b << 33));
| return 0;
|}
|$ gcc bit.c && ./a.out
`--

There *is* difference, isn't it?

It's undefined, and the results with your example depend on the gcc
version and optimization level.

E.g. with gcc 4.1, there is *no* difference any more if you turn on
optimization.

cu
Adrian

--

"Is there not promise of rain?" Ling Tan asked suddenly out
of the darkness. There had been need of rain for many days.
"Only a promise," Lao Er said.
Pearl S. Buck - Dragon Seed

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