Date: Tue, 3 Apr 2007 20:02:53 +0200 From: Stefan Farfeleder <stefan@fafoe.narf.at> To: Andriy Gapon <avg@icyb.net.ua> Cc: freebsd-hackers@freebsd.org Subject: Re: strange bit-shifting Message-ID: <20070403180253.GG907@lizard.fafoe.narf.at> In-Reply-To: <46128D47.50109@icyb.net.ua> References: <46128D47.50109@icyb.net.ua>
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On Tue, Apr 03, 2007 at 08:22:15PM +0300, Andriy Gapon wrote:
>
> $ cat test_shl.c
> #include <stdint.h>
> #include <stdio.h>
>
> int main()
> {
> uint64_t l;
>
> l = 0;
> l--;
> printf("%.16lX\n", l);
> l <<= 64;
> printf("%.16lX\n", l);
> return 0;
> }
>
> $ cc test_shl.c -o test_shl
> test_shl.c: In function `main':
> test_shl.c:11: warning: left shift count >= width of type
> $ ./test_shl
> FFFFFFFFFFFFFFFF
> FFFFFFFFFFFFFFFF
> $ uname -srm
> FreeBSD 6.2-RELEASE-p2 amd64
> $ gcc -v
> Using built-in specs.
> Configured with: FreeBSD/amd64 system compiler
> Thread model: posix
> gcc version 3.4.6 [FreeBSD] 20060305
>
> What gives ? It looks like shift is actually done not by specified
> number of bits but by that number modulo 64.
> Please also mind that the same thing happens if I use a variable instead
> of a constant in that expression.
The behaviour is undefined and you even got a warning from GCC.
C99 6.5.7:
# The integer promotions are performed on each of the operands. The type
# of the result is that of the promoted left operand. If the value of
# the right operand is negative or is greater than or equal to the width
# of the promoted left operand, the behavior is undefined.
Stefan
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