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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