Date: Wed, 13 Feb 2008 02:14:40 +0100 From: deeptech71@gmail.com To: freebsd-chat@freebsd.org Subject: bits wrap when leftshifting non-constant amounts Message-ID: <47B24480.9020001@gmail.com>
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My gcc 3.4.6 behaves weirdly when left shifting, and I couldn't find any
info on this.
the program:
#include <stdio.h>
int main( void ) {
unsigned n;
for( n = 13; n < 100; n += 7 ) {
printf( "0x42f1u << %u = %u\n", n, 0x42f1u << n );
}
return 0;
}
the output:
0x42f1u << 13 = 140386304
0x42f1u << 20 = 789577728
0x42f1u << 27 = 2281701376
0x42f1u << 34 = 68548
0x42f1u << 41 = 8774144
0x42f1u << 48 = 1123090432
0x42f1u << 55 = 2021654528
0x42f1u << 62 = 1073741824
0x42f1u << 69 = 548384
0x42f1u << 76 = 70193152
0x42f1u << 83 = 394788864
0x42f1u << 90 = 3288334336
0x42f1u << 97 = 34274
When I left shift a constant amount, it works. That is:
( 1u << 34u ) == 0
But using a variable:
unsigned lsh = 34;
( 1u << lsh ) == 4 !!!
It seems that lsh is first moduloed with the width of int. What the hell?
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