Date: Tue, 03 Apr 2007 20:22:15 +0300 From: Andriy Gapon <avg@icyb.net.ua> To: freebsd-hackers@freebsd.org Subject: strange bit-shifting Message-ID: <46128D47.50109@icyb.net.ua>
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$ 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. -- Andriy Gapon
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