On Sep 6, 2011, at 11:19 PM, Ron Wilson wrote:
>
> The asm() calls are being used to invoke the rotate instructions. C
> does not have operators for peforming bitwise rotation of an operand
> and emulating a rotate using shifts is messy and very ineficient:
>
> unsigned int rotateLeft(unsigned int x, unsigned int n)
> {
> unsigned int t;
>
> t = x >> (SIZEOFINT - n);
> return ((x << n) | t);
> }
FYI,
$ gcc -O2 -S rotate.c
$ cat rotate.s
…
_rotateLeft:
Leh_func_begin1:
pushq %rbp
Ltmp0:
movq %rsp, %rbp
Ltmp1:
movb %sil, %cl
movl %edi, %eax
roll %cl, %eax
popq %rbp
ret
Leh_func_end1:
..
$ gcc --version
i686-apple-darwin11-llvm-gcc-4.2 (GCC) 4.2.1 (Based on Apple Inc. build 5658)
(LLVM build 2335.15.00)
(This is GCC with LLVM backend (default on OS X Lion)).
--
Dmitry Chestnykh
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