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Micro-optimize scst_calc_block_shift()

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Replace a loop by a single instruction, namely bsr (bit scan reverse).
From the disassembler output of scst_calc_block_shift() on an x86_64
system:

   0x0000000000001493 <+35>:    bsr    %edx,%ecx

Signed-off-by: Bart Van Assche <bvanassche@acm.org>



git-svn-id: http://svn.code.sf.net/p/scst/svn/trunk@4134 d57e44dd-8a1f-0410-8b47-8ef2f437770f
This commit is contained in:
Vladislav Bolkhovitin
2012-02-17 01:43:21 +00:00
parent 659e6ea6e8
commit ab25cb45dc
1 changed files with 4 additions and 9 deletions
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13
scst/src/scst_lib.c
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@@ -5377,19 +5377,14 @@ out_err:
*/
int scst_calc_block_shift(int sector_size)
{
int block_shift = 0;
int t;
int block_shift;
if (sector_size == 0)
sector_size = 512;
t = sector_size;
while (1) {
if ((t & 1) != 0)
break;
t >>= 1;
block_shift++;
}
block_shift = ilog2(sector_size);
WARN_ON(1 << block_shift != sector_size);
if (block_shift < 9) {
PRINT_ERROR("Wrong sector size %d", sector_size);
block_shift = -1;