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SubjectRe: [PATCH] block: simplify blksize_bits() implementation
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On 10/26/22 08:14, Dawei Li wrote:
> Convert current looping-based implementation into bit operation,
> which can bring improvement for:
>
> 1) bitops is more efficient for its arch-level optimization.

As far as I know blksize_bits() is not used in the hot path so
performance of this function is not critical.

> 2) Given that blksize_bits() is inline, _if_ @size is compile-time
> constant, it's possible that order_base_2() _may_ make output
> compile-time evaluated, depending on code context and compiler behavior.
>
> Signed-off-by: Dawei Li <set_pte_at@outlook.com>
> ---
> include/linux/blkdev.h | 7 +------
> 1 file changed, 1 insertion(+), 6 deletions(-)
>
> diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
> index 50e358a19d98..117061c8b9a1 100644
> --- a/include/linux/blkdev.h
> +++ b/include/linux/blkdev.h
> @@ -1349,12 +1349,7 @@ static inline int blk_rq_aligned(struct request_queue *q, unsigned long addr,
> /* assumes size > 256 */
> static inline unsigned int blksize_bits(unsigned int size)
> {
> - unsigned int bits = 8;
> - do {
> - bits++;
> - size >>= 1;
> - } while (size > 256);
> - return bits;
> + return size > 512 ? order_base_2(size) : 9;
> }

How about optimizing this function even further by eliminating the
ternary operator, e.g. as follows (untested)?

return order_base_2(size >> SECTOR_SHIFT) + SECTOR_SHIFT;

Thanks,

Bart.

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