From 795ee49c1a28d1b3eeb2b463f18d557700fc6153 Mon Sep 17 00:00:00 2001 From: Pavel Begunkov Date: Sat, 30 Nov 2019 23:23:52 +0300 Subject: block: optimise bvec_iter_advance() bvec_iter_advance() is quite popular, but compilers fail to do proper alias analysis and optimise it good enough. The assembly is checked for gcc 9.2, x86-64. - remove @iter->bi_size from min(...), as it's always less than @bytes. Modify at the beginning and forget about it. - the compiler isn't able to collapse memory dependencies and remove writes in the loop. Help it by explicitely using local vars. Signed-off-by: Arvind Sankar Signed-off-by: Pavel Begunkov Signed-off-by: Jens Axboe --- include/linux/bvec.h | 22 ++++++++++------------ 1 file changed, 10 insertions(+), 12 deletions(-) diff --git a/include/linux/bvec.h b/include/linux/bvec.h index a032f01e928c..679a42253170 100644 --- a/include/linux/bvec.h +++ b/include/linux/bvec.h @@ -87,26 +87,24 @@ struct bvec_iter_all { static inline bool bvec_iter_advance(const struct bio_vec *bv, struct bvec_iter *iter, unsigned bytes) { + unsigned int idx = iter->bi_idx; + if (WARN_ONCE(bytes > iter->bi_size, "Attempted to advance past end of bvec iter\n")) { iter->bi_size = 0; return false; } - while (bytes) { - const struct bio_vec *cur = bv + iter->bi_idx; - unsigned len = min3(bytes, iter->bi_size, - cur->bv_len - iter->bi_bvec_done); - - bytes -= len; - iter->bi_size -= len; - iter->bi_bvec_done += len; + iter->bi_size -= bytes; + bytes += iter->bi_bvec_done; - if (iter->bi_bvec_done == cur->bv_len) { - iter->bi_bvec_done = 0; - iter->bi_idx++; - } + while (bytes && bytes >= bv[idx].bv_len) { + bytes -= bv[idx].bv_len; + idx++; } + + iter->bi_idx = idx; + iter->bi_bvec_done = bytes; return true; } -- cgit v1.2.3