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authorPavel Begunkov2019-11-30 23:23:52 +0300
committerJens Axboe2019-12-03 07:36:38 -0700
commit795ee49c1a28d1b3eeb2b463f18d557700fc6153 (patch)
tree37a0074d7559534cd68eb5d58883f5ab27f3bf7d
parent8cdda87a4414092cd210e766189cf0353a844861 (diff)
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 <nivedita@alum.mit.edu> Signed-off-by: Pavel Begunkov <asml.silence@gmail.com> Signed-off-by: Jens Axboe <axboe@kernel.dk>
-rw-r--r--include/linux/bvec.h22
1 files 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;
}