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: Pavel Begunkov <asml.silence@xxxxxxxxx>
---
v2: simplify code (Arvind Sankar)
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;
}
--
2.24.0
