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>
---
include/linux/bvec.h | 23 ++++++++++++++---------
1 file changed, 14 insertions(+), 9 deletions(-)
diff --git a/include/linux/bvec.h b/include/linux/bvec.h
index a032f01e928c..7b2f05faae14 100644
--- a/include/linux/bvec.h
+++ b/include/linux/bvec.h
@@ -87,26 +87,31 @@ struct bvec_iter_all {
static inline bool bvec_iter_advance(const struct bio_vec *bv,
struct bvec_iter *iter, unsigned bytes)
{
+ unsigned int done = iter->bi_bvec_done;
+ 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;
}
+ iter->bi_size -= bytes;
+
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);
+ const struct bio_vec *cur = bv + idx;
+ unsigned int len = min(bytes, cur->bv_len - done);
bytes -= len;
- iter->bi_size -= len;
- iter->bi_bvec_done += len;
-
- if (iter->bi_bvec_done == cur->bv_len) {
- iter->bi_bvec_done = 0;
- iter->bi_idx++;
+ done += len;
+ if (done == cur->bv_len) {
+ idx++;
+ done = 0;
}
}
+
+ iter->bi_idx = idx;
+ iter->bi_bvec_done = done;
return true;
}
--
2.24.0
