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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>
This commit is contained in:
Pavel Begunkov 2019-11-30 23:23:52 +03:00 committed by Jens Axboe
parent 8cdda87a44
commit 795ee49c1a
1 changed files with 10 additions and 12 deletions
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22
include/linux/bvec.h
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@ -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);
iter->bi_size -= bytes;
bytes += iter->bi_bvec_done;
bytes -= len;
iter->bi_size -= len;
iter->bi_bvec_done += len;
while (bytes && bytes >= bv[idx].bv_len) {
bytes -= bv[idx].bv_len;
idx++;
}
if (iter->bi_bvec_done == cur->bv_len) {
iter->bi_bvec_done = 0;
iter->bi_idx++;
}
}
iter->bi_idx = idx;
iter->bi_bvec_done = bytes;
return true;
}