block: optimise bvec_iter_advance() (795ee49c) · Commits · 戴 / test

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>

include/linux/bvec.h

+10 −12

Original line number	Diff line number	Diff line
		@@ -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;
		}

Admin message