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: NArvind Sankar <nivedita@alum.mit.edu> Signed-off-by: NPavel Begunkov <asml.silence@gmail.com> Signed-off-by: NJens Axboe <axboe@kernel.dk>
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