From: Al Viro > Sent: 09 January 2021 17:04 > > On Sat, Jan 09, 2021 at 04:09:08PM +0000, Pavel Begunkov wrote: > > On 06/12/2020 16:01, Pavel Begunkov wrote: > > > On 21/11/2020 14:37, Pavel Begunkov wrote: > > >> The problem here is that iov_iter_is_*() helpers check types for > > >> equality, but all iterate_* helpers do bitwise ands. This confuses > > >> compilers, so even if some cases were handled separately with > > >> iov_iter_is_*(), corresponding ifs in iterate*() right after are not > > >> eliminated. > > >> > > >> E.g. iov_iter_npages() first handles discards, but iterate_all_kinds() > > >> still checks for discard iter type and generates unreachable code down > > >> the line. > > > > > > Ping. This one should be pretty simple > > > > Ping please. Any doubts about this patch? > > Sorry, had been buried in other crap. I'm really not fond of the > bitmap use; if anything, I would rather turn iterate_and_advance() et.al. > into switches... That loses any optimisations in the order of the comparisons. The bitmap also allows different groups to be optimised for in different code paths. > How about moving the READ/WRITE part into MSB? Checking is just as fast > (if not faster - check for sign vs. checking bit 0). And turn the > types into straight (dense) enum. Does any code actually look at the fields as a pair? Would it even be better to use separate bytes? Even growing the on-stack structure by a word won't really matter. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
