On 09/01/2021 21:49, David Laight wrote: > 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. You still can have a fast path and even retoss ITER_* for convenience. Other use cases are not important at the current state. > >> 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. u8 type, rw; That won't bloat the struct. I like the idea. If used together compilers can treat it as u16. btw there is a 4B hole just after for x64. -- Pavel Begunkov
