RE: [PATCH 2/2] iov_iter: optimise iter type checking

[David Laight <David.Laight@ACULAB.COM>]

From: Pavel Begunkov
> Sent: 13 December 2020 22:33
> 
> On 11/12/2020 02:01, Al Viro wrote:
> > On Thu, Nov 19, 2020 at 05:12:44PM +0000, Pavel Begunkov wrote:
> >> On 19/11/2020 17:03, Christoph Hellwig wrote:
> >>> On Thu, Nov 19, 2020 at 03:29:43PM +0000, 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
> >>>> a compiler, so even if some cases were handled separately with
> >>>> iov_iter_is_*(), it can't eliminate and skip unreachable branches in
> >>>> following iterate*().
> >>>
> >>> I think we need to kill the iov_iter_is_* helpers, renumber to not do
> >>> the pointless bitmask and just check for equality (might turn into a
> >>> bunch of nice switch statements actually).
> >>
> >> There are uses like below though, and that would also add some overhead
> >> on iov_iter_type(), so it's not apparent to me which version would be
> >> cleaner/faster in the end. But yeah, we can experiment after landing
> >> this patch.
> >>
> >> if (type & (ITER_BVEC|ITER_KVEC))
> >
> > There are exactly 3 such places, and all of them would've been just as well
> > with case ITER_BVEC: case ITER_KVEC: ... in a switch.
> >
> > Hmm...  I wonder which would work better:
> >
> > enum iter_type {
> >         ITER_IOVEC = 0,
> >         ITER_KVEC = 2,
> >         ITER_BVEC = 4,
> >         ITER_PIPE = 6,
> >         ITER_DISCARD = 8,
> > };
> > iov_iter_type(iter)	(((iter)->type) & ~1)
> > iov_iter_rw(iter)	(((iter)->type) & 1)
> >
> > or
> >
> > enum iter_type {
> >         ITER_IOVEC,
> >         ITER_KVEC,
> >         ITER_BVEC,
> >         ITER_PIPE,
> >         ITER_DISCARD,
> > };
> > iov_iter_type(iter)	(((iter)->type) & (~0U>>1))
> > // callers of iov_iter_rw() are almost all comparing with explicit READ or WRITE
> > iov_iter_rw(iter)	(((iter)->type) & ~(~0U>>1) ? WRITE : READ)
> > with places like iov_iter_kvec() doing
> > 	i->type = ITER_KVEC | ((direction == WRITE) ? BIT(31) : 0);
> >
> > Preferences?
> 
> For the bitmask version (with this patch) we have most of
> iov_iter_type() completely optimised out. E.g. identical
> 
> iov_iter_type(i) & ITER_IOVEC <=> iter->type & ITER_IOVEC
> 
> It's also nice to have iov_iter_rw() to be just
> (type & 1), operations with which can be optimised in a handful of ways.
> 
> Unless the compiler would be able to heavily optimise switches,
> e.g. to out-of-memory/calculation-based jump tables, that I doubt,
> I'd personally leave it be. Though, not like it should matter much.

The advantage of the bit-masks is that the 'usual' options can
be tested for together. So the code can be (for example):
	if (likely(iter->type & (ITER_IOVEC | ITER_PIPE) {
		if (likely((iter->type & ITER_IOVEC)) {
			... code for iovec
		} else [
			... code for pipe
		}
	} else if (iter->type & ITER_BVEC) {
		... code for bvec
	} else if (iter->type & ITER_KVEC) {
		.. code for kvec
	} else {
		.. must be discard
	}

I'm not sure of the best order though.
You might want 3 bits in the first test.

	David

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