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?
