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

[Pavel Begunkov]

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))

-- 
Pavel Begunkov