On 14/12/2020 10:28, David Laight wrote:
> 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):
Well, you can do that for the non-bitwise case as well.
In a simpler form but should be enough.
enum { ITER_IOVEC = 1, ITER_BVEC = 2, ... }
if (type <= ITER_BVEC) {
if (iovec) ...
if (bvec) ...
} else { ... }
> 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
> }
--
Pavel Begunkov
