Re: [PATCH] Fix RTL simplifications of FFS, POPCOUNT and PARITY.

[Jeff Law <jeffreyalaw@gmail.com>]

On 1/2/23 10:22, Jakub Jelinek wrote:
> On Mon, Jan 02, 2023 at 09:20:33AM -0700, Jeff Law wrote:
>>>> In fact Raphael and I were about to submit a patch which takes advantage of
>>>> that capability to improve the code slightly for risc-v.
>>>
>>> Just use a pattern with zero_extend or sign_extend around it or subreg of
>>> it?
>> If it were only that easy ;(  In the bowels of the simplifications the
>> zero_extension turns into either a pair of shifts or an AND with a mask (I
>> forget which offhand).  I'm sure we *can* work around this in the target,
>> but it'll be ugly.
>>
>> The documentation definitely needs to be updated.  I looked at the whole
>> family a few weeks ago and my recollection was they all need to be fixed
>> (ffs, clrsb, clz, ctz, popcount & parity) if the defined semantics are that
>> the input and output operand modes must match.
> 
> When I look at the documentation of all the above, all of them have
> "The mode of @var{x} must be @var{m} or @code{VOIDmode}."
M is the mode on the insn (ie clzsi2).  I don't see a reference to
                                  ^^
@var{x}.  Weird.

jeff