[PATCH] P0556R3 Integral power-of-2 operations, P0553R2 Bit operations

[Jonathan Wakely]

On 04/07/18 10:26 +0200, Jakub Jelinek wrote:
>On Wed, Jul 04, 2018 at 09:14:04AM +0100, Jonathan Wakely wrote:
>> > Unfortunately it is not correct if _Nd is not power of two.
>> > E.g. for __sN 1, -1U % 20 is 15, not 19.
>> > So it would need to be
>> >      return (__x << __sN) | (__x >> ((_Nd - __sN) % _Nd));
>> > Unfortunately, our rotate pattern recognizer handles
>> >      return (__x << __sN) | (__x >> ((-__sN) % _Nd));
>> > or
>> >      return (__x << __sN) | (__x >> ((-__sN) & (_Nd - 1)));
>> > but doesn't handle the _Nd - __sN case.
>> > Is this C++17+ only?  Then perhaps
>>
>> The std::rotr and std::rotl functions are C++2a only, but I've added
>> the __rotr and __rotl versions for our own internal use in C++14 and
>> later.
>>
>> In practice I have no internal use for rotr and rotl, so I could
>> remove the __rot[rl] forms. However, won't ((_Nd & (_Nd - 1)) optimize
>> to a constant even without if-constexpr? I'll check.
>
>It should, sure.
>
>Anyway, I guess my C tests didn't test properly what happens in your
>functions.  We actually do optimize:
>unsigned long long foo (unsigned long long __x, unsigned int __s)
>{
>  constexpr int _Nd = 64;
>  unsigned int _sN = __s % _Nd;
>  return (__x << _sN) | (__x >> ((_Nd - _sN) % _Nd));
>}
>properly, just don't do it if it is:
>unsigned long long foo (unsigned long long __x, unsigned int __s)
>{
>  int _Nd = 64;
>  unsigned int _sN = __s % _Nd;
>  return (__x << _sN) | (__x >> ((_Nd - _sN) % _Nd));
>}
>Apparently, the (64 - x) & 63 optimization to -x & 63 is only done
>somewhere in the FEs and not in match.pd, which is something we should fix.
>
>But with constexpr _Nd you actually can use
>      return (__x << __sN) | (__x >> ((_Nd - __sN) % _Nd));
>unconditionally.

OK here's what I've just tested and committed to trunk.


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