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From: "jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/99396] std::rotl and std::rotr Does not
 convert into ROTATE on the gimple level
Date: Fri, 05 Mar 2021 09:36:25 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D99396

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jakub at gcc dot gnu.org

--- Comment #6 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
There is absolutely no reason why libstdc++ should use any intrinsics for t=
he
rotates, gcc recognizes a lot of patterns into rotates.
Just not the extra verbose one used in libstdc++.
The comment in gcc says:
/* Recognize rotation patterns.  Return true if a transformation
   applied, otherwise return false.

   We are looking for X with unsigned type T with bitsize B, OP being
   +, | or ^, some type T2 wider than T.  For:
   (X << CNT1) OP (X >> CNT2)                           iff CNT1 + CNT2 =3D=
=3D B
   ((T) ((T2) X << CNT1)) OP ((T) ((T2) X >> CNT2))     iff CNT1 + CNT2 =3D=
=3D B

   transform these into:
   X r<< CNT1

   Or for:
   (X << Y) OP (X >> (B - Y))
   (X << (int) Y) OP (X >> (int) (B - Y))
   ((T) ((T2) X << Y)) OP ((T) ((T2) X >> (B - Y)))
   ((T) ((T2) X << (int) Y)) OP ((T) ((T2) X >> (int) (B - Y)))
   (X << Y) | (X >> ((-Y) & (B - 1)))
   (X << (int) Y) | (X >> (int) ((-Y) & (B - 1)))
   ((T) ((T2) X << Y)) | ((T) ((T2) X >> ((-Y) & (B - 1))))
   ((T) ((T2) X << (int) Y)) | ((T) ((T2) X >> (int) ((-Y) & (B - 1))))

   transform these into:
   X r<< Y

   Or for:
   (X << (Y & (B - 1))) | (X >> ((-Y) & (B - 1)))
   (X << (int) (Y & (B - 1))) | (X >> (int) ((-Y) & (B - 1)))
   ((T) ((T2) X << (Y & (B - 1)))) | ((T) ((T2) X >> ((-Y) & (B - 1))))
   ((T) ((T2) X << (int) (Y & (B - 1)))) \
     | ((T) ((T2) X >> (int) ((-Y) & (B - 1))))

   transform these into:
   X r<< (Y & (B - 1))

   Note, in the patterns with T2 type, the type of OP operands
   might be even a signed type, but should have precision B.
   Expressions with & (B - 1) should be recognized only if B is
   a power of 2.  */

but libstdc++ does e.g.
      constexpr auto _Nd =3D __gnu_cxx::__int_traits<_Tp>::__digits;
      const int __r =3D __s % _Nd;
      if (__r =3D=3D 0)
        return __x;
      else if (__r > 0)
        return (__x << __r) | (__x >> ((_Nd - __r) % _Nd));
      else
        return (__x >> -__r) | (__x << ((_Nd + __r) % _Nd)); // rotr(x, -r)
So, can't it e.g. use
      constexpr auto _Nd =3D __gnu_cxx::__int_traits<_Tp>::__digits;
      const auto __r =3D static_cast<unsigned int>(__s);
      return (__x << (__r % _Nd)) | (__x >> ((-__r) % _Nd));
?=