https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99396
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 the
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 == B
((T) ((T2) X << CNT1)) OP ((T) ((T2) X >> CNT2)) iff CNT1 + CNT2 == 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 = __gnu_cxx::__int_traits<_Tp>::__digits;
const int __r = __s % _Nd;
if (__r == 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 = __gnu_cxx::__int_traits<_Tp>::__digits;
const auto __r = static_cast<unsigned int>(__s);
return (__x << (__r % _Nd)) | (__x >> ((-__r) % _Nd));
?
