libstdc++: Improve std::rot[lr] [PR99396]

libstdc++: Improve std::rot[lr] [PR99396]

As can be seen on:

unsigned char f1 (unsigned char x, int y) { return std::rotl (x, y); }
unsigned char f2 (unsigned char x, int y) { return std::rotr (x, y); }
unsigned short f3 (unsigned short x, int y) { return std::rotl (x, y); }
unsigned short f4 (unsigned short x, int y) { return std::rotr (x, y); }
unsigned int f5 (unsigned int x, int y) { return std::rotl (x, y); }
unsigned int f6 (unsigned int x, int y) { return std::rotr (x, y); }
unsigned long int f7 (unsigned long int x, int y) { return std::rotl (x, y); }
unsigned long int f8 (unsigned long int x, int y) { return std::rotr (x, y); }
unsigned long long int f9 (unsigned long long int x, int y) { return std::rotl (x, y); }
unsigned long long int f10 (unsigned long long int x, int y) { return std::rotr (x, y); }
//unsigned __int128 f11 (unsigned __int128 x, int y) { return std::rotl (x, y); }
//unsigned __int128 f12 (unsigned __int128 x, int y) { return std::rotr (x, y); }

constexpr auto a = std::rotl (1234U, 0);
constexpr auto b = std::rotl (1234U, 5);
constexpr auto c = std::rotl (1234U, -5);
constexpr auto d = std::rotl (1234U, -__INT_MAX__ - 1);
the current <bit> definitions of std::__rot[lr] aren't pattern recognized
as rotates, they are too long/complex for that, starting with signed modulo,
special case for 0 and different cases for positive and negative.

For types with power of two bits the following patch adds definitions that
the compiler can pattern recognize and turn e.g. on x86_64 into ro[lr][bwlq]
instructions.  For weirdo types like unsigned __int20 etc. it keeps the
current definitions.

2021-03-06  Jakub Jelinek  <jakub@redhat.com>

	PR libstdc++/99396
	* include/std/bit (__rotl, __rotr): Add optimized variants for power of
	two _Nd which the compiler can pattern match the rotates.