[gcc r11-7542] libstdc++: Improve std::rot[lr] [PR99396]

[gcc r11-7542] libstdc++: Improve std::rot[lr] [PR99396]

[Jakub Jelinek]

https://gcc.gnu.org/g:84185598dc7470bad4e7f8c22b64e3c944efb670

commit r11-7542-g84185598dc7470bad4e7f8c22b64e3c944efb670
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Sat Mar 6 11:11:30 2021 +0100

    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.

Diff:
---
 libstdc++-v3/include/std/bit | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/libstdc++-v3/include/std/bit b/libstdc++-v3/include/std/bit
index 1a6f240e692..8638a02c8a6 100644
--- a/libstdc++-v3/include/std/bit
+++ b/libstdc++-v3/include/std/bit
@@ -68,6 +68,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     __rotl(_Tp __x, int __s) noexcept
     {
       constexpr auto _Nd = __gnu_cxx::__int_traits<_Tp>::__digits;
+      if _GLIBCXX17_CONSTEXPR ((_Nd & (_Nd - 1)) == 0)
+	{
+	  // Variant for power of two _Nd which the compiler can
+	  // easily pattern match.
+	  constexpr unsigned __uNd = _Nd;
+	  const unsigned __r = __s;
+	  return (__x << (__r % __uNd)) | (__x >> ((-__r) % __uNd));
+	}
       const int __r = __s % _Nd;
       if (__r == 0)
 	return __x;
@@ -82,6 +90,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     __rotr(_Tp __x, int __s) noexcept
     {
       constexpr auto _Nd = __gnu_cxx::__int_traits<_Tp>::__digits;
+      if _GLIBCXX17_CONSTEXPR ((_Nd & (_Nd - 1)) == 0)
+	{
+	  // Variant for power of two _Nd which the compiler can
+	  // easily pattern match.
+	  constexpr unsigned __uNd = _Nd;
+	  const unsigned __r = __s;
+	  return (__x >> (__r % __uNd)) | (__x << ((-__r) % __uNd));
+	}
       const int __r = __s % _Nd;
       if (__r == 0)
 	return __x;