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From: Jakub Jelinek <jakub@gcc.gnu.org> To: gcc-cvs@gcc.gnu.org, libstdc++-cvs@gcc.gnu.org Subject: [gcc r11-7542] libstdc++: Improve std::rot[lr] [PR99396] Date: Sat, 6 Mar 2021 10:12:48 +0000 (GMT) [thread overview] Message-ID: <20210306101248.4270C388C025@sourceware.org> (raw) 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;
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