From 1603fafcf8cd45da71dd6368d48a60abafc60c3b Mon Sep 17 00:00:00 2001
From: Alex Guteniev <gutenev@gmail.com>
Date: Wed, 1 Dec 2021 21:45:55 +0200
Subject: [PATCH 1/3] make sure we are rotating in the right direction

---
 .../test.cpp                                  | 24 +++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/tests/std/tests/P0553R4_bit_rotating_and_counting_functions/test.cpp b/tests/std/tests/P0553R4_bit_rotating_and_counting_functions/test.cpp
index 95eb50bd0c3..07a57e53a58 100644
--- a/tests/std/tests/P0553R4_bit_rotating_and_counting_functions/test.cpp
+++ b/tests/std/tests/P0553R4_bit_rotating_and_counting_functions/test.cpp
@@ -86,9 +86,21 @@ template <typename T>
 constexpr bool test_rotl() {
     constexpr int digits  = numeric_limits<T>::digits;
     constexpr auto maxval = numeric_limits<T>::max();
+
+    constexpr T every_fourth[4] = {
+        static_cast<T>(0x1111'1111'1111'1111ULL),
+        static_cast<T>(0x2222'2222'2222'2222ULL),
+        static_cast<T>(0x4444'4444'4444'4444ULL),
+        static_cast<T>(0x8888'8888'8888'8888ULL),
+    };
+
     for (int i = 0; i < digits * 2; ++i) {
         assert(rotl(maxval, i) == maxval);
         assert(rotl(T{0}, i) == 0);
+
+        for (int j = 0; j < 4; ++j) {
+            assert(rotl(every_fourth[j], i) == every_fourth[(i + j) % 4]);
+        }
     }
     return true;
 }
@@ -97,9 +109,21 @@ template <typename T>
 constexpr bool test_rotr() {
     constexpr int digits  = numeric_limits<T>::digits;
     constexpr auto maxval = numeric_limits<T>::max();
+
+    constexpr T every_fourth[4] = {
+        static_cast<T>(0x1111'1111'1111'1111ULL),
+        static_cast<T>(0x2222'2222'2222'2222ULL),
+        static_cast<T>(0x4444'4444'4444'4444ULL),
+        static_cast<T>(0x8888'8888'8888'8888ULL),
+    };
+
     for (int i = 0; i < digits * 2; ++i) {
         assert(rotr(maxval, i) == maxval);
         assert(rotr(T{0}, i) == 0);
+
+        for (int j = 0; j < 4; ++j) {
+            assert(rotr(every_fourth[3 - j], i) == every_fourth[3 - ((i + j) % 4)]);
+        }
     }
     return true;
 }

From bf7b413ddd60da16992ae539b01fc7314d4ce4b9 Mon Sep 17 00:00:00 2001
From: Alex Guteniev <gutenev@gmail.com>
Date: Wed, 1 Dec 2021 21:46:38 +0200
Subject: [PATCH 2/3] Some intrinsics

---
 stl/inc/bit | 24 ++++++++++++++++++++++--
 1 file changed, 22 insertions(+), 2 deletions(-)

diff --git a/stl/inc/bit b/stl/inc/bit
index b412a53b968..520b20e1a7a 100644
--- a/stl/inc/bit
+++ b/stl/inc/bit
@@ -92,7 +92,17 @@ _NODISCARD constexpr _Ty rotr(_Ty _Val, int _Rotation) noexcept;
 template <class _Ty, enable_if_t<_Is_standard_unsigned_integer<_Ty>, int> = 0>
 _NODISCARD constexpr _Ty rotl(const _Ty _Val, const int _Rotation) noexcept {
     constexpr auto _Digits = numeric_limits<_Ty>::digits;
-    const auto _Remainder  = _Rotation % _Digits;
+
+    if (!_STD is_constant_evaluated()) {
+        if constexpr (_Digits == 64) {
+            return _rotl64(_Val, _Rotation);
+        } else if constexpr (_Digits == 32) {
+            return _rotl(_Val, _Rotation);
+        }
+        // TRANSITION: fallback to non-intrinsic case until <intrin0.h> changes
+    }
+
+    const auto _Remainder = _Rotation % _Digits;
     if (_Remainder > 0) {
         return static_cast<_Ty>(
             static_cast<_Ty>(_Val << _Remainder) | static_cast<_Ty>(_Val >> (_Digits - _Remainder)));
@@ -106,7 +116,17 @@ _NODISCARD constexpr _Ty rotl(const _Ty _Val, const int _Rotation) noexcept {
 template <class _Ty, enable_if_t<_Is_standard_unsigned_integer<_Ty>, int> _Enabled>
 _NODISCARD constexpr _Ty rotr(const _Ty _Val, const int _Rotation) noexcept {
     constexpr auto _Digits = numeric_limits<_Ty>::digits;
-    const auto _Remainder  = _Rotation % _Digits;
+
+    if (!_STD is_constant_evaluated()) {
+        if constexpr (_Digits == 64) {
+            return _rotr64(_Val, _Rotation);
+        } else if constexpr (_Digits == 32) {
+            return _rotr(_Val, _Rotation);
+        }
+        // TRANSITION: fallback to non-intrinsic case until <intrin0.h> changes
+    }
+
+    const auto _Remainder = _Rotation % _Digits;
     if (_Remainder > 0) {
         return static_cast<_Ty>(
             static_cast<_Ty>(_Val >> _Remainder) | static_cast<_Ty>(_Val << (_Digits - _Remainder)));

From b05e2e99e8bb3c9af992a482ded5db7c2d0c6529 Mon Sep 17 00:00:00 2001
From: Alex Guteniev <gutenev@gmail.com>
Date: Wed, 1 Dec 2021 21:52:04 +0200
Subject: [PATCH 3/3] simplify test

---
 .../tests/P0553R4_bit_rotating_and_counting_functions/test.cpp  | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tests/std/tests/P0553R4_bit_rotating_and_counting_functions/test.cpp b/tests/std/tests/P0553R4_bit_rotating_and_counting_functions/test.cpp
index 07a57e53a58..c73cccfa76b 100644
--- a/tests/std/tests/P0553R4_bit_rotating_and_counting_functions/test.cpp
+++ b/tests/std/tests/P0553R4_bit_rotating_and_counting_functions/test.cpp
@@ -122,7 +122,7 @@ constexpr bool test_rotr() {
         assert(rotr(T{0}, i) == 0);
 
         for (int j = 0; j < 4; ++j) {
-            assert(rotr(every_fourth[3 - j], i) == every_fourth[3 - ((i + j) % 4)]);
+            assert(rotr(every_fourth[(i + j) % 4], i) == every_fourth[j]);
         }
     }
     return true;