diff --git a/llvm/include/llvm/ADT/bit.h b/llvm/include/llvm/ADT/bit.h
--- a/llvm/include/llvm/ADT/bit.h
+++ b/llvm/include/llvm/ADT/bit.h
@@ -16,6 +16,7 @@
 
 #include <cstdint>
 #include <cstring>
+#include <limits>
 #include <type_traits>
 
 namespace llvm {
@@ -40,6 +41,181 @@
   return (Value != 0) && ((Value & (Value - 1)) == 0);
 }
 
+#ifdef _MSC_VER
+// Declare these intrinsics manually rather including intrin.h. It's very
+// expensive, and bit.h is popular via MathExtras.h.
+// #include <intrin.h>
+extern "C" {
+unsigned char _BitScanForward(unsigned long *_Index, unsigned long _Mask);
+unsigned char _BitScanForward64(unsigned long *_Index, unsigned __int64 _Mask);
+unsigned char _BitScanReverse(unsigned long *_Index, unsigned long _Mask);
+unsigned char _BitScanReverse64(unsigned long *_Index, unsigned __int64 _Mask);
+}
+#endif
+
+namespace detail {
+template <typename T, std::size_t SizeOfT> struct TrailingZerosCounter {
+  static unsigned count(T Val) {
+    if (!Val)
+      return std::numeric_limits<T>::digits;
+    if (Val & 0x1)
+      return 0;
+
+    // Bisection method.
+    unsigned ZeroBits = 0;
+    T Shift = std::numeric_limits<T>::digits >> 1;
+    T Mask = std::numeric_limits<T>::max() >> Shift;
+    while (Shift) {
+      if ((Val & Mask) == 0) {
+        Val >>= Shift;
+        ZeroBits |= Shift;
+      }
+      Shift >>= 1;
+      Mask >>= Shift;
+    }
+    return ZeroBits;
+  }
+};
+
+#if defined(__GNUC__) || defined(_MSC_VER)
+template <typename T> struct TrailingZerosCounter<T, 4> {
+  static unsigned count(T Val) {
+    if (Val == 0)
+      return 32;
+
+#if __has_builtin(__builtin_ctz) || defined(__GNUC__)
+    return __builtin_ctz(Val);
+#elif defined(_MSC_VER)
+    unsigned long Index;
+    _BitScanForward(&Index, Val);
+    return Index;
+#endif
+  }
+};
+
+#if !defined(_MSC_VER) || defined(_M_X64)
+template <typename T> struct TrailingZerosCounter<T, 8> {
+  static unsigned count(T Val) {
+    if (Val == 0)
+      return 64;
+
+#if __has_builtin(__builtin_ctzll) || defined(__GNUC__)
+    return __builtin_ctzll(Val);
+#elif defined(_MSC_VER)
+    unsigned long Index;
+    _BitScanForward64(&Index, Val);
+    return Index;
+#endif
+  }
+};
+#endif
+#endif
+} // namespace detail
+
+/// Count number of 0's from the least significant bit to the most
+///   stopping at the first 1.
+///
+/// Only unsigned integral types are allowed.
+///
+/// Returns std::numeric_limits<T>::digits on an input of 0.
+template <typename T> int countr_zero(T Val) {
+  static_assert(std::is_unsigned_v<T>,
+                "Only unsigned integral types are allowed.");
+  return llvm::detail::TrailingZerosCounter<T, sizeof(T)>::count(Val);
+}
+
+namespace detail {
+template <typename T, std::size_t SizeOfT> struct LeadingZerosCounter {
+  static unsigned count(T Val) {
+    if (!Val)
+      return std::numeric_limits<T>::digits;
+
+    // Bisection method.
+    unsigned ZeroBits = 0;
+    for (T Shift = std::numeric_limits<T>::digits >> 1; Shift; Shift >>= 1) {
+      T Tmp = Val >> Shift;
+      if (Tmp)
+        Val = Tmp;
+      else
+        ZeroBits |= Shift;
+    }
+    return ZeroBits;
+  }
+};
+
+#if defined(__GNUC__) || defined(_MSC_VER)
+template <typename T> struct LeadingZerosCounter<T, 4> {
+  static unsigned count(T Val) {
+    if (Val == 0)
+      return 32;
+
+#if __has_builtin(__builtin_clz) || defined(__GNUC__)
+    return __builtin_clz(Val);
+#elif defined(_MSC_VER)
+    unsigned long Index;
+    _BitScanReverse(&Index, Val);
+    return Index ^ 31;
+#endif
+  }
+};
+
+#if !defined(_MSC_VER) || defined(_M_X64)
+template <typename T> struct LeadingZerosCounter<T, 8> {
+  static unsigned count(T Val) {
+    if (Val == 0)
+      return 64;
+
+#if __has_builtin(__builtin_clzll) || defined(__GNUC__)
+    return __builtin_clzll(Val);
+#elif defined(_MSC_VER)
+    unsigned long Index;
+    _BitScanReverse64(&Index, Val);
+    return Index ^ 63;
+#endif
+  }
+};
+#endif
+#endif
+} // namespace detail
+
+/// Count number of 0's from the most significant bit to the least
+///   stopping at the first 1.
+///
+/// Only unsigned integral types are allowed.
+///
+/// Returns std::numeric_limits<T>::digits on an input of 0.
+template <typename T> int countl_zero(T Val) {
+  static_assert(std::is_unsigned_v<T>,
+                "Only unsigned integral types are allowed.");
+  return llvm::detail::LeadingZerosCounter<T, sizeof(T)>::count(Val);
+}
+
+/// Count the number of ones from the most significant bit to the first
+/// zero bit.
+///
+/// Ex. countl_one(0xFF0FFF00) == 8.
+/// Only unsigned integral types are allowed.
+///
+/// Returns std::numeric_limits<T>::digits on an input of all ones.
+template <typename T> int countl_one(T Value) {
+  static_assert(std::is_unsigned_v<T>,
+                "Only unsigned integral types are allowed.");
+  return llvm::countl_zero<T>(~Value);
+}
+
+/// Count the number of ones from the least significant bit to the first
+/// zero bit.
+///
+/// Ex. countr_one(0x00FF00FF) == 8.
+/// Only unsigned integral types are allowed.
+///
+/// Returns std::numeric_limits<T>::digits on an input of all ones.
+template <typename T> int countr_one(T Value) {
+  static_assert(std::is_unsigned_v<T>,
+                "Only unsigned integral types are allowed.");
+  return llvm::countr_zero<T>(~Value);
+}
+
 namespace detail {
 template <typename T, std::size_t SizeOfT> struct PopulationCounter {
   static int count(T Value) {
diff --git a/llvm/include/llvm/Support/MathExtras.h b/llvm/include/llvm/Support/MathExtras.h
--- a/llvm/include/llvm/Support/MathExtras.h
+++ b/llvm/include/llvm/Support/MathExtras.h
@@ -14,7 +14,6 @@
 #define LLVM_SUPPORT_MATHEXTRAS_H
 
 #include "llvm/ADT/bit.h"
-#include "llvm/Support/Compiler.h"
 #include <cassert>
 #include <climits>
 #include <cstdint>
@@ -22,18 +21,6 @@
 #include <limits>
 #include <type_traits>
 
-#ifdef _MSC_VER
-// Declare these intrinsics manually rather including intrin.h. It's very
-// expensive, and MathExtras.h is popular.
-// #include <intrin.h>
-extern "C" {
-unsigned char _BitScanForward(unsigned long *_Index, unsigned long _Mask);
-unsigned char _BitScanForward64(unsigned long *_Index, unsigned __int64 _Mask);
-unsigned char _BitScanReverse(unsigned long *_Index, unsigned long _Mask);
-unsigned char _BitScanReverse64(unsigned long *_Index, unsigned __int64 _Mask);
-}
-#endif
-
 namespace llvm {
 
 /// The behavior an operation has on an input of 0.
@@ -80,65 +67,6 @@
                 phif        = 1.61803399F; // (0x1.9e377aP+0) https://oeis.org/A001622
 } // namespace numbers
 
-namespace detail {
-template <typename T, std::size_t SizeOfT> struct TrailingZerosCounter {
-  static unsigned count(T Val) {
-    if (!Val)
-      return std::numeric_limits<T>::digits;
-    if (Val & 0x1)
-      return 0;
-
-    // Bisection method.
-    unsigned ZeroBits = 0;
-    T Shift = std::numeric_limits<T>::digits >> 1;
-    T Mask = std::numeric_limits<T>::max() >> Shift;
-    while (Shift) {
-      if ((Val & Mask) == 0) {
-        Val >>= Shift;
-        ZeroBits |= Shift;
-      }
-      Shift >>= 1;
-      Mask >>= Shift;
-    }
-    return ZeroBits;
-  }
-};
-
-#if defined(__GNUC__) || defined(_MSC_VER)
-template <typename T> struct TrailingZerosCounter<T, 4> {
-  static unsigned count(T Val) {
-    if (Val == 0)
-      return 32;
-
-#if __has_builtin(__builtin_ctz) || defined(__GNUC__)
-    return __builtin_ctz(Val);
-#elif defined(_MSC_VER)
-    unsigned long Index;
-    _BitScanForward(&Index, Val);
-    return Index;
-#endif
-  }
-};
-
-#if !defined(_MSC_VER) || defined(_M_X64)
-template <typename T> struct TrailingZerosCounter<T, 8> {
-  static unsigned count(T Val) {
-    if (Val == 0)
-      return 64;
-
-#if __has_builtin(__builtin_ctzll) || defined(__GNUC__)
-    return __builtin_ctzll(Val);
-#elif defined(_MSC_VER)
-    unsigned long Index;
-    _BitScanForward64(&Index, Val);
-    return Index;
-#endif
-  }
-};
-#endif
-#endif
-} // namespace detail
-
 /// Count number of 0's from the least significant bit to the most
 ///   stopping at the first 1.
 ///
@@ -148,62 +76,8 @@
 template <typename T> unsigned countTrailingZeros(T Val) {
   static_assert(std::is_unsigned_v<T>,
                 "Only unsigned integral types are allowed.");
-  return llvm::detail::TrailingZerosCounter<T, sizeof(T)>::count(Val);
-}
-
-namespace detail {
-template <typename T, std::size_t SizeOfT> struct LeadingZerosCounter {
-  static unsigned count(T Val) {
-    if (!Val)
-      return std::numeric_limits<T>::digits;
-
-    // Bisection method.
-    unsigned ZeroBits = 0;
-    for (T Shift = std::numeric_limits<T>::digits >> 1; Shift; Shift >>= 1) {
-      T Tmp = Val >> Shift;
-      if (Tmp)
-        Val = Tmp;
-      else
-        ZeroBits |= Shift;
-    }
-    return ZeroBits;
-  }
-};
-
-#if defined(__GNUC__) || defined(_MSC_VER)
-template <typename T> struct LeadingZerosCounter<T, 4> {
-  static unsigned count(T Val) {
-    if (Val == 0)
-      return 32;
-
-#if __has_builtin(__builtin_clz) || defined(__GNUC__)
-    return __builtin_clz(Val);
-#elif defined(_MSC_VER)
-    unsigned long Index;
-    _BitScanReverse(&Index, Val);
-    return Index ^ 31;
-#endif
-  }
-};
-
-#if !defined(_MSC_VER) || defined(_M_X64)
-template <typename T> struct LeadingZerosCounter<T, 8> {
-  static unsigned count(T Val) {
-    if (Val == 0)
-      return 64;
-
-#if __has_builtin(__builtin_clzll) || defined(__GNUC__)
-    return __builtin_clzll(Val);
-#elif defined(_MSC_VER)
-    unsigned long Index;
-    _BitScanReverse64(&Index, Val);
-    return Index ^ 63;
-#endif
-  }
-};
-#endif
-#endif
-} // namespace detail
+  return llvm::countr_zero(Val);
+}
 
 /// Count number of 0's from the most significant bit to the least
 ///   stopping at the first 1.
@@ -214,7 +88,7 @@
 template <typename T> unsigned countLeadingZeros(T Val) {
   static_assert(std::is_unsigned_v<T>,
                 "Only unsigned integral types are allowed.");
-  return llvm::detail::LeadingZerosCounter<T, sizeof(T)>::count(Val);
+  return llvm::countl_zero(Val);
 }
 
 /// Get the index of the first set bit starting from the least
@@ -465,7 +339,7 @@
 template <typename T> unsigned countLeadingOnes(T Value) {
   static_assert(std::is_unsigned_v<T>,
                 "Only unsigned integral types are allowed.");
-  return countLeadingZeros<T>(~Value);
+  return llvm::countl_one<T>(Value);
 }
 
 /// Count the number of ones from the least significant bit to the first
@@ -478,7 +352,7 @@
 template <typename T> unsigned countTrailingOnes(T Value) {
   static_assert(std::is_unsigned_v<T>,
                 "Only unsigned integral types are allowed.");
-  return countTrailingZeros<T>(~Value);
+  return llvm::countr_one<T>(Value);
 }
 
 /// Count the number of set bits in a value.
diff --git a/llvm/unittests/ADT/BitTest.cpp b/llvm/unittests/ADT/BitTest.cpp
--- a/llvm/unittests/ADT/BitTest.cpp
+++ b/llvm/unittests/ADT/BitTest.cpp
@@ -48,6 +48,102 @@
   EXPECT_TRUE(llvm::has_single_bit(static_cast<uint16_t>(kValueS16)));
 }
 
+TEST(BitTest, CountlZero) {
+  uint8_t Z8 = 0;
+  uint16_t Z16 = 0;
+  uint32_t Z32 = 0;
+  uint64_t Z64 = 0;
+  EXPECT_EQ(8, llvm::countl_zero(Z8));
+  EXPECT_EQ(16, llvm::countl_zero(Z16));
+  EXPECT_EQ(32, llvm::countl_zero(Z32));
+  EXPECT_EQ(64, llvm::countl_zero(Z64));
+
+  uint8_t NZ8 = 42;
+  uint16_t NZ16 = 42;
+  uint32_t NZ32 = 42;
+  uint64_t NZ64 = 42;
+  EXPECT_EQ(2, llvm::countl_zero(NZ8));
+  EXPECT_EQ(10, llvm::countl_zero(NZ16));
+  EXPECT_EQ(26, llvm::countl_zero(NZ32));
+  EXPECT_EQ(58, llvm::countl_zero(NZ64));
+
+  EXPECT_EQ(8, llvm::countl_zero(0x00F000FFu));
+  EXPECT_EQ(8, llvm::countl_zero(0x00F12345u));
+  for (unsigned i = 0; i <= 30; ++i) {
+    EXPECT_EQ(int(31 - i), llvm::countl_zero(1u << i));
+  }
+
+  EXPECT_EQ(8, llvm::countl_zero(0x00F1234500F12345ULL));
+  EXPECT_EQ(1, llvm::countl_zero(1ULL << 62));
+  for (unsigned i = 0; i <= 62; ++i) {
+    EXPECT_EQ(int(63 - i), llvm::countl_zero(1ULL << i));
+  }
+}
+
+TEST(BitTest, CountrZero) {
+  uint8_t Z8 = 0;
+  uint16_t Z16 = 0;
+  uint32_t Z32 = 0;
+  uint64_t Z64 = 0;
+  EXPECT_EQ(8, llvm::countr_zero(Z8));
+  EXPECT_EQ(16, llvm::countr_zero(Z16));
+  EXPECT_EQ(32, llvm::countr_zero(Z32));
+  EXPECT_EQ(64, llvm::countr_zero(Z64));
+
+  uint8_t NZ8 = 42;
+  uint16_t NZ16 = 42;
+  uint32_t NZ32 = 42;
+  uint64_t NZ64 = 42;
+  EXPECT_EQ(1, llvm::countr_zero(NZ8));
+  EXPECT_EQ(1, llvm::countr_zero(NZ16));
+  EXPECT_EQ(1, llvm::countr_zero(NZ32));
+  EXPECT_EQ(1, llvm::countr_zero(NZ64));
+}
+
+TEST(BitTest, CountlOne) {
+  for (int i = 30; i >= 0; --i) {
+    // Start with all ones and unset some bit.
+    EXPECT_EQ(31 - i, llvm::countl_one(0xFFFFFFFF ^ (1 << i)));
+  }
+  for (int i = 62; i >= 0; --i) {
+    // Start with all ones and unset some bit.
+    EXPECT_EQ(63 - i, llvm::countl_one(0xFFFFFFFFFFFFFFFFULL ^ (1LL << i)));
+  }
+  for (int i = 30; i >= 0; --i) {
+    // Start with all ones and unset some bit.
+    EXPECT_EQ(31 - i, llvm::countl_one(0xFFFFFFFF ^ (1 << i)));
+  }
+}
+
+TEST(BitTest, CountrOne) {
+  uint8_t AllOnes8 = ~(uint8_t)0;
+  uint16_t AllOnes16 = ~(uint16_t)0;
+  uint32_t AllOnes32 = ~(uint32_t)0;
+  uint64_t AllOnes64 = ~(uint64_t)0;
+  EXPECT_EQ(8, llvm::countr_one(AllOnes8));
+  EXPECT_EQ(16, llvm::countr_one(AllOnes16));
+  EXPECT_EQ(32, llvm::countr_one(AllOnes32));
+  EXPECT_EQ(64, llvm::countr_one(AllOnes64));
+
+  uint8_t X8 = 6;
+  uint16_t X16 = 6;
+  uint32_t X32 = 6;
+  uint64_t X64 = 6;
+  EXPECT_EQ(0, llvm::countr_one(X8));
+  EXPECT_EQ(0, llvm::countr_one(X16));
+  EXPECT_EQ(0, llvm::countr_one(X32));
+  EXPECT_EQ(0, llvm::countr_one(X64));
+
+  uint8_t Y8 = 23;
+  uint16_t Y16 = 23;
+  uint32_t Y32 = 23;
+  uint64_t Y64 = 23;
+  EXPECT_EQ(3, llvm::countr_one(Y8));
+  EXPECT_EQ(3, llvm::countr_one(Y16));
+  EXPECT_EQ(3, llvm::countr_one(Y32));
+  EXPECT_EQ(3, llvm::countr_one(Y64));
+}
+
 TEST(BitTest, PopCount) {
   EXPECT_EQ(0, llvm::popcount(0U));
   EXPECT_EQ(0, llvm::popcount(0ULL));