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
@@ -66,7 +66,9 @@
 /// Only unsigned integral types are allowed.
 ///
 /// Returns std::numeric_limits<T>::digits on an input of 0.
-template <typename T> unsigned countTrailingZeros(T Val) {
+template <typename T>
+LLVM_DEPRECATED("Use llvm::countr_zero instead.", "llvm::countr_zero")
+unsigned countTrailingZeros(T Val) {
   static_assert(std::is_unsigned_v<T>,
                 "Only unsigned integral types are allowed.");
   return llvm::countr_zero(Val);
@@ -78,7 +80,9 @@
 /// Only unsigned integral types are allowed.
 ///
 /// Returns std::numeric_limits<T>::digits on an input of 0.
-template <typename T> unsigned countLeadingZeros(T Val) {
+template <typename T>
+LLVM_DEPRECATED("Use llvm::countl_zero instead.", "llvm::countl_zero")
+unsigned countLeadingZeros(T Val) {
   static_assert(std::is_unsigned_v<T>,
                 "Only unsigned integral types are allowed.");
   return llvm::countl_zero(Val);
@@ -301,7 +305,9 @@
 /// Only unsigned integral types are allowed.
 ///
 /// Returns std::numeric_limits<T>::digits on an input of all ones.
-template <typename T> unsigned countLeadingOnes(T Value) {
+template <typename T>
+LLVM_DEPRECATED("Use llvm::countl_one instead.", "llvm::countl_one")
+unsigned countLeadingOnes(T Value) {
   static_assert(std::is_unsigned_v<T>,
                 "Only unsigned integral types are allowed.");
   return llvm::countl_one<T>(Value);
@@ -314,7 +320,9 @@
 /// Only unsigned integral types are allowed.
 ///
 /// Returns std::numeric_limits<T>::digits on an input of all ones.
-template <typename T> unsigned countTrailingOnes(T Value) {
+template <typename T>
+LLVM_DEPRECATED("Use llvm::countr_one instead.", "llvm::countr_one")
+unsigned countTrailingOnes(T Value) {
   static_assert(std::is_unsigned_v<T>,
                 "Only unsigned integral types are allowed.");
   return llvm::countr_one<T>(Value);
@@ -324,6 +332,7 @@
 /// Ex. countPopulation(0xF000F000) = 8
 /// Returns 0 if the word is zero.
 template <typename T>
+LLVM_DEPRECATED("Use llvm::popcount instead.", "llvm::popcount")
 inline unsigned countPopulation(T Value) {
   static_assert(std::is_unsigned_v<T>,
                 "Only unsigned integral types are allowed.");
diff --git a/llvm/unittests/Support/MathExtrasTest.cpp b/llvm/unittests/Support/MathExtrasTest.cpp
--- a/llvm/unittests/Support/MathExtrasTest.cpp
+++ b/llvm/unittests/Support/MathExtrasTest.cpp
@@ -13,58 +13,6 @@
 
 namespace {
 
-TEST(MathExtras, countTrailingZeros) {
-  uint8_t Z8 = 0;
-  uint16_t Z16 = 0;
-  uint32_t Z32 = 0;
-  uint64_t Z64 = 0;
-  EXPECT_EQ(8u, countTrailingZeros(Z8));
-  EXPECT_EQ(16u, countTrailingZeros(Z16));
-  EXPECT_EQ(32u, countTrailingZeros(Z32));
-  EXPECT_EQ(64u, countTrailingZeros(Z64));
-
-  uint8_t NZ8 = 42;
-  uint16_t NZ16 = 42;
-  uint32_t NZ32 = 42;
-  uint64_t NZ64 = 42;
-  EXPECT_EQ(1u, countTrailingZeros(NZ8));
-  EXPECT_EQ(1u, countTrailingZeros(NZ16));
-  EXPECT_EQ(1u, countTrailingZeros(NZ32));
-  EXPECT_EQ(1u, countTrailingZeros(NZ64));
-}
-
-TEST(MathExtras, countLeadingZeros) {
-  uint8_t Z8 = 0;
-  uint16_t Z16 = 0;
-  uint32_t Z32 = 0;
-  uint64_t Z64 = 0;
-  EXPECT_EQ(8u, countLeadingZeros(Z8));
-  EXPECT_EQ(16u, countLeadingZeros(Z16));
-  EXPECT_EQ(32u, countLeadingZeros(Z32));
-  EXPECT_EQ(64u, countLeadingZeros(Z64));
-
-  uint8_t NZ8 = 42;
-  uint16_t NZ16 = 42;
-  uint32_t NZ32 = 42;
-  uint64_t NZ64 = 42;
-  EXPECT_EQ(2u, countLeadingZeros(NZ8));
-  EXPECT_EQ(10u, countLeadingZeros(NZ16));
-  EXPECT_EQ(26u, countLeadingZeros(NZ32));
-  EXPECT_EQ(58u, countLeadingZeros(NZ64));
-
-  EXPECT_EQ(8u, countLeadingZeros(0x00F000FFu));
-  EXPECT_EQ(8u, countLeadingZeros(0x00F12345u));
-  for (unsigned i = 0; i <= 30; ++i) {
-    EXPECT_EQ(31 - i, countLeadingZeros(1u << i));
-  }
-
-  EXPECT_EQ(8u, countLeadingZeros(0x00F1234500F12345ULL));
-  EXPECT_EQ(1u, countLeadingZeros(1ULL << 62));
-  for (unsigned i = 0; i <= 62; ++i) {
-    EXPECT_EQ(63 - i, countLeadingZeros(1ULL << i));
-  }
-}
-
 TEST(MathExtras, onesMask) {
   EXPECT_EQ(0U, maskLeadingOnes<uint8_t>(0));
   EXPECT_EQ(0U, maskTrailingOnes<uint8_t>(0));
@@ -220,21 +168,6 @@
   EXPECT_EQ(CTLog2<1ULL << 15>(), 15U);
 }
 
-TEST(MathExtras, countLeadingOnes) {
-  for (int i = 30; i >= 0; --i) {
-    // Start with all ones and unset some bit.
-    EXPECT_EQ(31u - i, countLeadingOnes(0xFFFFFFFF ^ (1 << i)));
-  }
-  for (int i = 62; i >= 0; --i) {
-    // Start with all ones and unset some bit.
-    EXPECT_EQ(63u - i, countLeadingOnes(0xFFFFFFFFFFFFFFFFULL ^ (1LL << i)));
-  }
-  for (int i = 30; i >= 0; --i) {
-    // Start with all ones and unset some bit.
-    EXPECT_EQ(31u - i, countLeadingOnes(0xFFFFFFFF ^ (1 << i)));
-  }
-}
-
 TEST(MathExtras, FloatBits) {
   static const float kValue = 5632.34f;
   EXPECT_FLOAT_EQ(kValue, BitsToFloat(FloatToBits(kValue)));