diff --git a/libc/src/__support/CMakeLists.txt b/libc/src/__support/CMakeLists.txt
--- a/libc/src/__support/CMakeLists.txt
+++ b/libc/src/__support/CMakeLists.txt
@@ -15,6 +15,12 @@
     ctype_utils.h
 )
 
+add_header_library(
+  find_leading_one
+  HDRS
+    find_leading_one.h
+)
+
 add_header_library(
   high_precision_decimal
   HDRS
diff --git a/libc/src/__support/FPUtil/CMakeLists.txt b/libc/src/__support/FPUtil/CMakeLists.txt
--- a/libc/src/__support/FPUtil/CMakeLists.txt
+++ b/libc/src/__support/FPUtil/CMakeLists.txt
@@ -15,10 +15,12 @@
     PolyEval.h
     UInt.h
     XFloat.h
+    Hypot.h
   DEPENDS
     libc.include.math
     libc.include.errno
     libc.include.fenv
     libc.src.__support.common
+    libc.src.__support.find_leading_one
     libc.src.__support.CPP.standalone_cpp
 )
diff --git a/libc/src/__support/FPUtil/Hypot.h b/libc/src/__support/FPUtil/Hypot.h
--- a/libc/src/__support/FPUtil/Hypot.h
+++ b/libc/src/__support/FPUtil/Hypot.h
@@ -12,48 +12,11 @@
 #include "BasicOperations.h"
 #include "FPBits.h"
 #include "src/__support/CPP/TypeTraits.h"
+#include "src/__support/find_leading_one.h"
 
 namespace __llvm_libc {
 namespace fputil {
 
-namespace internal {
-
-template <typename T>
-static inline T find_leading_one(T mant, int &shift_length);
-
-template <>
-inline uint32_t find_leading_one<uint32_t>(uint32_t mant, int &shift_length) {
-  shift_length = 0;
-  constexpr int NSTEPS = 5;
-  constexpr uint32_t BOUNDS[NSTEPS] = {1 << 16, 1 << 8, 1 << 4, 1 << 2, 1 << 1};
-  constexpr int SHIFTS[NSTEPS] = {16, 8, 4, 2, 1};
-  for (int i = 0; i < NSTEPS; ++i) {
-    if (mant >= BOUNDS[i]) {
-      shift_length += SHIFTS[i];
-      mant >>= SHIFTS[i];
-    }
-  }
-  return 1U << shift_length;
-}
-
-template <>
-inline uint64_t find_leading_one<uint64_t>(uint64_t mant, int &shift_length) {
-  shift_length = 0;
-  constexpr int NSTEPS = 6;
-  constexpr uint64_t BOUNDS[NSTEPS] = {1ULL << 32, 1ULL << 16, 1ULL << 8,
-                                       1ULL << 4,  1ULL << 2,  1ULL << 1};
-  constexpr int SHIFTS[NSTEPS] = {32, 16, 8, 4, 2, 1};
-  for (int i = 0; i < NSTEPS; ++i) {
-    if (mant >= BOUNDS[i]) {
-      shift_length += SHIFTS[i];
-      mant >>= SHIFTS[i];
-    }
-  }
-  return 1ULL << shift_length;
-}
-
-} // namespace internal
-
 template <typename T> struct DoubleLength;
 
 template <> struct DoubleLength<uint16_t> { using Type = uint32_t; };
@@ -178,7 +141,8 @@
     a_mant |= ONE;
     y_mant_width = MantissaWidth<T>::VALUE + 1;
   } else {
-    leading_one = internal::find_leading_one(a_mant, y_mant_width);
+    y_mant_width = __llvm_libc::internal::find_leading_one(a_mant);
+    leading_one = UIntType(1) << y_mant_width;
     a_exp = 1;
   }
 
diff --git a/libc/src/__support/find_leading_one.h b/libc/src/__support/find_leading_one.h
new file mode 100644
--- /dev/null
+++ b/libc/src/__support/find_leading_one.h
@@ -0,0 +1,32 @@
+//===-- Utility for finding position of leading 1 in integer ----*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_SRC_SUPPORT_FIND_LEADING_ONE_H
+#define LLVM_LIBC_SRC_SUPPORT_FIND_LEADING_ONE_H
+
+#include "src/__support/architectures.h"
+
+namespace __llvm_libc {
+namespace internal {
+/// Finds the position of the most-significant 1 bit in the given integer.
+///
+/// \param value the integer to scan. Should be a positive number.
+///
+/// \returns the position of the bit as an offset from the LSB (e.g., passing
+//   0b0101 would return 2). Returns 0 on domain error (e.g., if 0 is passed).
+template <typename T> static inline int find_leading_one(T value);
+} // namespace internal
+} // namespace __llvm_libc
+
+#if defined(LLVM_LIBC_ARCH_X86_64)
+#include "src/__support/x86_64/find_leading_one.h"
+#else
+#include "src/__support/generic/find_leading_one.h"
+#endif
+
+#endif // LLVM_LIBC_SRC_SUPPORT_FIND_LEADING_ONE_H
diff --git a/libc/src/__support/generic/find_leading_one.h b/libc/src/__support/generic/find_leading_one.h
new file mode 100644
--- /dev/null
+++ b/libc/src/__support/generic/find_leading_one.h
@@ -0,0 +1,46 @@
+//===-- Utility for finding position of leading 1 in integer ----*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_SRC_SUPPORT_GENERIC_FIND_LEADING_ONE_H
+#define LLVM_LIBC_SRC_SUPPORT_GENERIC_FIND_LEADING_ONE_H
+
+#include "src/__support/CPP/TypeTraits.h"
+#include <stdint.h>
+
+template <>
+inline int __llvm_libc::internal::find_leading_one<uint32_t>(uint32_t value) {
+  int shift_length = 0;
+  constexpr int NSTEPS = 5;
+  constexpr uint32_t BOUNDS[NSTEPS] = {1 << 16, 1 << 8, 1 << 4, 1 << 2, 1 << 1};
+  constexpr int SHIFTS[NSTEPS] = {16, 8, 4, 2, 1};
+  for (int i = 0; i < NSTEPS; ++i) {
+    if (value >= BOUNDS[i]) {
+      shift_length += SHIFTS[i];
+      value >>= SHIFTS[i];
+    }
+  }
+  return shift_length;
+}
+
+template <>
+inline int __llvm_libc::internal::find_leading_one<uint64_t>(uint64_t value) {
+  int shift_length = 0;
+  constexpr int NSTEPS = 6;
+  constexpr uint64_t BOUNDS[NSTEPS] = {1ULL << 32, 1ULL << 16, 1ULL << 8,
+                                       1ULL << 4,  1ULL << 2,  1ULL << 1};
+  constexpr int SHIFTS[NSTEPS] = {32, 16, 8, 4, 2, 1};
+  for (int i = 0; i < NSTEPS; ++i) {
+    if (value >= BOUNDS[i]) {
+      shift_length += SHIFTS[i];
+      value >>= SHIFTS[i];
+    }
+  }
+  return shift_length;
+}
+
+#endif // LLVM_LIBC_SRC_SUPPORT_GENERIC_FIND_LEADING_ONE_H
diff --git a/libc/src/__support/x86_64/find_leading_one.h b/libc/src/__support/x86_64/find_leading_one.h
new file mode 100644
--- /dev/null
+++ b/libc/src/__support/x86_64/find_leading_one.h
@@ -0,0 +1,36 @@
+//===-- Utility for finding position of leading 1 in integer ----*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_SRC_SUPPORT_X86_64_FIND_LEADING_ONE_H
+#define LLVM_LIBC_SRC_SUPPORT_X86_64_FIND_LEADING_ONE_H
+
+#include "src/__support/CPP/TypeTraits.h"
+#include <stdint.h>
+
+#if !defined(LLVM_LIBC_ARCH_X86_64)
+#error Must be LLVM_LIBC_ARCH_X86_64 to include this file
+#endif
+
+template <typename T>
+inline int __llvm_libc::internal::find_leading_one(T value) {
+  static_assert(
+      cpp::IsSame<T, uint32_t>::Value || cpp::IsSame<T, uint64_t>::Value, "");
+
+  if (value == 0) {
+    return 0;
+  }
+
+  __asm__("bsr %[value], %[value]"
+          : [value] "=r"(value)
+          : "0"(value)
+          : "flags"); // Clobbers ZF
+
+  return static_cast<int>(value);
+}
+
+#endif // LLVM_LIBC_SRC_SUPPORT_X86_64_FIND_LEADING_ONE_H
diff --git a/libc/test/src/__support/CMakeLists.txt b/libc/test/src/__support/CMakeLists.txt
--- a/libc/test/src/__support/CMakeLists.txt
+++ b/libc/test/src/__support/CMakeLists.txt
@@ -10,6 +10,16 @@
     libc.src.__support.common
 )
 
+add_libc_unittest(
+  find_leading_one_test
+  SUITE
+    libc_support_unittests
+  SRCS
+    find_leading_one_test.cpp
+  DEPENDS
+    libc.src.__support.find_leading_one
+)
+
 add_libc_unittest(
   high_precision_decimal_test
   SUITE
diff --git a/libc/test/src/__support/find_leading_one_test.cpp b/libc/test/src/__support/find_leading_one_test.cpp
new file mode 100644
--- /dev/null
+++ b/libc/test/src/__support/find_leading_one_test.cpp
@@ -0,0 +1,44 @@
+//===-- Unittests for find_leading_one ------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "src/__support/find_leading_one.h"
+#include "utils/UnitTest/Test.h"
+#include <climits>
+
+namespace __llvm_libc {
+
+struct LlvmLibcFindLeadingOne : testing::Test {
+  template <typename T> void exhaustive_test() {
+    for (std::size_t i = 0; i < sizeof(T) * CHAR_BIT; i++) {
+      const T value = static_cast<T>(1ULL << i);
+      EXPECT_EQ(internal::find_leading_one(value), static_cast<int>(i));
+    }
+  }
+};
+
+TEST_F(LlvmLibcFindLeadingOne, Zero) {
+  EXPECT_EQ(internal::find_leading_one(0U), 0);
+}
+
+TEST_F(LlvmLibcFindLeadingOne, One) {
+  EXPECT_EQ(internal::find_leading_one(1U), 0);
+}
+
+TEST_F(LlvmLibcFindLeadingOne, Two) {
+  EXPECT_EQ(internal::find_leading_one(2U), 1);
+}
+
+TEST_F(LlvmLibcFindLeadingOne, Three) {
+  EXPECT_EQ(internal::find_leading_one(3U), 1);
+}
+
+TEST_F(LlvmLibcFindLeadingOne, Exhaustive32Bit) { exhaustive_test<uint32_t>(); }
+
+TEST_F(LlvmLibcFindLeadingOne, Exhaustive64Bit) { exhaustive_test<uint64_t>(); }
+
+} // namespace __llvm_libc