fixup math extras

Author: nickdesaulniers

libc/src/__support/UInt.h

@@ -1229,6 +1229,49 @@ LIBC_INLINE constexpr cpp::enable_if_t<is_big_int_v<T>, T> mask_leading_ones() {
   return out;
 }
 
+// Specialization of count_zeros ('math_extras.h') for BigInt.
+template <typename T>
+[[nodiscard]]
+LIBC_INLINE constexpr cpp::enable_if_t<is_big_int_v<T>, int>
+count_zeros(T value) {
+  return cpp::popcount(~value);
+}
+
+// Specialization of first_leading_zero ('math_extras.h') for BigInt.
+template <typename T>
+[[nodiscard]]
+LIBC_INLINE constexpr cpp::enable_if_t<is_big_int_v<T>, int>
+first_leading_zero(T value) {
+  return value == cpp::numeric_limits<T>::max() ? 0
+                                                : cpp::countl_one(value) + 1;
+}
+
+// Specialization of first_leading_one ('math_extras.h') for BigInt.
+template <typename T>
+[[nodiscard]]
+LIBC_INLINE constexpr cpp::enable_if_t<is_big_int_v<T>, int>
+first_leading_one(T value) {
+  return first_leading_zero(~value);
+}
+
+// Specialization of first_trailing_zero ('math_extras.h') for BigInt.
+template <typename T>
+[[nodiscard]]
+LIBC_INLINE constexpr cpp::enable_if_t<is_big_int_v<T>, int>
+first_trailing_zero(T value) {
+  return value == cpp::numeric_limits<T>::max() ? 0
+                                                : cpp::countr_zero(~value) + 1;
+}
+
+// Specialization of first_trailing_one ('math_extras.h') for BigInt.
+template <typename T>
+[[nodiscard]]
+LIBC_INLINE constexpr cpp::enable_if_t<is_big_int_v<T>, int>
+first_trailing_one(T value) {
+  return value == cpp::numeric_limits<T>::max() ? 0
+                                                : cpp::countr_zero(value) + 1;
+}
+
 } // namespace LIBC_NAMESPACE
 
 #endif // LLVM_LIBC_SRC___SUPPORT_UINT_H

libc/test/src/__support/math_extras_test.cpp

@@ -6,7 +6,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-#include "src/__support/UInt128.h" // UInt128
+#include "src/__support/UInt128.h" // UInt<128>
 #include "src/__support/integer_literals.h"
 #include "src/__support/math_extras.h"
 #include "test/UnitTest/Test.h"
@@ -19,7 +19,7 @@ using UnsignedTypesNoBigInt = testing::TypeList<
     __uint128_t,
 #endif // LIBC_TYPES_HAS_INT128
     unsigned char, unsigned short, unsigned int, unsigned long,
-    unsigned long long>;
+    unsigned long long, UInt<128>>;
 
 TEST(LlvmLibcBlockMathExtrasTest, mask_trailing_ones) {
   EXPECT_EQ(0_u8, (mask_leading_ones<uint8_t, 0>()));