fix bug

Author: c8ef

libc/src/__support/big_int.h

@@ -1383,8 +1383,9 @@ first_trailing_zero(T value) {
 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;
+  return (value == 0 || value == cpp::numeric_limits<T>::max())
+             ? 0
+             : cpp::countr_zero(value) + 1;
 }
 
 } // namespace LIBC_NAMESPACE_DECL

libc/src/__support/math_extras.h

@@ -146,8 +146,9 @@ first_trailing_zero(T value) {
 template <typename T>
 [[nodiscard]] LIBC_INLINE constexpr cpp::enable_if_t<cpp::is_unsigned_v<T>, int>
 first_trailing_one(T value) {
-  return value == cpp::numeric_limits<T>::max() ? 0
-                                                : cpp::countr_zero(value) + 1;
+  return (value == 0 || value == cpp::numeric_limits<T>::max())
+             ? 0
+             : cpp::countr_zero(value) + 1;
 }
 
 template <typename T>

libc/test/src/__support/math_extras_test.cpp

@@ -95,6 +95,7 @@ TYPED_TEST(LlvmLibcBitTest, FirstTrailingZero, UnsignedTypesNoBigInt) {
 }
 
 TYPED_TEST(LlvmLibcBitTest, FirstTrailingOne, UnsignedTypesNoBigInt) {
+  EXPECT_EQ(first_trailing_one<T>(static_cast<T>(0)), 0);
   EXPECT_EQ(first_trailing_one<T>(cpp::numeric_limits<T>::max()), 0);
   for (int i = 0U; i != cpp::numeric_limits<T>::digits; ++i) {
     auto lhs = T(T(1) << size_t(i));