[libc] first_trailing_one(0) should be 0. (#130155)

Fix this minor bug. See more context at #129892.

Author: c8ef

libc/src/__support/big_int.h

@@ -1380,8 +1380,7 @@ 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 ? 0 : cpp::countr_zero(value) + 1;
 }
 
 } // namespace LIBC_NAMESPACE_DECL

libc/src/__support/math_extras.h

@@ -146,8 +146,7 @@ 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 ? 0 : cpp::countr_zero(value) + 1;
 }
 
 template <typename T>

libc/test/src/__support/math_extras_test.cpp

@@ -91,7 +91,8 @@ TYPED_TEST(LlvmLibcBitTest, FirstTrailingZero, UnsignedTypesNoBigInt) {
 }
 
 TYPED_TEST(LlvmLibcBitTest, FirstTrailingOne, UnsignedTypesNoBigInt) {
-  EXPECT_EQ(first_trailing_one<T>(cpp::numeric_limits<T>::max()), 0);
+  EXPECT_EQ(first_trailing_one<T>(static_cast<T>(0)), 0);
+  EXPECT_EQ(first_trailing_one<T>(cpp::numeric_limits<T>::max()), 1);
   for (int i = 0U; i != cpp::numeric_limits<T>::digits; ++i)
     EXPECT_EQ(first_trailing_one<T>(T(1) << i), i + 1);
 }

libc/test/src/stdbit/stdc_first_trailing_one_uc_test.cpp

@@ -11,7 +11,7 @@
 #include "test/UnitTest/Test.h"
 
 TEST(LlvmLibcStdcFirstTrailingOneUcTest, ALL) {
-  EXPECT_EQ(LIBC_NAMESPACE::stdc_first_trailing_one_uc(UCHAR_MAX), 0U);
+  EXPECT_EQ(LIBC_NAMESPACE::stdc_first_trailing_one_uc(UCHAR_MAX), 1U);
 }
 
 TEST(LlvmLibcStdcFirstTrailingOneUcTest, OneHot) {

libc/test/src/stdbit/stdc_first_trailing_one_ui_test.cpp

@@ -11,7 +11,7 @@
 #include "test/UnitTest/Test.h"
 
 TEST(LlvmLibcStdcFirstTrailingOneUiTest, ALL) {
-  EXPECT_EQ(LIBC_NAMESPACE::stdc_first_trailing_one_ui(UINT_MAX), 0U);
+  EXPECT_EQ(LIBC_NAMESPACE::stdc_first_trailing_one_ui(UINT_MAX), 1U);
 }
 
 TEST(LlvmLibcStdcFirstTrailingOneUiTest, OneHot) {

libc/test/src/stdbit/stdc_first_trailing_one_ul_test.cpp

@@ -11,7 +11,7 @@
 #include "test/UnitTest/Test.h"
 
 TEST(LlvmLibcStdcFirstTrailingOneUlTest, ALL) {
-  EXPECT_EQ(LIBC_NAMESPACE::stdc_first_trailing_one_ul(ULONG_MAX), 0U);
+  EXPECT_EQ(LIBC_NAMESPACE::stdc_first_trailing_one_ul(ULONG_MAX), 1U);
 }
 
 TEST(LlvmLibcStdcFirstTrailingOneUlTest, OneHot) {

libc/test/src/stdbit/stdc_first_trailing_one_ull_test.cpp

@@ -11,7 +11,7 @@
 #include "test/UnitTest/Test.h"
 
 TEST(LlvmLibcStdcFirstTrailingOneUllTest, ALL) {
-  EXPECT_EQ(LIBC_NAMESPACE::stdc_first_trailing_one_ull(ULLONG_MAX), 0U);
+  EXPECT_EQ(LIBC_NAMESPACE::stdc_first_trailing_one_ull(ULLONG_MAX), 1U);
 }
 
 TEST(LlvmLibcStdcFirstTrailingOneUllTest, OneHot) {

libc/test/src/stdbit/stdc_first_trailing_one_us_test.cpp

@@ -11,7 +11,7 @@
 #include "test/UnitTest/Test.h"
 
 TEST(LlvmLibcStdcFirstTrailingOneUsTest, ALL) {
-  EXPECT_EQ(LIBC_NAMESPACE::stdc_first_trailing_one_us(USHRT_MAX), 0U);
+  EXPECT_EQ(LIBC_NAMESPACE::stdc_first_trailing_one_us(USHRT_MAX), 1U);
 }
 
 TEST(LlvmLibcStdcFirstTrailingOneUsTest, OneHot) {