[libc-commits] [libc] dd9a2f0 - [libc] `first_trailing_one(0)` should be `0`. (#130155)

[via libc-commits]

Author: Connector Switch
Date: 2025-03-08T00:11:43+08:00
New Revision: dd9a2f0677312beb6785c2790b54836959046e96

URL: https://github.com/llvm/llvm-project/commit/dd9a2f0677312beb6785c2790b54836959046e96
DIFF: https://github.com/llvm/llvm-project/commit/dd9a2f0677312beb6785c2790b54836959046e96.diff

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

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

Added: 
    

Modified: 
    libc/src/__support/big_int.h
    libc/src/__support/math_extras.h
    libc/test/src/__support/math_extras_test.cpp
    libc/test/src/stdbit/stdc_first_trailing_one_uc_test.cpp
    libc/test/src/stdbit/stdc_first_trailing_one_ui_test.cpp
    libc/test/src/stdbit/stdc_first_trailing_one_ul_test.cpp
    libc/test/src/stdbit/stdc_first_trailing_one_ull_test.cpp
    libc/test/src/stdbit/stdc_first_trailing_one_us_test.cpp

Removed: 
    


################################################################################
diff  --git a/libc/src/__support/big_int.h b/libc/src/__support/big_int.h
index e726a094b5dac..f680a76d82a7d 100644
--- a/libc/src/__support/big_int.h
+++ b/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

diff  --git a/libc/src/__support/math_extras.h b/libc/src/__support/math_extras.h
index 6f4a006aad270..47df2a43250c8 100644
--- a/libc/src/__support/math_extras.h
+++ b/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>

diff  --git a/libc/test/src/__support/math_extras_test.cpp b/libc/test/src/__support/math_extras_test.cpp
index 08c090017c1a1..9b1a3dd86731c 100644
--- a/libc/test/src/__support/math_extras_test.cpp
+++ b/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);
 }

diff  --git a/libc/test/src/stdbit/stdc_first_trailing_one_uc_test.cpp b/libc/test/src/stdbit/stdc_first_trailing_one_uc_test.cpp
index 5ca4cfca1b479..f507e9ac12c2a 100644
--- a/libc/test/src/stdbit/stdc_first_trailing_one_uc_test.cpp
+++ b/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) {

diff  --git a/libc/test/src/stdbit/stdc_first_trailing_one_ui_test.cpp b/libc/test/src/stdbit/stdc_first_trailing_one_ui_test.cpp
index 137c8a42e407d..3109d6888d534 100644
--- a/libc/test/src/stdbit/stdc_first_trailing_one_ui_test.cpp
+++ b/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) {

diff  --git a/libc/test/src/stdbit/stdc_first_trailing_one_ul_test.cpp b/libc/test/src/stdbit/stdc_first_trailing_one_ul_test.cpp
index 3fc1f3f16c60d..8d1f39acff8d3 100644
--- a/libc/test/src/stdbit/stdc_first_trailing_one_ul_test.cpp
+++ b/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) {

diff  --git a/libc/test/src/stdbit/stdc_first_trailing_one_ull_test.cpp b/libc/test/src/stdbit/stdc_first_trailing_one_ull_test.cpp
index 5719e09a5120a..12ef0537e3281 100644
--- a/libc/test/src/stdbit/stdc_first_trailing_one_ull_test.cpp
+++ b/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) {

diff  --git a/libc/test/src/stdbit/stdc_first_trailing_one_us_test.cpp b/libc/test/src/stdbit/stdc_first_trailing_one_us_test.cpp
index 46c69acfb7f07..6a506e6cc9632 100644
--- a/libc/test/src/stdbit/stdc_first_trailing_one_us_test.cpp
+++ b/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) {