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

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Merged
merged 3 commits into from
Mar 7, 2025
Merged

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

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afe4ff0
fix bug
c8ef Mar 6, 2025
4467cf6
address review comments
c8ef Mar 7, 2025
f855488
Merge branch 'main' into ctz
c8ef Mar 7, 2025
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3 changes: 1 addition & 2 deletions libc/src/__support/big_int.h
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Original file line number Diff line number Diff line change
Expand Up @@ -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
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@lntue lntue Mar 6, 2025

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it should be just value == 0. When value is not 0, cpp::countr_zero(value) + 1 should be correct.

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@c8ef c8ef Mar 7, 2025

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It will fail with inputs like USHRT_MAX because we need it to return 0, but cpp::countr_zero(USHRT_MAX) + 1 = 1.

: cpp::countr_zero(value) + 1;
return value == 0 ? 0 : cpp::countr_zero(value) + 1;
}

} // namespace LIBC_NAMESPACE_DECL
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3 changes: 1 addition & 2 deletions libc/src/__support/math_extras.h
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Expand Up @@ -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>
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3 changes: 2 additions & 1 deletion libc/test/src/__support/math_extras_test.cpp
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Expand Up @@ -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);
}
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2 changes: 1 addition & 1 deletion libc/test/src/stdbit/stdc_first_trailing_one_uc_test.cpp
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Expand Up @@ -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) {
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2 changes: 1 addition & 1 deletion libc/test/src/stdbit/stdc_first_trailing_one_ui_test.cpp
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Expand Up @@ -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) {
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2 changes: 1 addition & 1 deletion libc/test/src/stdbit/stdc_first_trailing_one_ul_test.cpp
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Expand Up @@ -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) {
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2 changes: 1 addition & 1 deletion libc/test/src/stdbit/stdc_first_trailing_one_ull_test.cpp
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Expand Up @@ -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) {
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2 changes: 1 addition & 1 deletion libc/test/src/stdbit/stdc_first_trailing_one_us_test.cpp
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Expand Up @@ -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) {
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