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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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5 changes: 3 additions & 2 deletions libc/src/__support/big_int.h
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Original file line number Diff line number Diff line change
Expand Up @@ -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
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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 || value == cpp::numeric_limits<T>::max())
? 0
: cpp::countr_zero(value) + 1;
}

} // namespace LIBC_NAMESPACE_DECL
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5 changes: 3 additions & 2 deletions libc/src/__support/math_extras.h
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Expand Up @@ -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())
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@lntue lntue Mar 6, 2025

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it should be just value == 0, the other case is cpp::countr_zero(value) + 1

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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.

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@lntue lntue Mar 7, 2025

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Why should it return 0 with USHRT_MAX? The least significant index of the first bit 1 of USHRT_MAX is 0, so according to the spec, stdc_first_trailing_one_us(USHRT_MAX) should be 1. Hence cpp::countr_zero(USHRT_MAX) + 1 is the correct. If our tests do not reflect that, then the tests also need to be updated.

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@c8ef c8ef Mar 7, 2025

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Got it finally. Sorry for being careless. I'll update this tonight.

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

template <typename T>
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1 change: 1 addition & 0 deletions libc/test/src/__support/math_extras_test.cpp
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Expand Up @@ -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));
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