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[libc++] Simplify some of the <bit> functions #160267

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Merged
merged 1 commit into from
Sep 24, 2025
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[libc++] Simplify some of the <bit> functions #160267

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2 changes: 1 addition & 1 deletion libcxx/include/__bit/countl.h
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Expand Up @@ -37,7 +37,7 @@ template <__unsigned_integer _Tp>

template <__unsigned_integer _Tp>
[[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr int countl_one(_Tp __t) noexcept {
return __t != numeric_limits<_Tp>::max() ? std::countl_zero(static_cast<_Tp>(~__t)) : numeric_limits<_Tp>::digits;
return std::countl_zero(static_cast<_Tp>(~__t));
}

#endif // _LIBCPP_STD_VER >= 20
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2 changes: 1 addition & 1 deletion libcxx/include/__bit/countr.h
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Expand Up @@ -37,7 +37,7 @@ template <__unsigned_integer _Tp>

template <__unsigned_integer _Tp>
[[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr int countr_one(_Tp __t) noexcept {
return __t != numeric_limits<_Tp>::max() ? std::countr_zero(static_cast<_Tp>(~__t)) : numeric_limits<_Tp>::digits;
return std::countr_zero(static_cast<_Tp>(~__t));
}

#endif // _LIBCPP_STD_VER >= 20
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2 changes: 1 addition & 1 deletion libcxx/include/__bit/has_single_bit.h
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Expand Up @@ -25,7 +25,7 @@ _LIBCPP_BEGIN_NAMESPACE_STD

template <__unsigned_integer _Tp>
[[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr bool has_single_bit(_Tp __t) noexcept {
return __t != 0 && (((__t & (__t - 1)) == 0));
return __t != 0 && ((__t & (__t - 1)) == 0);
}

_LIBCPP_END_NAMESPACE_STD
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41 changes: 15 additions & 26 deletions libcxx/include/__bit/rotate.h
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Expand Up @@ -22,46 +22,35 @@ _LIBCPP_BEGIN_NAMESPACE_STD
// Writing two full functions for rotl and rotr makes it easier for the compiler
// to optimize the code. On x86 this function becomes the ROL instruction and
// the rotr function becomes the ROR instruction.
template <class _Tp>
_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 _Tp __rotl(_Tp __x, int __s) _NOEXCEPT {
static_assert(__is_unsigned_integer_v<_Tp>, "__rotl requires an unsigned integer type");

#if _LIBCPP_STD_VER >= 20

template <__unsigned_integer _Tp>
[[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _Tp rotl(_Tp __t, int __cnt) noexcept {
const int __n = numeric_limits<_Tp>::digits;
int __r = __s % __n;
int __r = __cnt % __n;

if (__r == 0)
return __x;
return __t;

if (__r > 0)
return (__x << __r) | (__x >> (__n - __r));
return (__t << __r) | (__t >> (__n - __r));

return (__x >> -__r) | (__x << (__n + __r));
return (__t >> -__r) | (__t << (__n + __r));
}

template <class _Tp>
_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 _Tp __rotr(_Tp __x, int __s) _NOEXCEPT {
static_assert(__is_unsigned_integer_v<_Tp>, "__rotr requires an unsigned integer type");
template <__unsigned_integer _Tp>
[[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _Tp rotr(_Tp __t, int __cnt) noexcept {
const int __n = numeric_limits<_Tp>::digits;
int __r = __s % __n;
int __r = __cnt % __n;

if (__r == 0)
return __x;
return __t;

if (__r > 0)
return (__x >> __r) | (__x << (__n - __r));

return (__x << -__r) | (__x >> (__n + __r));
}
return (__t >> __r) | (__t << (__n - __r));

#if _LIBCPP_STD_VER >= 20

template <__unsigned_integer _Tp>
[[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _Tp rotl(_Tp __t, int __cnt) noexcept {
return std::__rotl(__t, __cnt);
}

template <__unsigned_integer _Tp>
[[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _Tp rotr(_Tp __t, int __cnt) noexcept {
return std::__rotr(__t, __cnt);
return (__t << -__r) | (__t >> (__n + __r));
}

#endif // _LIBCPP_STD_VER >= 20
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3 changes: 0 additions & 3 deletions libcxx/test/libcxx/numerics/bit.ops.pass.cpp
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Expand Up @@ -11,18 +11,15 @@

#include <__bit/bit_log2.h>
#include <__bit/countl.h>
#include <__bit/rotate.h>
#include <cassert>

#include "test_macros.h"

TEST_CONSTEXPR_CXX14 bool test() {
const unsigned v = 0x12345678;

ASSERT_SAME_TYPE(unsigned, decltype(std::__rotr(v, 3)));
ASSERT_SAME_TYPE(int, decltype(std::__countl_zero(v)));

assert(std::__rotr(v, 3) == 0x02468acfU);
assert(std::__countl_zero(v) == 3);

#if TEST_STD_VER > 17
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