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[libcxx] Use generic builtins for popcount, clz and ctz #86563

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merged 7 commits into from Apr 11, 2024
Merged

[libcxx] Use generic builtins for popcount, clz and ctz #86563

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f4ca119
[libcxx] Use generic builtins for popcount, clz and ctz
marcauberer Mar 25, 2024
3535509
Shortcuts that benefit from generic builtins
marcauberer Mar 25, 2024
227d89e
Simplify
marcauberer Mar 25, 2024
41eb681
Do cleanup and remove non-feasible usages
marcauberer Mar 26, 2024
6d969ba
Fix typo
marcauberer Mar 27, 2024
bf43981
Avoid duplicate signatures
marcauberer Mar 28, 2024
d3c2847
Use optional second parameter of c{l|t}z builtins
marcauberer Mar 29, 2024
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11 changes: 11 additions & 0 deletions libcxx/include/__bit/countl.h
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Expand Up @@ -6,6 +6,9 @@
//
//===----------------------------------------------------------------------===//

// TODO: __builtin_clzg is available since Clang 19 and GCC 14. When support for older versions is dropped, we can
// refactor this code to exclusively use __builtin_clzg.

#ifndef _LIBCPP___BIT_COUNTL_H
#define _LIBCPP___BIT_COUNTL_H

Expand Down Expand Up @@ -38,6 +41,9 @@ _LIBCPP_NODISCARD inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR int __libcpp_cl

#ifndef _LIBCPP_HAS_NO_INT128
inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR int __libcpp_clz(__uint128_t __x) _NOEXCEPT {
# if __has_builtin(__builtin_clzg)
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return __builtin_clzg(__x);
# else
// The function is written in this form due to C++ constexpr limitations.
// The algorithm:
// - Test whether any bit in the high 64-bits is set
Expand All @@ -49,12 +55,16 @@ inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR int __libcpp_clz(__uint128_t __x)
// zeros in the high 64-bits.
return ((__x >> 64) == 0) ? (64 + __builtin_clzll(static_cast<unsigned long long>(__x)))
: __builtin_clzll(static_cast<unsigned long long>(__x >> 64));
# endif
}
#endif // _LIBCPP_HAS_NO_INT128

template <class _Tp>
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_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 int __countl_zero(_Tp __t) _NOEXCEPT {
static_assert(__libcpp_is_unsigned_integer<_Tp>::value, "__countl_zero requires an unsigned integer type");
#if __has_builtin(__builtin_clzg)
return __builtin_clzg(__t, numeric_limits<_Tp>::digits);
#else // __has_builtin(__builtin_clzg)
if (__t == 0)
return numeric_limits<_Tp>::digits;

Expand All @@ -79,6 +89,7 @@ _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 int __countl_zero(_Tp __t) _
}
return __ret + __iter;
}
#endif // __has_builtin(__builtin_clzg)
}

#if _LIBCPP_STD_VER >= 20
Expand Down
8 changes: 7 additions & 1 deletion libcxx/include/__bit/countr.h
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Expand Up @@ -6,6 +6,9 @@
//
//===----------------------------------------------------------------------===//

// TODO: __builtin_ctzg is available since Clang 19 and GCC 14. When support for older versions is dropped, we can
// refactor this code to exclusively use __builtin_ctzg.

#ifndef _LIBCPP___BIT_COUNTR_H
#define _LIBCPP___BIT_COUNTR_H

Expand Down Expand Up @@ -37,9 +40,11 @@ _LIBCPP_NODISCARD inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR int __libcpp_ct

template <class _Tp>
_LIBCPP_NODISCARD _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 int __countr_zero(_Tp __t) _NOEXCEPT {
#if __has_builtin(__builtin_ctzg)
return __builtin_ctzg(__t, numeric_limits<_Tp>::digits);
#else // __has_builtin(__builtin_ctzg)
if (__t == 0)
return numeric_limits<_Tp>::digits;

if (sizeof(_Tp) <= sizeof(unsigned int))
return std::__libcpp_ctz(static_cast<unsigned int>(__t));
else if (sizeof(_Tp) <= sizeof(unsigned long))
Expand All @@ -55,6 +60,7 @@ _LIBCPP_NODISCARD _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 int __coun
}
return __ret + std::__libcpp_ctz(static_cast<unsigned long long>(__t));
}
#endif // __has_builtin(__builtin_ctzg)
}

#if _LIBCPP_STD_VER >= 20
Expand Down
7 changes: 7 additions & 0 deletions libcxx/include/__bit/popcount.h
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Expand Up @@ -6,6 +6,9 @@
//
//===----------------------------------------------------------------------===//

// TODO: __builtin_popcountg is available since Clang 19 and GCC 14. When support for older versions is dropped, we can
// refactor this code to exclusively use __builtin_popcountg.

#ifndef _LIBCPP___BIT_POPCOUNT_H
#define _LIBCPP___BIT_POPCOUNT_H

Expand Down Expand Up @@ -39,6 +42,9 @@ inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR int __libcpp_popcount(unsigned lo

template <__libcpp_unsigned_integer _Tp>
_LIBCPP_NODISCARD_EXT _LIBCPP_HIDE_FROM_ABI constexpr int popcount(_Tp __t) noexcept {
# if __has_builtin(__builtin_popcountg)
return __builtin_popcountg(__t);
# else // __has_builtin(__builtin_popcountg)
if (sizeof(_Tp) <= sizeof(unsigned int))
return std::__libcpp_popcount(static_cast<unsigned int>(__t));
else if (sizeof(_Tp) <= sizeof(unsigned long))
Expand All @@ -53,6 +59,7 @@ _LIBCPP_NODISCARD_EXT _LIBCPP_HIDE_FROM_ABI constexpr int popcount(_Tp __t) noex
}
return __ret;
}
# endif // __has_builtin(__builtin_popcountg)
}

#endif // _LIBCPP_STD_VER >= 20
Expand Down