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[libc++] Use _BitInt and __builtin_popcountg in bitset::count() #160679

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Merged
merged 1 commit into from
Sep 25, 2025
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[libc++] Use _BitInt and __builtin_popcountg in bitset::count() #160679

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11 changes: 10 additions & 1 deletion libcxx/include/bitset
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Original file line number Diff line number Diff line change
Expand Up @@ -867,7 +867,16 @@ bitset<_Size>::to_string(char __zero, char __one) const {

template <size_t _Size>
inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 size_t bitset<_Size>::count() const _NOEXCEPT {
return static_cast<size_t>(std::count(__base::__make_iter(0), __base::__make_iter(_Size), true));
# if defined(_LIBCPP_COMPILER_CLANG_BASED) && !defined(_LIBCPP_CXX03_LANG)
if constexpr (_Size == 0) {
return 0;
} else if constexpr (_Size <= __base::__bits_per_word) {
return __builtin_popcountg(static_cast<unsigned _BitInt(_Size)>(__base::__first_));
} else
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Do we really need this else? Can't we have

#if defined(CLANG)
if constexpr (...) {
  // A
} else if constexpr (...) {
 // B
}
#endif

return std::count(...);

Seems a bit simpler and equivalent. Are you worried about the mere presence of return std::count(...) preventing inlining?

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We don't need the else, but it improves compile times quite a bit, since we avoid instantiating std::count altogether with it. IMO the tiny simplification isn't worth the compile time increase.

# endif
{
return static_cast<size_t>(std::count(__base::__make_iter(0), __base::__make_iter(_Size), true));
}
}

template <size_t _Size>
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