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

[Nikolas Klauser via libcxx-commits]

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@@ -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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philnik777 wrote:

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.

https://github.com/llvm/llvm-project/pull/160679