[libcxx-commits] [libcxx] [libc++] Optimize std::has_single_bit (PR #133063)

[Nikolas Klauser via libcxx-commits]

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@@ -25,7 +25,7 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 
 template <__libcpp_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 ^ (__t - 1)) > __t - 1;
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philnik777 wrote:

I think `__builtin_popcountg(__t) == 1` would be better. That results in identical code and is obviously correct (and if some backend wants to handle this case specially it's trivial to match the IR).

https://github.com/llvm/llvm-project/pull/133063