[libc++] Optimize std::has_single_bit v2.0

Alcaro · web-flow · commit 144f83e43eca · 2025-11-05T13:03:01.000+01:00
diff --git a/libcxx/include/__bit/has_single_bit.h b/libcxx/include/__bit/has_single_bit.h
@@ -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 ^ (__t - 1)) > __t - 1;
+  return __builtin_popcountg(__t) == 1;
 }
 
 _LIBCPP_END_NAMESPACE_STD

Original file line number	Diff line number	Diff line change
`@@ -25,7 +25,7 @@ _LIBCPP_BEGIN_NAMESPACE_STD`
`25`	`25`
`26`	`26`	`template <__libcpp_unsigned_integer _Tp>`
`27`	`27`	`[[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr bool has_single_bit(_Tp __t) noexcept {`
`28`		`- return (__t ^ (__t - 1)) > __t - 1;`
	`28`	`+ return __builtin_popcountg(__t) == 1;`
`29`	`29`	`}`
`30`	`30`
`31`	`31`	`_LIBCPP_END_NAMESPACE_STD`