[libc++] Optimize std::has_single_bit

Alcaro · web-flow · commit 549d11d551f8 · 2025-03-26T10:46:19.000+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 != 0 && (((__t & (__t - 1)) == 0));
+  return (__t ^ (__t - 1)) > __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 != 0 && (((__t & (__t - 1)) == 0));`
	`28`	`+ return (__t ^ (__t - 1)) > __t - 1;`
`29`	`29`	`}`
`30`	`30`
`31`	`31`	`_LIBCPP_END_NAMESPACE_STD`