[libc++] Optimize std::has_single_bit (llvm#133063)

Alcaro · aadeshps-mcw · commit 132d2c999fce · 2025-11-26T15:54:06.000+05:30
Clang translates most implementations of has_single_bit to `(v ^ (v-1)) > v-1` - except the one definition libc++ actually uses. Proof of correctness: https://godbolt.org/z/d61bxW4r1 (Could also be fixed by teaching Clang to optimize better, but making source match output feels clearer to me. And it improves unoptimized performance.)
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 <__unsigned_integer _Tp>
 [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr bool has_single_bit(_Tp __t) noexcept {
-  return __t != 0 && ((__t & (__t - 1)) == 0);
+  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 <__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 __builtin_popcountg(__t) == 1;`
`29`	`29`	`}`
`30`	`30`
`31`	`31`	`_LIBCPP_END_NAMESPACE_STD`