This reimplements std::has_single_bit using the well-known bit-twiddilng trick[1], which is much faster than popcount on x86_64.
Note that when __x is signed and maximally negative then this implementation invokes UB due to signed overflow, whereas the previous implementation would return true. This isn't a problem for has_single_bit because it accepts only unsigned types, but it is a potential problem for the unconstrained __has_single_bit. Should __has_single_bit continue to handle this non-standard case correctly for sake of backwards compatibility? Tested on x86_64-pc-linux-gnu. [1]: http://www.graphics.stanford.edu/~seander/bithacks.html#DetermineIfPowerOf2 libstdc++-v3/ChangeLog: * include/std/bit (__has_single_bit): Define in terms of bitwise-and, not popcount. --- libstdc++-v3/include/std/bit | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/libstdc++-v3/include/std/bit b/libstdc++-v3/include/std/bit index ef19d649e32..621ee4a9b95 100644 --- a/libstdc++-v3/include/std/bit +++ b/libstdc++-v3/include/std/bit @@ -316,7 +316,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION template<typename _Tp> constexpr bool __has_single_bit(_Tp __x) noexcept - { return std::__popcount(__x) == 1; } + { return __x != 0 && (__x & (__x - 1)) == 0; } template<typename _Tp> constexpr _Tp -- 2.36.0.rc2.10.g1ac7422e39
