On Thu, Apr 14, 2022 at 2:59 PM Jonathan Wakely <jwak...@redhat.com> wrote: > > On Thu, 14 Apr 2022 at 19:17, Patrick Palka via Libstdc++ > <libstd...@gcc.gnu.org> wrote: > > > > This reimplements std::has_single_bit using the well-known bit-twiddilng > > trick[1], which is much faster than popcount on x86_64. > > Is that always true for all microarchitectures? We have > https://gcc.gnu.org/PR97759 on this topic, and I think we agreed that > the compiler should match the popcount pattern and Do The Right Thing > for the target and current -march.
Whoops, I completely forgot that we had a PR about this! Makes sense to fix this on the compiler side instead. > > If we're confident it's always better, that PR number should go in the > changelog. > > > 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? > > No. The extensions have the same preconditions as the corresponding > standard functions, we just don't check them. The code using them is > internal to the library and should only use unsigned types. Users > relying on the extensions need to meet those preconditions too. Understood, thanks! > > > 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 > > >
