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Re: [PATCH] libstdc++: Optimize C++20 comparison category types
Am Do., 6. Feb. 2020 um 15:28 Uhr schrieb Jonathan Wakely <jwakely@redhat.com>:
>
> On 06/02/20 13:53 +0000, Jonathan Wakely wrote:
> >On 06/02/20 13:40 +0000, Jonathan Wakely wrote:
> >>This reduces sizeof(std::partial_ordering) and optimizes conversion and
> >>comparison operators to avoid conditional branches where possible.
> >>
> >> * libsupc++/compare (__cmp_cat::_Ncmp::unordered): Change value to 2.
> >> (partial_ordering::_M_is_ordered): Remove data member.
> >> (partial_ordering): Use second bit of _M_value for unordered. Adjust
> >> comparison operators.
> >> (weak_ordering::operator partial_ordering): Simplify to remove
> >> branches.
> >> (operator<=>(unspecified, weak_ordering)): Likewise.
> >> (strong_ordering::operator partial_ordering): Likewise.
> >> (strong_ordering::operator weak_ordering): Likewise.
> >> (operator<=>(unspecified, strong_ordering)): Likewise.
> >> * testsuite/18_support/comparisons/categories/partialord.cc: New test.
> >> * testsuite/18_support/comparisons/categories/strongord.cc: New test.
> >> * testsuite/18_support/comparisons/categories/weakord.cc: New test.
> >>
> >>Tested powerpc64le-linux and x86_64-linux.
> >>
> >>This is an ABI change for the partial_ordering type, but that is why I
> >>think we should do it now, not after GCC 10 is released. The sooner
> >>the better, before these types are being widely used.
> >>
> >>I plan to commit this in the next 12 hours or so, unless there are
> >>(valid :-) objections.
> >>
> >>Thanks to Barry Revzin for pointing out there was room for these
> >>operators to be improved.
> >
> >We could also change the int _M_value data member of all three
> >comparison category types to be a signed char instead of int. That
> >would reduce the size further.
>
> Or maybe std::int_fast8_t is the right type here.
I like this suggestion.
- Daniel