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. It probably doesn't matter for most uses, only when one of the types is used as a data member and the smaller type would allow a more compact layout. I'm not sure how common such uses will be, but I suppose it's plausible somebody could have a function returning a std::tuple<std::partial_ordering, char> which would benefit. Anybody want to argue for or against making them 8 bits?
