Re: [PATCH] libstdc++: Optimize C++20 comparison category types

[Jonathan Wakely <jwakely at redhat dot com>]

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?