__vector equality compare: LDC vs {GDC, DMD}

[Bruce Carneal]

On Wednesday, 27 July 2022 at 08:04:03 UTC, kinke wrote:
> On Wednesday, 27 July 2022 at 03:50:50 UTC, Bruce Carneal wrote:
>> That said, I am curious: is this behavior intentional or 
>> accidental?
>
> Intentional (based on the front-end type of (in)equality and 
> identity expressions being a scalar bool, no special case for 
> vectors): 
> https://github.com/ldc-developers/ldc/commit/db8797e1ced40d555baa3007e6a5f6d05e29b58d

OK. You will be aware of all of these but for future reference 
here are six things the compiler could do when presented with a 
vector relOp:

1) error out (the current GDC and DMD choice)
2) produce a scalar boolean (the current LDC choice for 
eql/not-eql)
3) produce a bit-per-lane reduction to a scalar (as provided by 
ballot intrinsics)
4) produce a vector of one byte booleans
5) produce a full lane width vector of "booleans" (with value 
zero or one)
6) produce a full lane width vector of masks (all zeros or all 
ones)

As noted in the linked to material, 6) is popular.  Fortunately 6)
is reliably and efficiently synthesized by LDC and GDC from a 
simple known-bounds for loop over a ternary.