Mark WaveGetLaneCount as constexpr

[Jasper-Bekkers]

Currently one can't easily do this; instead you'd have to create a new shader permutation for every supported wave size.

groupshared uint stuff[WaveGetLaneCount()];

[simondeschenes]

I would also like to see this supported.

[llvm-beanz]

WaveGetLaneCount can't be a constexpr. It isn't a compile-time known value. It should be known in the driver compiler but it isn't in the frontend so it can't be constexpr.

The use case of being able to construct a wave-width array could potentially be addressed another way, but constexpr isn't the answer here.

[MarijnS95]

@llvm-beanz perhaps leave this open (and retitle) for that "other way"?

[llvm-beanz]

@MarijnS95, I think the correct approach for this is to add some sort of support for specialization constants to DXIL which we have tracked in the hlsl-specs repo issue #16.

While at first glance these may seem slightly orthogonal, both require runtime & driver initialized constants, which is what this is too. We have the same challenge with #4290.

So, I think the TL;DR here is that this will all be rolled into specialization constant support, and we don't really need multiple issues tracking it.

[MarijnS95]

@llvm-beanz that sounds like the perfect match for specialization constants, as long as it is tracked somewhere that WaveGetLaneCount() should be treated as such? Seems that (treating driver-defined constants as specialization constants) is covered in microsoft/hlsl-specs#16 (comment).