Adds WaveShuffle intrinsic (#1298)

* Support for WaveReadLaneAt with dynamic (but uniform across Wave) on Vk by enabling VK1.4.
Fixed wave-lane-at.slang test to test with laneId that is uniform across the Wave.

* Added WaveShuffle intrinsic.
Test for WaveShuffle intrinsic.

* Added some documentation on WaveShuffle

* Fix that version required for subgroupBroadcast to be non constexpr is actually 1.5

1 files changed, 11 insertions, 0 deletions

diff --git a/docs/target-compatibility.md b/docs/target-compatibility.md
index e8edeeaf5..6967f7454 100644
--- a/docs/target-compatibility.md
+++ b/docs/target-compatibility.md
@@ -20,6 +20,7 @@ Items with ^ means there is some discussion about support later in the document
 | SM6.0 Wave Intrinsics       |     No       |   Yes        |  Partial   |     Yes       |    No
 | SM6.0 Quad Intrinsics       |     No       |   Yes        |   No +     |     No        |    No
 | SM6.5 Wave Intrinsics       |     No       |   Yes ^      |   No +     |     Yes       |    No
+| WaveShuffle                 |     No       |   Limited ^  |   Yes +    |     Yes       |    No
 | Tesselation                 |     Yes ^    |   Yes ^      |   No +     |     No        |    No
 | Graphics Pipeline           |     Yes      |   Yes        |   Yes      |     No        |    No
 | Ray Tracing DXR 1.0         |     No       |   Yes ^      |   Yes ^    |     No        |    No
@@ -56,6 +57,16 @@ tex.GetDimensions is the GetDimensions method on 'texture' objects. This is not
 
 SM6.5 Wave Intrinsics are supported, but requires a downstream DXC compiler that supports SM6.5. As it stands the DXC shipping with windows does not. 
 
+## WaveShuffle
+
+WaveShuffle is an intrinsic added to the Slang stdlibrary to expose the glsl `subgroupShuffle` intrinsics and allow loosened requirements on laneId. 
+
+`HLSL` uses `WaveReadLaneAt` and this requires the `laneId` must be 'dynamically uniform' across the wave. WaveShuffle has the same functionality but relaxes this restriction. 
+
+`WaveReadLaneAt` most obviously maps to `subgroupBroadcast` in GLSL. This has the extra restriction the index must be compile time consts. With SPIR-V 1.5 it is allowed to be 'dynamically uniform', but doesn't work on current glslang.
+
+NOTE! That using WaveShuffle to target `HLSL` will produce `WaveReadLaneAt` - that means strictly speaking the restriction *still applies*, and the correct behavior will only be seen on hardware that allows the loosed requirements of laneId, on hardware that does not result of `WaveShuffle` is the same as `WaveReadLaneId` which is undefined. 
+
 ## Tesselation
 
 Although tesselation stages should work on D3D11 and D3D12 they are not tested within our test framework, and may have problems.