Add support for GetAttributeAtVertex for D3D (#1725)

This operation was added along with the `SV_Barycentrics` system-value input, and allows for a `nointerpolation` varying input to a fragment shader to be fetched at a specific vertex index within the primitive that is causing the fragment shader to be invoked.

This change adds support for the new operations in the standard library, and also includes a test case to make sure that we emit it correctly when producing HLSL/DXIL.

This change also includes a small bug fix to our emission logic for function parameters so that we properly emit layout-related attributes for varying parameters declared directly on an entry point.
(Note that most attribute end up being declared in `struct` types in existing HLSL shaders, and our IR passes produce only global variables for attributes on GLSL; the only case this affects is inidividual scalar/vector attributes declared declared as entry-point parameters, when outputting HLSL)

Note that this change only adds support for the new function on the HLSL/DXIL path, and doesn't yet add any cross-compilation support for GLSL/SPIR-V. The reason for this is that the equivalent GLSL feature(s) appear to use a different model to the HLSL version, and we need to invent a suitable approach to align them to make portable code possible.

2 files changed, 30 insertions, 0 deletions

diff --git a/tests/pipeline/rasterization/get-attribute-at-vertex.slang b/tests/pipeline/rasterization/get-attribute-at-vertex.slang
new file mode 100644
index 000000000..56fbcce78
--- /dev/null
+++ b/tests/pipeline/rasterization/get-attribute-at-vertex.slang
@@ -0,0 +1,16 @@
+// get-attribute-at-vertex.slang
+
+// Basic test for `GetAttributeAtVertex` function
+
+//TEST:CROSS_COMPILE:-target dxil -entry main -stage fragment -profile sm_6_1
+
+[shader("fragment")]
+void main(
+    nointerpolation float4 color : COLOR,
+    float3 bary : SV_Barycentrics,
+    out float4 result : SV_Target)
+{
+    result = bary.x * GetAttributeAtVertex(color, 0)
+           + bary.y * GetAttributeAtVertex(color, 1)
+           + bary.z * GetAttributeAtVertex(color, 2);
+}
diff --git a/tests/pipeline/rasterization/get-attribute-at-vertex.slang.hlsl b/tests/pipeline/rasterization/get-attribute-at-vertex.slang.hlsl
new file mode 100644
index 000000000..257b334bf
--- /dev/null
+++ b/tests/pipeline/rasterization/get-attribute-at-vertex.slang.hlsl
@@ -0,0 +1,14 @@
+// get-attribute-at-vertex.slang.hlsl
+
+//TEST_IGNORE_FILE:
+
+[shader("pixel")]
+void main(
+    nointerpolation vector<float,4> color_0 : COLOR,
+    vector<float,3> bary_0 : SV_BARYCENTRICS,
+    out vector<float,4> result_0 : SV_TARGET)
+{
+    result_0 = bary_0.x * GetAttributeAtVertex(color_0, (uint) int(0))
+             + bary_0.y * GetAttributeAtVertex(color_0, (uint) int(1))
+             + bary_0.z * GetAttributeAtVertex(color_0, (uint) int(2));
+}