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.

1 files changed, 12 insertions, 0 deletions

diff --git a/source/slang/slang-emit-c-like.cpp b/source/slang/slang-emit-c-like.cpp
index 0bd6efad3..d099af255 100644
--- a/source/slang/slang-emit-c-like.cpp
+++ b/source/slang/slang-emit-c-like.cpp
@@ -2663,6 +2663,18 @@ void CLikeSourceEmitter::emitSimpleFuncParamImpl(IRParam* param)
     auto paramName = getName(param);
     auto paramType = param->getDataType();
 
+    if(auto layoutDecoration = param->findDecoration<IRLayoutDecoration>() )
+    {
+        auto layout = as<IRVarLayout>(layoutDecoration->getLayout());
+        SLANG_ASSERT(layout);
+
+        if(layout->usesResourceKind(LayoutResourceKind::VaryingInput)
+            || layout->usesResourceKind(LayoutResourceKind::VaryingOutput))
+        {
+            emitInterpolationModifiers(param, paramType, layout);
+        }
+    }
+
     emitParamType(paramType, paramName);
     emitSemantics(param);
 }