Reflection: allow querying of semantics on varying input/output (#224)

This is functionality required to support a Falcor bug fix.

Most of the code to compute the right semantic name/index for a parameter was already present.
This change adds:

- Storage for semantic name/index on every `VarLayout`
  - Note: this is wasteful and should be optimized later

- A public API to query the semantic name/index
  - The contract is that this API returns `NULL` if the parameter had no semantic

- A bit of work in `parameter-binding.cpp` to attach semantics to varying input/output when traversing varying parameters.
  - Note: this is intentionally set up so that it associates semantics even with non-leaf parameters, so that an API user can query the semantic of a `struct` parameter and know that its members will be assigned sequential semantic indices from its starting value.

- Support for dumping this information in reflection tests

One notable thing that I did *not* change here is that the reflection test fixture doesn't report information on the output of an entry point, even though it really should. That should be fixed in a separate change, though, because it would affect many of the expected outputs.

4 files changed, 53 insertions, 0 deletions

diff --git a/source/slang/lower.cpp b/source/slang/lower.cpp
index 5708bab64..94738307b 100644
--- a/source/slang/lower.cpp
+++ b/source/slang/lower.cpp
@@ -3183,6 +3183,8 @@ struct LoweringVisitor
                     newFieldLayout->varDecl = fieldLayout->varDecl;
                     newFieldLayout->systemValueSemantic = fieldLayout->systemValueSemantic;
                     newFieldLayout->systemValueSemanticIndex = fieldLayout->systemValueSemanticIndex;
+                    newFieldLayout->semanticName = fieldLayout->semanticName;
+                    newFieldLayout->semanticIndex = fieldLayout->semanticIndex;
 
                     for (auto resInfo : fieldLayout->resourceInfos)
                     {
diff --git a/source/slang/parameter-binding.cpp b/source/slang/parameter-binding.cpp
index 4fbec5652..fed838e6b 100644
--- a/source/slang/parameter-binding.cpp
+++ b/source/slang/parameter-binding.cpp
@@ -1190,6 +1190,27 @@ static RefPtr<TypeLayout> processEntryPointParameter(
     EntryPointParameterState const& state,
     RefPtr<VarLayout>               varLayout)
 {
+    // If there is an available semantic name and index,
+    // then we should apply it to this parameter unconditionally
+    // (that is, not just if it is a leaf parameter).
+    auto optSemanticName    =  state.optSemanticName;
+    if (optSemanticName && varLayout)
+    {
+        // Always store semantics in upper-case for
+        // reflection information, since they are
+        // supposed to be case-insensitive and
+        // upper-case is the dominant convention.
+        String semanticName = *optSemanticName;
+        String sn = semanticName.ToUpper();
+
+        auto semanticIndex      = *state.ioSemanticIndex;
+
+        varLayout->semanticName = sn;
+        varLayout->semanticIndex = semanticIndex;
+        varLayout->flags |= VarLayoutFlag::HasSemantic;
+    }
+
+
     // Scalar and vector types are treated as outputs directly
     if(auto basicType = type->As<BasicExpressionType>())
     {
diff --git a/source/slang/reflection.cpp b/source/slang/reflection.cpp
index 435b4db3f..d7e8820c0 100644
--- a/source/slang/reflection.cpp
+++ b/source/slang/reflection.cpp
@@ -623,6 +623,28 @@ SLANG_API size_t spReflectionVariableLayout_GetSpace(SlangReflectionVariableLayo
     return info->space;
 }
 
+SLANG_API char const* spReflectionVariableLayout_GetSemanticName(SlangReflectionVariableLayout* inVarLayout)
+{
+    auto varLayout = convert(inVarLayout);
+    if(!varLayout) return 0;
+
+    if (!(varLayout->flags & Slang::VarLayoutFlag::HasSemantic))
+        return 0;
+
+    return varLayout->semanticName.Buffer();
+}
+
+SLANG_API size_t spReflectionVariableLayout_GetSemanticIndex(SlangReflectionVariableLayout* inVarLayout)
+{
+    auto varLayout = convert(inVarLayout);
+    if(!varLayout) return 0;
+
+    if (!(varLayout->flags & Slang::VarLayoutFlag::HasSemantic))
+        return 0;
+
+    return varLayout->semanticIndex;
+}
+
 
 // Shader Parameter Reflection
 
diff --git a/source/slang/type-layout.h b/source/slang/type-layout.h
index 971088aac..d078d9554 100644
--- a/source/slang/type-layout.h
+++ b/source/slang/type-layout.h
@@ -218,6 +218,7 @@ typedef unsigned int VarLayoutFlags;
 enum VarLayoutFlag : VarLayoutFlags
 {
     IsRedeclaration = 1 << 0, ///< This is a redeclaration of some shader parameter
+    HasSemantic = 1 << 1,
 };
 
 // A reified layout for a particular variable, field, etc.
@@ -241,6 +242,13 @@ public:
     String  systemValueSemantic;
     int     systemValueSemanticIndex;
 
+    // General cse semantic name and index
+    // TODO: this and the system-value field are redundant
+    // TODO: the `VarLayout` type is getting bloated; we need to not store this
+    // information unless actually required.
+    String  semanticName;
+    int     semanticIndex;
+
     // The start register(s) for any resources
     struct ResourceInfo
     {