Legalize the Entry-point for WGSL (shader-slang#5498)

* Legalize the Entry-point for WGSL

The return type of the entry-point needs to be legalized when targeting
WGSL.
This commit flattens the nested-structs of the return type and the input
parameters of the entry-point.

Most of code is copied from the legalization code for Metal. The
following functions are exactly same to the implementation for Metal or
almost same.
- flattenInputParameters() : 136 lines
- reportUnsupportedSystemAttribute() : 7 lines
- ensureResultStructHasUserSemantic() : 46 lines
- struct MapStructToFlatStruct : 176 lines
- flattenNestedStructs() : 95 lines
- maybeFlattenNestedStructs() : 42 lines
- _replaceAllReturnInst() : 19 lines
- _returnNonOverlappingAttributeIndex() : 16 lines
- _replaceAttributeOfLayout() : 23 lines
- tryConvertValue() : 41 lines
- legalizeSystemValueParameters() : 11 lines

They need to be refactored to reduce the duplication later.

The test case, `tests/compute/assoctype-lookup.slang`, had a bug that
the compute shader was trying to use the varying input/output with the
user defined semantics.

This commit removes the user defined semantics, because the compute
shaders cannot use the user defined semantics.

---------

Co-authored-by: slangbot <186143334+slangbot@users.noreply.github.com>

Author: jkwak-work

source/slang/slang-emit-c-like.cpp

@@ -3293,6 +3293,11 @@ void CLikeSourceEmitter::emitSemanticsUsingVarLayout(IRVarLayout* varLayout)
     }
 }
 
+void CLikeSourceEmitter::emitSemanticsPrefix(IRInst* inst)
+{
+    emitSemanticsPrefixImpl(inst);
+}
+
 void CLikeSourceEmitter::emitSemantics(IRInst* inst, bool allowOffsetLayout)
 {
     emitSemanticsImpl(inst, allowOffsetLayout);
@@ -3869,6 +3874,7 @@ void CLikeSourceEmitter::emitStructDeclarationsBlock(
                 emitPackOffsetModifier(fieldKey, fieldType, packOffsetDecoration);
             }
         }
+        emitSemanticsPrefix(fieldKey);
         emitStructFieldAttributes(structType, ff);
         emitMemoryQualifiers(fieldKey);
         emitType(fieldType, getName(fieldKey));

source/slang/slang-emit-c-like.h

@@ -355,6 +355,7 @@ class CLikeSourceEmitter : public SourceEmitterBase
     void diagnoseUnhandledInst(IRInst* inst);
     void emitInst(IRInst* inst);
 
+    void emitSemanticsPrefix(IRInst* inst);
     void emitSemantics(IRInst* inst, bool allowOffsets = false);
     void emitSemanticsUsingVarLayout(IRVarLayout* varLayout);
 
@@ -557,6 +558,7 @@ class CLikeSourceEmitter : public SourceEmitterBase
         SLANG_UNUSED(rate);
         SLANG_UNUSED(addressSpace);
     }
+    virtual void emitSemanticsPrefixImpl(IRInst* inst) { SLANG_UNUSED(inst); }
     virtual void emitSemanticsImpl(IRInst* inst, bool allowOffsetLayout)
     {
         SLANG_UNUSED(inst);

source/slang/slang-emit-wgsl.cpp

@@ -236,6 +236,32 @@ static bool isPowerOf2(const uint32_t n)
     return (n != 0U) && ((n - 1U) & n) == 0U;
 }
 
+bool WGSLSourceEmitter::maybeEmitSystemSemantic(IRInst* inst)
+{
+    if (auto sysSemanticDecor = inst->findDecoration<IRTargetSystemValueDecoration>())
+    {
+        m_writer->emit("@builtin(");
+        m_writer->emit(sysSemanticDecor->getSemantic());
+        m_writer->emit(")");
+        return true;
+    }
+    return false;
+}
+
+void WGSLSourceEmitter::emitSemanticsPrefixImpl(IRInst* inst)
+{
+    if (!maybeEmitSystemSemantic(inst))
+    {
+        if (auto semanticDecoration = inst->findDecoration<IRSemanticDecoration>())
+        {
+            m_writer->emit("@location(");
+            m_writer->emit(semanticDecoration->getSemanticIndex());
+            m_writer->emit(")");
+            return;
+        }
+    }
+}
+
 void WGSLSourceEmitter::emitStructFieldAttributes(IRStructType* structType, IRStructField* field)
 {
     // Tint emits errors unless we explicitly spell out the layout in some cases, so emit

source/slang/slang-emit-wgsl.h

@@ -38,6 +38,7 @@ class WGSLSourceEmitter : public CLikeSourceEmitter
     virtual void emitParamTypeImpl(IRType* type, const String& name) SLANG_OVERRIDE;
     virtual void _emitType(IRType* type, DeclaratorInfo* declarator) SLANG_OVERRIDE;
     virtual void emitFrontMatterImpl(TargetRequest* targetReq) SLANG_OVERRIDE;
+    virtual void emitSemanticsPrefixImpl(IRInst* inst) SLANG_OVERRIDE;
     virtual void emitStructFieldAttributes(IRStructType* structType, IRStructField* field)
         SLANG_OVERRIDE;
     virtual void emitCallArg(IRInst* inst) SLANG_OVERRIDE;
@@ -57,6 +58,8 @@ class WGSLSourceEmitter : public CLikeSourceEmitter
     void ensurePrelude(const char* preludeText);
 
 private:
+    bool maybeEmitSystemSemantic(IRInst* inst);
+
     // Emit the matrix type with 'rowCountWGSL' WGSL-rows and 'colCountWGSL' WGSL-columns
     void emitMatrixType(
         IRType* const elementType,