Legalize the Entry-point for WGSL (#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>

1 files changed, 26 insertions, 0 deletions

diff --git a/source/slang/slang-emit-wgsl.cpp b/source/slang/slang-emit-wgsl.cpp
index dea95c6ec..256697bc7 100644
--- a/source/slang/slang-emit-wgsl.cpp
+++ b/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