[WGSL] Serialize global overrides

https://bugs.webkit.org/show_bug.cgi?id=255600
rdar://108197601

Reviewed by Myles C. Maxfield.

Add serialization support for override declarations. Notice that we temporarily
disable the mangling of globals, which ironically wasn't being used yet, but
requires notifying the API about the mangling. I will be implementing that next
and will re-enable the mangling.

* Source/WebGPU/WGSL/MangleNames.cpp:
(WGSL::NameManglerVisitor::visit):
* Source/WebGPU/WGSL/Metal/MetalFunctionWriter.cpp:
(WGSL::Metal::FunctionDefinitionWriter::write):
(WGSL::Metal::FunctionDefinitionWriter::visit):
(WGSL::Metal::FunctionDefinitionWriter::visitGlobal):
(WGSL::Metal::FunctionDefinitionWriter::serializeVariable):

Canonical link: https://commits.webkit.org/263167@main

Source/WebGPU/WGSL/MangleNames.cpp

@@ -158,9 +158,11 @@ void NameManglerVisitor::visit(AST::Structure& structure)
     ASSERT_UNUSED(result, result.isNewEntry);
 }
 
-void NameManglerVisitor::visit(AST::Variable& variable)
+void NameManglerVisitor::visit(AST::Variable&)
 {
-    visitVariableDeclaration(variable, MangledName::Global);
+    // FIXME: we need to notify the API about these renames
+    // https://bugs.webkit.org/show_bug.cgi?id=250441
+    // visitVariableDeclaration(variable, MangledName::Global);
 }
 
 void NameManglerVisitor::visit(AST::VariableStatement& variable)

Source/WebGPU/WGSL/Metal/MetalFunctionWriter.cpp

@@ -96,19 +96,24 @@ class FunctionDefinitionWriter : public AST::Visitor {
 
 private:
     void visit(const Type*);
+    void visitGlobal(AST::Variable&);
+    void serializeVariable(AST::Variable&);
 
     StringBuilder& m_stringBuilder;
     CallGraph& m_callGraph;
     Indentation<4> m_indent { 0 };
     std::optional<AST::StructureRole> m_structRole;
     std::optional<AST::StageAttribute::Stage> m_entryPointStage;
     std::optional<String> m_suffix;
+    unsigned m_functionConstantIndex { 0 };
 };
 
 void FunctionDefinitionWriter::write()
 {
     for (auto& structure : m_callGraph.ast().structures())
         visit(structure);
+    for (auto& variable : m_callGraph.ast().variables())
+        visitGlobal(variable);
     for (auto& entryPoint : m_callGraph.entrypoints())
         visit(entryPoint.function);
 }
@@ -180,6 +185,22 @@ void FunctionDefinitionWriter::visit(AST::Structure& structDecl)
 }
 
 void FunctionDefinitionWriter::visit(AST::Variable& variable)
+{
+    serializeVariable(variable);
+    m_stringBuilder.append(";\n");
+}
+
+void FunctionDefinitionWriter::visitGlobal(AST::Variable& variable)
+{
+    if (variable.flavor() != AST::VariableFlavor::Override)
+        return;
+
+    m_stringBuilder.append("constant ");
+    serializeVariable(variable);
+    m_stringBuilder.append(" [[function_constant(", m_functionConstantIndex++, ")]];\n");
+}
+
+void FunctionDefinitionWriter::serializeVariable(AST::Variable& variable)
 {
     if (variable.maybeTypeName())
         visit(*variable.maybeTypeName());
@@ -193,7 +214,6 @@ void FunctionDefinitionWriter::visit(AST::Variable& variable)
         m_stringBuilder.append(" = ");
         visit(*variable.maybeInitializer());
     }
-    m_stringBuilder.append(";\n");
 }
 
 void FunctionDefinitionWriter::visit(AST::Attribute& attribute)