[msl-writer] Emit function constant information

This CL adds the code to emit function_constant attributes to the
generated MSL.

Bug: tint:153
Change-Id: I0e4f50257fde7e8db8f53e15bc9f460ebfc809ee
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/29520
Commit-Queue: Ryan Harrison <rharrison@chromium.org>
Reviewed-by: Ryan Harrison <rharrison@chromium.org>

src/ast/variable.cc

@@ -41,6 +41,11 @@ DecoratedVariable* Variable::AsDecorated() {
   return static_cast<DecoratedVariable*>(this);
 }
 
+const DecoratedVariable* Variable::AsDecorated() const {
+  assert(IsDecorated());
+  return static_cast<const DecoratedVariable*>(this);
+}
+
 bool Variable::IsDecorated() const {
   return false;
 }

src/ast/variable.h

@@ -141,6 +141,8 @@ class Variable : public Node {
 
   /// @returns the expression as a decorated variable
   DecoratedVariable* AsDecorated();
+  /// @returns the expression as a decorated variable
+  const DecoratedVariable* AsDecorated() const;
 
   /// @returns true if the name and path are both present
   bool IsValid() const override;

src/writer/msl/generator_impl.cc

@@ -1829,7 +1829,7 @@ bool GeneratorImpl::EmitVariable(ast::Variable* var, bool skip_constructor) {
 bool GeneratorImpl::EmitProgramConstVariable(const ast::Variable* var) {
   make_indent();
 
-  if (var->IsDecorated()) {
+  if (var->IsDecorated() && !var->AsDecorated()->HasConstantIdDecoration()) {
     error_ = "Decorated const values not valid";
     return false;
   }
@@ -1846,7 +1846,10 @@ bool GeneratorImpl::EmitProgramConstVariable(const ast::Variable* var) {
     out_ << " " << var->name();
   }
 
-  if (var->constructor() != nullptr) {
+  if (var->IsDecorated() && var->AsDecorated()->HasConstantIdDecoration()) {
+    out_ << " [[function_constant(" << var->AsDecorated()->constant_id()
+         << ")]]";
+  } else if (var->constructor() != nullptr) {
     out_ << " = ";
     if (!EmitExpression(var->constructor())) {
       return false;

src/writer/msl/generator_impl_module_constant_test.cc

@@ -16,6 +16,8 @@
 #include <vector>
 
 #include "gtest/gtest.h"
+#include "src/ast/constant_id_decoration.h"
+#include "src/ast/decorated_variable.h"
 #include "src/ast/float_literal.h"
 #include "src/ast/module.h"
 #include "src/ast/scalar_constructor_expression.h"
@@ -58,6 +60,25 @@ TEST_F(MslGeneratorImplTest, Emit_ModuleConstant) {
       "constant float pos[3] = {1.00000000f, 2.00000000f, 3.00000000f};\n");
 }
 
+TEST_F(MslGeneratorImplTest, Emit_SpecConstant) {
+  ast::type::F32Type f32;
+
+  ast::VariableDecorationList decos;
+  decos.push_back(std::make_unique<ast::ConstantIdDecoration>(23));
+
+  auto var = std::make_unique<ast::DecoratedVariable>(
+      std::make_unique<ast::Variable>("pos", ast::StorageClass::kNone, &f32));
+  var->set_decorations(std::move(decos));
+  var->set_is_const(true);
+  var->set_constructor(std::make_unique<ast::ScalarConstructorExpression>(
+      std::make_unique<ast::FloatLiteral>(&f32, 3.0f)));
+
+  ast::Module m;
+  GeneratorImpl g(&m);
+  ASSERT_TRUE(g.EmitProgramConstVariable(var.get())) << g.error();
+  EXPECT_EQ(g.result(), "constant float pos [[function_constant(23)]];\n");
+}
+
 }  // namespace
 }  // namespace msl
 }  // namespace writer