Add EmitVertexPointSizeTransform

EmitVertexPointSizeTransform is a Transformer that adds a PointSize builtin global output variable to the module which is assigned 1.0 as the new first statement for all vertex stage entry points.

If the module does not contain a vertex pipeline stage entry point then then this transformer is a no-op.

Bug: tint:321
Change-Id: I0e01236339d9fa1ceab3622af0931a1199c33b99
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/34561
Commit-Queue: Ben Clayton <bclayton@google.com>
Reviewed-by: dan sinclair <dsinclair@chromium.org>

src/ast/builtin.cc

@@ -59,6 +59,9 @@ std::ostream& operator<<(std::ostream& out, Builtin builtin) {
       out << "global_invocation_id";
       break;
     }
+    case Builtin::kPointSize: {
+      out << "pointsize";
+    }
   }
   return out;
 }

src/ast/builtin.h

@@ -31,7 +31,11 @@ enum class Builtin {
   kFragDepth,
   kLocalInvocationId,
   kLocalInvocationIdx,
-  kGlobalInvocationId
+  kGlobalInvocationId,
+
+  // Below are not currently WGSL builtins, but are included in this enum as
+  // they are used by certain backends.
+  kPointSize,
 };
 
 std::ostream& operator<<(std::ostream& out, Builtin builtin);

src/ast/module.cc

@@ -65,6 +65,15 @@ Function* Module::FindFunctionByNameAndStage(const std::string& name,
   return nullptr;
 }
 
+bool Module::HasStage(ast::PipelineStage stage) const {
+  for (auto* func : functions_) {
+    if (func->pipeline_stage() == stage) {
+      return true;
+    }
+  }
+  return false;
+}
+
 bool Module::IsValid() const {
   for (auto* var : global_variables_) {
     if (var == nullptr || !var->IsValid()) {

src/ast/module.h

@@ -83,6 +83,10 @@ class Module {
   /// @returns the associated function or nullptr if none exists
   Function* FindFunctionByNameAndStage(const std::string& name,
                                        PipelineStage stage) const;
+  /// @param stage the pipeline stage
+  /// @returns true if the module contains an entrypoint function with the given
+  /// stage
+  bool HasStage(PipelineStage stage) const;
 
   /// @returns true if all required fields in the AST are present.
   bool IsValid() const;