[msl-writer] Emit DiscardStatement.

This CL adds the DiscardStatement to the MSL backend.

Bug: tint:164
Change-Id: I1e201d3b6a167690af7bb799e1c93e48232f4a0a
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/25602
Reviewed-by: David Neto <dneto@google.com>

BUILD.gn

@@ -994,6 +994,7 @@ source_set("tint_unittests_msl_writer_src") {
     "src/writer/msl/generator_impl_cast_test.cc",
     "src/writer/msl/generator_impl_constructor_test.cc",
     "src/writer/msl/generator_impl_continue_test.cc",
+    "src/writer/msl/generator_impl_discard_test.cc",
     "src/writer/msl/generator_impl_entry_point_test.cc",
     "src/writer/msl/generator_impl_function_test.cc",
     "src/writer/msl/generator_impl_identifier_test.cc",

src/CMakeLists.txt

@@ -525,6 +525,7 @@ if(${TINT_BUILD_MSL_WRITER})
     writer/msl/generator_impl_cast_test.cc
     writer/msl/generator_impl_constructor_test.cc
     writer/msl/generator_impl_continue_test.cc
+    writer/msl/generator_impl_discard_test.cc
     writer/msl/generator_impl_entry_point_test.cc
     writer/msl/generator_impl_function_test.cc
     writer/msl/generator_impl_identifier_test.cc

src/writer/msl/generator_impl.cc

@@ -1389,6 +1389,14 @@ bool GeneratorImpl::EmitLoop(ast::LoopStatement* stmt) {
   return true;
 }
 
+bool GeneratorImpl::EmitDiscard(ast::DiscardStatement*) {
+  make_indent();
+  // TODO(dsinclair): Verify this is correct when the discard semantics are
+  // defined for WGSL (https://github.com/gpuweb/gpuweb/issues/361)
+  out_ << "discard_fragment();" << std::endl;
+  return true;
+}
+
 bool GeneratorImpl::EmitKill(ast::KillStatement*) {
   make_indent();
   // TODO(dsinclair): Verify this is correct when the kill semantics are defined
@@ -1509,6 +1517,9 @@ bool GeneratorImpl::EmitStatement(ast::Statement* stmt) {
   if (stmt->IsContinue()) {
     return EmitContinue(stmt->AsContinue());
   }
+  if (stmt->IsDiscard()) {
+    return EmitDiscard(stmt->AsDiscard());
+  }
   if (stmt->IsFallthrough()) {
     make_indent();
     out_ << "/* fallthrough */" << std::endl;

src/writer/msl/generator_impl.h

@@ -97,6 +97,10 @@ class GeneratorImpl : public TextGenerator {
   /// @param stmt the statement to emit
   /// @returns true if the statement was emitted successfully
   bool EmitContinue(ast::ContinueStatement* stmt);
+  /// Handles generating a discard statement
+  /// @param stmt the discard statement
+  /// @returns true if the statement was successfully emitted
+  bool EmitDiscard(ast::DiscardStatement* stmt);
   /// Handles generating an else statement
   /// @param stmt the statement to emit
   /// @returns true if the statement was emitted

src/writer/msl/generator_impl_discard_test.cc

@@ -0,0 +1,41 @@
+// Copyright 2020 The Tint Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "gtest/gtest.h"
+#include "src/ast/discard_statement.h"
+#include "src/ast/module.h"
+#include "src/writer/msl/generator_impl.h"
+
+namespace tint {
+namespace writer {
+namespace msl {
+namespace {
+
+using MslGeneratorImplTest = testing::Test;
+
+TEST_F(MslGeneratorImplTest, Emit_Discard) {
+  ast::DiscardStatement stmt;
+
+  ast::Module m;
+  GeneratorImpl g(&m);
+  g.increment_indent();
+
+  ASSERT_TRUE(g.EmitStatement(&stmt)) << g.error();
+  EXPECT_EQ(g.result(), "  discard_fragment();\n");
+}
+
+}  // namespace
+}  // namespace msl
+}  // namespace writer
+}  // namespace tint