[msl-writer] Emit kill statement.

This Cl adds emission of the Kill statement as `discard_fragment`. This
may need to be revised when the semantics of Kill are agreed upon.

Bug: tint:8
Change-Id: I2d09f09143b2acd0139d876e873e3c70abbc84a3
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/23841
Reviewed-by: David Neto <dneto@google.com>

BUILD.gn

@@ -892,6 +892,7 @@ source_set("tint_unittests_msl_writer_src") {
     "src/writer/msl/generator_impl_function_test.cc",
     "src/writer/msl/generator_impl_identifier_test.cc",
     "src/writer/msl/generator_impl_if_test.cc",
+    "src/writer/msl/generator_impl_kill_test.cc",
     "src/writer/msl/generator_impl_return_test.cc",
     "src/writer/msl/generator_impl_test.cc",
     "src/writer/msl/generator_impl_type_test.cc",

src/CMakeLists.txt

@@ -500,6 +500,7 @@ if(${TINT_BUILD_MSL_WRITER})
     writer/msl/generator_impl_function_test.cc
     writer/msl/generator_impl_identifier_test.cc
     writer/msl/generator_impl_if_test.cc
+    writer/msl/generator_impl_kill_test.cc
     writer/msl/generator_impl_return_test.cc
     writer/msl/generator_impl_test.cc
     writer/msl/generator_impl_type_test.cc

src/writer/msl/generator_impl.cc

@@ -347,6 +347,14 @@ bool GeneratorImpl::EmitIdentifier(ast::IdentifierExpression* expr) {
   return true;
 }
 
+bool GeneratorImpl::EmitKill(ast::KillStatement*) {
+  make_indent();
+  // TODO(dsinclair): Verify this is correct when the kill semantics are defined
+  // for WGSL (https://github.com/gpuweb/gpuweb/issues/361)
+  out_ << "discard_fragment();" << std::endl;
+  return true;
+}
+
 bool GeneratorImpl::EmitElse(ast::ElseStatement* stmt) {
   if (stmt->HasCondition()) {
     out_ << " else if (";
@@ -431,6 +439,9 @@ bool GeneratorImpl::EmitStatement(ast::Statement* stmt) {
   if (stmt->IsIf()) {
     return EmitIf(stmt->AsIf());
   }
+  if (stmt->IsKill()) {
+    return EmitKill(stmt->AsKill());
+  }
   if (stmt->IsReturn()) {
     return EmitReturn(stmt->AsReturn());
   }

src/writer/msl/generator_impl.h

@@ -80,6 +80,10 @@ class GeneratorImpl : public TextGenerator {
   /// @param stmt the statement to emit
   /// @returns true if the statement was successfully emitted
   bool EmitIf(ast::IfStatement* stmt);
+  /// Handles generating a kill statement
+  /// @param stmt the kill statement
+  /// @returns true if the statement was successfully emitted
+  bool EmitKill(ast::KillStatement* stmt);
   /// Handles a literal
   /// @param lit the literal to emit
   /// @returns true if the literal was successfully emitted

src/writer/msl/generator_impl_kill_test.cc

@@ -0,0 +1,39 @@
+// 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/kill_statement.h"
+#include "src/writer/msl/generator_impl.h"
+
+namespace tint {
+namespace writer {
+namespace msl {
+namespace {
+
+using MslGeneratorImplTest = testing::Test;
+
+TEST_F(MslGeneratorImplTest, Emit_Kill) {
+  ast::KillStatement k;
+
+  GeneratorImpl g;
+  g.increment_indent();
+
+  ASSERT_TRUE(g.EmitStatement(&k)) << g.error();
+  EXPECT_EQ(g.result(), "  discard_fragment();\n");
+}
+
+}  // namespace
+}  // namespace msl
+}  // namespace writer
+}  // namespace tint