[spirv-writer] Add DiscardStatement support.

This CL updates the spirv-writer to emit the DiscardStatement as an
OpKill.

Bug: tint:163
Change-Id: Ic2514ee8a4ef7ef0220fc2e1145f8df0c3d32069
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/25641
Reviewed-by: David Neto <dneto@google.com>

BUILD.gn

@@ -803,6 +803,7 @@ source_set("tint_unittests_spv_writer_src") {
     "src/writer/spirv/builder_call_test.cc",
     "src/writer/spirv/builder_cast_expression_test.cc",
     "src/writer/spirv/builder_constructor_expression_test.cc",
+    "src/writer/spirv/builder_discard_test.cc",
     "src/writer/spirv/builder_entry_point_test.cc",
     "src/writer/spirv/builder_function_test.cc",
     "src/writer/spirv/builder_function_variable_test.cc",

src/CMakeLists.txt

@@ -457,6 +457,7 @@ if(${TINT_BUILD_SPV_WRITER})
     writer/spirv/builder_call_test.cc
     writer/spirv/builder_cast_expression_test.cc
     writer/spirv/builder_constructor_expression_test.cc
+    writer/spirv/builder_discard_test.cc
     writer/spirv/builder_entry_point_test.cc
     writer/spirv/builder_function_test.cc
     writer/spirv/builder_function_variable_test.cc

src/writer/spirv/builder.cc

@@ -105,8 +105,8 @@ bool LastIsTerminator(const ast::StatementList& stmts) {
   }
 
   auto* last = stmts.back().get();
-  return last->IsBreak() || last->IsContinue() || last->IsReturn() ||
-         last->IsKill() || last->IsFallthrough();
+  return last->IsBreak() || last->IsContinue() || last->IsDiscard() ||
+         last->IsReturn() || last->IsKill() || last->IsFallthrough();
 }
 
 uint32_t IndexFromName(char name) {
@@ -273,6 +273,14 @@ bool Builder::GenerateContinueStatement(ast::ContinueStatement*) {
   return true;
 }
 
+// TODO(dsinclair): This is generating an OpKill but the semantics of kill
+// haven't been defined for WGSL yet. So, this may need to change.
+// https://github.com/gpuweb/gpuweb/issues/676
+bool Builder::GenerateDiscardStatement(ast::DiscardStatement*) {
+  push_function_inst(spv::Op::OpKill, {});
+  return true;
+}
+
 // TODO(dsinclair): This is generating an OpKill but the semantics of kill
 // haven't been defined for WGSL yet. So, this may need to change.
 // https://github.com/gpuweb/gpuweb/issues/676
@@ -1816,6 +1824,9 @@ bool Builder::GenerateStatement(ast::Statement* stmt) {
   if (stmt->IsContinue()) {
     return GenerateContinueStatement(stmt->AsContinue());
   }
+  if (stmt->IsDiscard()) {
+    return GenerateDiscardStatement(stmt->AsDiscard());
+  }
   if (stmt->IsFallthrough()) {
     // Do nothing here, the fallthrough gets handled by the switch code.
     return true;

src/writer/spirv/builder.h

@@ -175,6 +175,10 @@ class Builder {
   /// @param stmt the statement to generate
   /// @returns true if the statement was successfully generated
   bool GenerateContinueStatement(ast::ContinueStatement* stmt);
+  /// Generates a discard statement
+  /// @param stmt the statement to generate
+  /// @returns true if the statement was successfully generated
+  bool GenerateDiscardStatement(ast::DiscardStatement* stmt);
   /// Generates a kill statement
   /// @param stmt the statement to generate
   /// @returns true if the statement was successfully generated

src/writer/spirv/builder_discard_test.cc

@@ -0,0 +1,43 @@
+// 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/context.h"
+#include "src/type_determiner.h"
+#include "src/writer/spirv/builder.h"
+#include "src/writer/spirv/spv_dump.h"
+
+namespace tint {
+namespace writer {
+namespace spirv {
+namespace {
+
+using BuilderTest = testing::Test;
+
+TEST_F(BuilderTest, Discard) {
+  ast::DiscardStatement expr;
+
+  ast::Module mod;
+  Builder b(&mod);
+  b.push_function(Function{});
+  EXPECT_EQ(b.GenerateStatement(&expr), 1u) << b.error();
+  EXPECT_EQ(DumpInstructions(b.functions()[0].instructions()), R"(OpKill
+)");
+}
+
+}  // namespace
+}  // namespace spirv
+}  // namespace writer
+}  // namespace tint