add a pass to drop return values in set_local and store (#539)

2 files changed, 80 insertions, 0 deletions

diff --git a/src/passes/CMakeLists.txt b/src/passes/CMakeLists.txt
index 1c2a63da4..2b70fb9d4 100644
--- a/src/passes/CMakeLists.txt
+++ b/src/passes/CMakeLists.txt
@@ -2,6 +2,7 @@ SET(passes_SOURCES
   pass.cpp
   CoalesceLocals.cpp
   DeadCodeElimination.cpp
+  DropReturnValues.cpp
   LowerIfElse.cpp
   MergeBlocks.cpp
   Metrics.cpp
diff --git a/src/passes/DropReturnValues.cpp b/src/passes/DropReturnValues.cpp
new file mode 100644
index 000000000..b89de3011
--- /dev/null
+++ b/src/passes/DropReturnValues.cpp
@@ -0,0 +1,79 @@
+/*
+ * Copyright 2016 WebAssembly Community Group participants
+ *
+ * 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.
+ */
+
+//
+// Stops using return values from set_local and store nodes.
+//
+
+#include <wasm.h>
+#include <pass.h>
+#include <ast_utils.h>
+#include <wasm-builder.h>
+
+namespace wasm {
+
+struct DropReturnValues : public WalkerPass<PostWalker<DropReturnValues, Visitor<DropReturnValues>>> {
+  bool isFunctionParallel() { return true; }
+
+  std::vector<Expression*> expressionStack;
+
+  void visitSetLocal(SetLocal* curr) {
+    if (ExpressionAnalyzer::isResultUsed(expressionStack, getFunction())) {
+      Builder builder(*getModule());
+      replaceCurrent(builder.makeSequence(
+        curr,
+        builder.makeGetLocal(curr->index, curr->type)
+      ));
+    }
+  }
+
+  void visitStore(Store* curr) {
+    if (ExpressionAnalyzer::isResultUsed(expressionStack, getFunction())) {
+      Index index = getFunction()->getNumLocals();
+      getFunction()->vars.emplace_back(curr->type);
+      Builder builder(*getModule());
+      replaceCurrent(builder.makeSequence(
+        builder.makeSequence(
+          builder.makeSetLocal(index, curr->value),
+          curr
+        ),
+        builder.makeGetLocal(index, curr->type)
+      ));
+      curr->value = builder.makeGetLocal(index, curr->type);
+    }
+  }
+
+  static void visitPre(DropReturnValues* self, Expression** currp) {
+    self->expressionStack.push_back(*currp);
+  }
+
+  static void visitPost(DropReturnValues* self, Expression** currp) {
+    self->expressionStack.pop_back();
+  }
+
+  static void scan(DropReturnValues* self, Expression** currp) {
+    self->pushTask(visitPost, currp);
+
+    WalkerPass<PostWalker<DropReturnValues, Visitor<DropReturnValues>>>::scan(self, currp);
+
+    self->pushTask(visitPre, currp);
+  }
+};
+
+static RegisterPass<DropReturnValues> registerPass("drop-return-values", "stops relying on return values from set_local and store");
+
+} // namespace wasm
+