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

Author: kripken

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

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
+

test/passes/drop-return-values.txt

@@ -0,0 +1,31 @@
+(module
+  (memory 10)
+  (func $0
+    (local $x i32)
+    (local $1 i32)
+    (i32.add
+      (block
+        (set_local $x
+          (i32.const 10)
+        )
+        (get_local $x)
+      )
+      (i32.const 20)
+    )
+    (i32.add
+      (block
+        (block
+          (set_local $1
+            (i32.const 40)
+          )
+          (i32.store
+            (i32.const 30)
+            (get_local $1)
+          )
+        )
+        (get_local $1)
+      )
+      (i32.const 50)
+    )
+  )
+)

test/passes/drop-return-values.wast

@@ -0,0 +1,9 @@
+(module
+  (memory 10)
+  (func
+    (local $x i32)
+    (i32.add (set_local $x (i32.const 10)) (i32.const 20))
+    (i32.add (i32.store (i32.const 30) (i32.const 40)) (i32.const 50))
+  )
+)
+