6 files changed, 111 insertions, 37 deletions
diff --git a/src/passes/CMakeLists.txt b/src/passes/CMakeLists.txt
index 9e69cbae2..2ccf5f040 100644
--- a/src/passes/CMakeLists.txt
+++ b/src/passes/CMakeLists.txt
@@ -11,6 +11,7 @@ SET(passes_SOURCES
   NameList.cpp
   OptimizeInstructions.cpp
   PostEmscripten.cpp
+  Precompute.cpp
   Print.cpp
   RemoveImports.cpp
   RemoveMemory.cpp
diff --git a/src/passes/Precompute.cpp b/src/passes/Precompute.cpp
new file mode 100644
index 000000000..db6999cbd
--- /dev/null
+++ b/src/passes/Precompute.cpp
@@ -0,0 +1,103 @@
+/*
+ * 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.
+ */
+
+//
+// Removes dead, i.e. unreachable, code.
+//
+// We keep a record of when control flow is reachable. When it isn't, we
+// kill (turn into unreachable). We then fold away entire unreachable
+// expressions.
+//
+// When dead code causes an operation to not happen, like a store, a call
+// or an add, we replace with a block with a list of what does happen.
+// That isn't necessarily smaller, but blocks are friendlier to other
+// optimizations: blocks can be merged and eliminated, and they clearly
+// have no side effects.
+//
+
+#include <wasm.h>
+#include <pass.h>
+#include <wasm-builder.h>
+#include <wasm-interpreter.h>
+
+namespace wasm {
+
+// Execute an expression by itself. Errors if we hit anything we need anything not in the expression itself standalone.
+class StandaloneExpressionRunner : public ExpressionRunner<StandaloneExpressionRunner> {
+public:
+  struct NonstandaloneException {}; // TODO: use a flow with a special name, as this is likely very slow
+
+  Flow visitLoop(Loop* curr) {
+    // loops might be infinite, so must be careful
+    // but we can't tell if non-infinite, since we don't have state, so loops are just impossible to optimize for now
+    throw NonstandaloneException();
+  }
+
+  Flow visitCall(Call* curr) {
+    throw NonstandaloneException();
+  }
+  Flow visitCallImport(CallImport* curr) {
+    throw NonstandaloneException();
+  }
+  Flow visitCallIndirect(CallIndirect* curr) {
+    throw NonstandaloneException();
+  }
+  Flow visitGetLocal(GetLocal *curr) {
+    throw NonstandaloneException();
+  }
+  Flow visitSetLocal(SetLocal *curr) {
+    throw NonstandaloneException();
+  }
+  Flow visitLoad(Load *curr) {
+    throw NonstandaloneException();
+  }
+  Flow visitStore(Store *curr) {
+    throw NonstandaloneException();
+  }
+  Flow visitHost(Host *curr) {
+    throw NonstandaloneException();
+  }
+
+  void trap(const char* why) override {
+    throw NonstandaloneException();
+  }
+};
+
+struct Precompute : public WalkerPass<PostWalker<Precompute, UnifiedExpressionVisitor<Precompute>>> {
+  bool isFunctionParallel() override { return true; }
+
+  Pass* create() override { return new Precompute; }
+
+  void visitExpression(Expression* curr) {
+    if (curr->is<Const>()) return;
+    // try to evaluate this into a const
+    Flow flow;
+    try {
+      flow = StandaloneExpressionRunner().visit(curr);
+    } catch (StandaloneExpressionRunner::NonstandaloneException& e) {
+      return;
+    }
+    if (flow.breaking()) return; // TODO: can create a break as a replacement
+    if (isConcreteWasmType(flow.value.type)) {
+      replaceCurrent(Builder(*getModule()).makeConst(flow.value));
+    }
+  }
+};
+
+static RegisterPass<Precompute> registerPass("precompute", "computes compile-time evaluatable expressions");
+
+} // namespace wasm
+
diff --git a/src/passes/pass.cpp b/src/passes/pass.cpp
index babb4635b..c5315a044 100644
--- a/src/passes/pass.cpp
+++ b/src/passes/pass.cpp
@@ -63,6 +63,7 @@ void PassRunner::addDefaultOptimizationPasses() {
   add("remove-unused-brs");
   add("remove-unused-names");
   add("optimize-instructions");
+  add("precompute");
   add("simplify-locals");
   add("vacuum"); // previous pass creates garbage
   add("remove-unused-brs"); // simplify-locals opens opportunities for phi optimizations
@@ -80,6 +81,7 @@ void PassRunner::addDefaultFunctionOptimizationPasses() {
   add("remove-unused-brs");
   add("remove-unused-names");
   add("optimize-instructions");
+  add("precompute");
   add("simplify-locals");
   add("vacuum"); // previous pass creates garbage
   add("remove-unused-brs"); // simplify-locals opens opportunities for phi optimizations
diff --git a/src/wasm-builder.h b/src/wasm-builder.h
index ae8d5fa4c..2eb051251 100644
--- a/src/wasm-builder.h
+++ b/src/wasm-builder.h
@@ -134,6 +134,7 @@ public:
     return ret;
   }
   Const* makeConst(Literal value) {
+    assert(isConcreteWasmType(value.type));
     auto* ret = wasm.allocator.alloc<Const>();
     ret->value = value;
     ret->type = value.type;
diff --git a/src/wasm-interpreter.h b/src/wasm-interpreter.h
index 55edbdfa0..f7e213b93 100644
--- a/src/wasm-interpreter.h
+++ b/src/wasm-interpreter.h
@@ -39,8 +39,7 @@ using namespace cashew;
 
 // Utilities
 
-IString WASM("wasm"),
-        RETURN_FLOW("*return:)*");
+extern Name WASM, RETURN_FLOW;
 
 enum {
   maxCallDepth = 250
@@ -492,41 +491,6 @@ public:
   }
 };
 
-// Execute an expression by itself. Errors if we hit anything we need anything not in the expression itself standalone.
-class StandaloneExpressionRunner : public ExpressionRunner<StandaloneExpressionRunner> {
-public:
-  struct NonstandaloneException {};
-
-  Flow visitCall(Call* curr) {
-    throw NonstandaloneException();
-  }
-  Flow visitCallImport(CallImport* curr) {
-    throw NonstandaloneException();
-  }
-  Flow visitCallIndirect(CallIndirect* curr) {
-    throw NonstandaloneException();
-  }
-  Flow visitGetLocal(GetLocal *curr) {
-    throw NonstandaloneException();
-  }
-  Flow visitSetLocal(SetLocal *curr) {
-    throw NonstandaloneException();
-  }
-  Flow visitLoad(Load *curr) {
-    throw NonstandaloneException();
-  }
-  Flow visitStore(Store *curr) {
-    throw NonstandaloneException();
-  }
-  Flow visitHost(Host *curr) {
-    throw NonstandaloneException();
-  }
-
-  void trap(const char* why) {
-    throw NonstandaloneException();
-  }
-};
-
 //
 // An instance of a WebAssembly module, which can execute it via AST interpretation.
 //
diff --git a/src/wasm.cpp b/src/wasm.cpp
index 48a0f276a..dd06a8d14 100644
--- a/src/wasm.cpp
+++ b/src/wasm.cpp
@@ -20,6 +20,9 @@
 
 namespace wasm {
 
+Name WASM("wasm"),
+     RETURN_FLOW("*return:)*");
+
 struct TypeSeeker : public PostWalker<TypeSeeker, Visitor<TypeSeeker>> {
   Expression* target; // look for this one
   Name targetName;