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diff --git a/src/passes/Precompute.cpp b/src/passes/Precompute.cpp
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+/*
+ * 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
+