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diff --git a/src/passes/PostEmscripten.cpp b/src/passes/PostEmscripten.cpp
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+/*
+ * Copyright 2015 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.
+ */
+
+//
+// Misc optimizations that are useful for and/or are only valid for
+// emscripten output.
+//
+
+#include <wasm.h>
+#include <pass.h>
+
+namespace wasm {
+
+struct PostEmscripten : public WalkerPass<WasmWalker<PostEmscripten>> {
+  // When we have a Load from a local value (typically a GetLocal) plus a constant offset,
+  // we may be able to fold it in.
+  // The semantics of the Add are to wrap, while wasm offset semantics purposefully do
+  // not wrap. So this is not always safe to do. For example, a load may depend on
+  // wrapping via
+  //      (2^32 - 10) + 100   =>  wrap and load from address 90
+  // Without wrapping, we get something too large, and an error. *However*, for
+  // asm2wasm output coming from Emscripten, we allocate the lowest 1024 for mapped
+  // globals. Mapped globals are simple types (i32, float or double), always
+  // accessed directly by a single constant. Therefore if we see (..) + K where
+  // K is less then 1024, then if it wraps, it wraps into [0, 1024) which is at best
+  // a mapped global, but it can't be because they are accessed directly (at worst,
+  // it's 0 or an unused section of memory that was reserved for mapped globlas).
+  // Thus it is ok to optimize such small constants into Load offsets.
+  void visitLoad(Load *curr) {
+    if (curr->offset) return;
+    auto add = curr->ptr->dyn_cast<Binary>();
+    if (!add || add->op != Add) return;
+    assert(add->type == i32);
+    auto c = add->right->dyn_cast<Const>();
+    if (!c) {
+      c = add->left->dyn_cast<Const>();
+      if (c) {
+        // if one is a const, it's ok to swap
+        add->left = add->right;
+        add->right = c;
+      }
+    }
+    if (!c) return;
+    auto value = c->value.geti32();
+    if (value >= 0 && value < 1024) {
+      // foldable, by the above logic
+      curr->ptr = add->left;
+      curr->offset = value;
+    }
+  }
+};
+
+static RegisterPass<PostEmscripten> registerPass("post-emscripten", "miscellaneous optimizations for Emscripten-generated code");
+
+} // namespace wasm