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diff --git a/src/passes/SpillPointers.cpp b/src/passes/SpillPointers.cpp
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+++ b/src/passes/SpillPointers.cpp
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+/*
+ * Copyright 2017 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.
+ */
+
+
+//
+// Spills values that might be pointers to the C stack. This allows
+// Boehm-style GC to see them properly.
+//
+// To reduce the overhead of the extra operations added here, you
+// should probably run optimizations after doing it.
+// TODO: add a dead store elimination pass, which would help here
+//
+//  * There is currently no check that there is enough stack space.
+//
+
+#include "wasm.h"
+#include "pass.h"
+#include "cfg/liveness-traversal.h"
+#include "wasm-builder.h"
+#include "abi/abi.h"
+#include "abi/stack.h"
+
+namespace wasm {
+
+struct SpillPointers : public WalkerPass<LivenessWalker<SpillPointers, Visitor<SpillPointers>>> {
+  bool isFunctionParallel() override { return true; }
+
+  Pass* create() override { return new SpillPointers; }
+
+  // a mapping of the pointers to all the spillable things. We need to know
+  // how to replace them, and as we spill we may modify them. This map
+  // gives us, for an Expression** seen during the walk (and placed in the
+  // basic block, which is what we iterate on for efficiency) => the
+  // current actual pointer, which may have moded
+  std::unordered_map<Expression**, Expression**> actualPointers;
+
+  // note calls in basic blocks
+  template<typename T>
+  void visitSpillable(T* curr) {
+     // if in unreachable code, ignore
+    if (!currBasicBlock) return;
+    auto* pointer = getCurrentPointer();
+    currBasicBlock->contents.actions.emplace_back(pointer);
+    actualPointers[pointer] = pointer; // starts out as correct, may change later
+  }
+
+  void visitCall(Call* curr) {
+    visitSpillable(curr);
+  }
+  void visitCallImport(CallImport* curr) {
+    visitSpillable(curr);
+  }
+  void visitCallIndirect(CallIndirect* curr) {
+    visitSpillable(curr);
+  }
+
+  // main entry point
+
+  void doWalkFunction(Function* func) {
+    super::doWalkFunction(func);
+    spillPointers();
+  }
+
+  // map pointers to their offset in the spill area
+  typedef std::unordered_map<Index, Index> PointerMap;
+  
+  void spillPointers() {
+    // we only care about possible pointers
+    auto* func = getFunction();
+    PointerMap pointerMap;
+    for (Index i = 0; i < func->getNumLocals(); i++) {
+      if (func->getLocalType(i) == ABI::PointerType) {
+        auto offset = pointerMap.size() * getWasmTypeSize(ABI::PointerType);
+        pointerMap[i] = offset;
+      }
+    }
+    // find calls and spill around them
+    bool spilled = false;
+    Index spillLocal = -1;
+    for (auto& curr : basicBlocks) {
+      if (liveBlocks.count(curr.get()) == 0) continue; // ignore dead blocks
+      auto& liveness = curr->contents;
+      auto& actions = liveness.actions;
+      Index lastCall = -1;
+      for (Index i = 0; i < actions.size(); i++) {
+        auto& action = liveness.actions[i];
+        if (action.isOther()) {
+          lastCall = i;
+        }
+      }
+      if (lastCall == Index(-1)) continue; // nothing to see here
+      // scan through the block, spilling around the calls
+      // TODO: we can filter on pointerMap everywhere
+      LocalSet live = liveness.end;
+      for (int i = int(actions.size()) - 1; i >= 0; i--) {
+        auto& action = actions[i];
+        if (action.isGet()) {
+          live.insert(action.index);
+        } else if (action.isSet()) {
+          live.erase(action.index);
+        } else if (action.isOther()) {
+          std::vector<Index> toSpill;
+          for (auto index : live) {
+            if (pointerMap.count(index) > 0) {
+              toSpill.push_back(index);
+            }
+          }
+          if (!toSpill.empty()) {
+            // we now have a call + the information about which locals
+            // should be spilled
+            if (!spilled) {
+              // prepare stack support: get a pointer to stack space big enough for all our data
+              spillLocal = Builder::addVar(func, ABI::PointerType);
+              spilled = true;
+            }
+            auto* pointer = actualPointers[action.origin]; // the origin was seen at walk, but the thing may have moved
+            spillPointersAroundCall(pointer, toSpill, spillLocal, pointerMap, func, getModule());
+          }
+        } else {
+          WASM_UNREACHABLE();
+        }
+      }
+    }
+    if (spilled) {
+      // get the stack space, and set the local to it
+      ABI::getStackSpace(spillLocal, func, getWasmTypeSize(ABI::PointerType) * pointerMap.size(), *getModule());
+    }
+  }
+
+  void spillPointersAroundCall(Expression** origin, std::vector<Index>& toSpill, Index spillLocal, PointerMap& pointerMap, Function* func, Module* module) {
+    auto* call = *origin;
+    if (call->type == unreachable) return; // the call is never reached anyhow, ignore
+    Builder builder(*module);
+    auto* block = builder.makeBlock();
+    // move the operands into locals, as we must spill after they are executed
+    auto handleOperand = [&](Expression*& operand) {
+      auto temp = builder.addVar(func, operand->type);
+      auto* set = builder.makeSetLocal(temp, operand);
+      block->list.push_back(set);
+      block->finalize();
+      if (actualPointers.count(&operand) > 0) {
+        // this is something we track, and it's moving - update
+        actualPointers[&operand] = &set->value;
+      }
+      operand = builder.makeGetLocal(temp, operand->type);
+    };
+    if (call->is<Call>()) {
+      for (auto*& operand : call->cast<Call>()->operands) {
+        handleOperand(operand);
+      }
+    } else if (call->is<CallImport>()) {
+      for (auto*& operand : call->cast<CallImport>()->operands) {
+        handleOperand(operand);
+      }
+    } else if (call->is<CallIndirect>()) {
+      for (auto*& operand : call->cast<CallIndirect>()->operands) {
+        handleOperand(operand);
+      }
+      handleOperand(call->cast<CallIndirect>()->target);
+    } else {
+      WASM_UNREACHABLE();
+    }
+    // add the spills
+    for (auto index : toSpill) {
+      block->list.push_back(builder.makeStore(
+        getWasmTypeSize(ABI::PointerType),
+        pointerMap[index],
+        getWasmTypeSize(ABI::PointerType),
+        builder.makeGetLocal(spillLocal, ABI::PointerType),
+        builder.makeGetLocal(index, ABI::PointerType),
+        ABI::PointerType
+      ));
+    }
+    // add the (modified) call
+    block->list.push_back(call);
+    block->finalize();
+    *origin = block;
+  }
+};
+
+Pass *createSpillPointersPass() {
+  return new SpillPointers();
+}
+
+} // namespace wasm