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diff --git a/src/passes/CoalesceLocals.cpp b/src/passes/CoalesceLocals.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.
+ */
+
+
+//
+// Coalesce locals, in order to reduce the total number of locals. This
+// is similar to register allocation, however, there is never any
+// spilling, and there isn't a fixed number of locals.
+//
+
+
+#include <memory>
+#include <unordered_set>
+
+#include "wasm.h"
+#include "pass.h"
+#include "ast_utils.h"
+#include "cfg/cfg-traversal.h"
+
+namespace wasm {
+
+// A set of locals. This is optimized for comparisons,
+// mergings, and iteration on elements, assuming that there
+// may be a great many potential elements but actual sets
+// may be fairly small. Specifically, we use a sorted
+// vector.
+struct LocalSet : std::vector<Index> {
+  LocalSet() {}
+
+  LocalSet merge(const LocalSet& other) const {
+    LocalSet ret;
+    ret.resize(size() + other.size());
+    Index i = 0, j = 0, t = 0;
+    while (i < size() && j < other.size()) {
+      auto left = (*this)[i];
+      auto right = other[j];
+      if (left < right) {
+        ret[t++] = left;
+        i++;
+      } else if (left > right) {
+        ret[t++] = right;
+        j++;
+      } else {
+        ret[t++] = left;
+        i++;
+        j++;
+      }
+    }
+    while (i < size()) {
+      ret[t++] = (*this)[i];
+      i++;
+    }
+    while (j < other.size()) {
+      ret[t++] = other[j];
+      j++;
+    }
+    ret.resize(t);
+    return ret;
+  }
+
+  // TODO: binary search in all the following
+
+  void insert(Index x) {
+    for (size_t i = 0; i < size(); i++) {
+      auto seen = (*this)[i];
+      if (seen == x) return;
+      if (seen > x) {
+        resize(size() + 1);
+        for (size_t j = size() - 1; j > i; j--) {
+          (*this)[j] = (*this)[j - 1];
+        }
+        (*this)[i] = x;
+        return;
+      }
+    }
+    // we didn't find anything larger
+    push_back(x);
+  }
+
+  bool erase(Index x) {
+    for (size_t i = 0; i < size(); i++) {
+      if ((*this)[i] == x) {
+        for (size_t j = i + 1; j < size(); j++) {
+          (*this)[j - 1] = (*this)[j];
+        }
+        resize(size() - 1);
+        return true;
+      }
+    }
+    return false;
+  }
+
+  bool has(Index x) {
+    for (size_t i = 0; i < size(); i++) {
+      if ((*this)[i] == x) return true;
+    }
+    return false;
+  }
+
+  void verify() const {
+    for (size_t i = 1; i < size(); i++) {
+      assert((*this)[i - 1] < (*this)[i]);
+    }
+  }
+
+  void dump(const char* str = nullptr) const {
+    std::cout << "LocalSet " << (str ? str : "") << ": ";
+    for (auto x : *this) std::cout << x << " ";
+    std::cout << '\n';
+  }
+};
+
+// a liveness-relevant action
+struct Action {
+  enum What {
+    Get, Set
+  };
+  What what;
+  Index index; // the local index read or written
+  Expression** origin; // the origin
+  bool effective; // whether a store is actually effective, i.e., may be read
+
+  Action(What what, Index index, Expression** origin) : what(what), index(index), origin(origin), effective(false) {}
+
+  bool isGet() { return what == Get; }
+  bool isSet() { return what == Set; }
+};
+
+// information about liveness in a basic block
+struct Liveness {
+  LocalSet start, end; // live locals at the start and end
+  std::vector<Action> actions; // actions occurring in this block
+
+  void dump(Function* func) {
+    if (actions.empty()) return;
+    std::cout << "    actions:\n";
+    for (auto& action : actions) {
+      std::cout << "      " << (action.isGet() ? "get" : "set") << " " << func->getLocalName(action.index) << "\n";
+    }
+  }
+};
+
+struct CoalesceLocals : public WalkerPass<CFGWalker<CoalesceLocals, Visitor<CoalesceLocals>, Liveness>> {
+  bool isFunctionParallel() { return true; }
+
+  Index numLocals;
+
+  // cfg traversal work
+
+  static void doVisitGetLocal(CoalesceLocals* self, Expression** currp) {
+    auto* curr = (*currp)->cast<GetLocal>();
+    self->currBasicBlock->contents.actions.emplace_back(Action::Get, curr->index, currp);
+  }
+
+  static void doVisitSetLocal(CoalesceLocals* self, Expression** currp) {
+    auto* curr = (*currp)->cast<SetLocal>();
+    self->currBasicBlock->contents.actions.emplace_back(Action::Set, curr->index, currp);
+  }
+
+  // main entry point
+
+  void walk(Expression*& root) {
+    numLocals = getFunction()->getNumLocals();
+    // collect initial liveness info
+    WalkerPass<CFGWalker<CoalesceLocals, Visitor<CoalesceLocals>, Liveness>>::walk(root);
+    // ignore links to dead blocks, so they don't confuse us and we can see their stores are all ineffective
+    liveBlocks = findLiveBlocks();
+    unlinkDeadBlocks(liveBlocks);
+#ifdef CFG_DEBUG
+    dumpCFG("the cfg", getFunction());
+#endif
+    // flow liveness across blocks
+    flowLiveness();
+    // pick new indices
+    auto indices = pickIndices();
+    // apply indices
+    applyIndices(indices, root);
+  }
+
+  // inteference state
+
+  std::vector<bool> interferences;
+  std::unordered_set<BasicBlock*> liveBlocks;
+
+  void interfere(Index i, Index j) {
+    if (i == j) return;
+#ifdef CFG_DEBUG
+    if (!interferences[i + j * numLocals]) {
+      std::cout << getFunction()->name << ": interfere " << getFunction()->getLocalName(std::min(i, j)) << " : " << getFunction()->getLocalName(std::max(i, j)) << "\n";
+    }
+#endif
+    interferences[i + j * numLocals] = 1;
+    interferences[j + i * numLocals] = 1;
+  }
+
+  void flowLiveness() {
+    interferences.resize(numLocals * numLocals);
+    std::fill(interferences.begin(), interferences.end(), 0);
+    // keep working while stuff is flowing
+    std::vector<BasicBlock*> queue; // TODO set to avoid inserting same block more than once at a time period. TODO optimize, an order might be more efficient
+    for (auto& curr : basicBlocks) {
+      if (liveBlocks.count(curr.get()) == 0) continue; // ignore dead blocks
+      queue.push_back(curr.get());
+      // do the first scan through the block, starting with nothing live at the end, and updating the liveness at the start
+      scanLivenessThroughActions(curr->contents.actions, curr->contents.start);
+    }
+    // at every point in time, we assume we already noted interferences between things already known alive at the end, and scanned back throught the block using that
+    while (queue.size() > 0) {
+      auto* curr = queue.back(); // TODO: order?
+      queue.pop_back();
+      LocalSet live;
+      if (!mergeStartsAndCheckChange(curr->out, curr->contents.end, live)) continue;
+#ifdef CFG_DEBUG
+      std::cout << "change noticed at end of " << debugIds[curr] << " from " << curr->contents.end.size() << " to " << live.size() << " (out of " << numLocals << ")\n";
+#endif
+      assert(curr->contents.end.size() < live.size());
+      curr->contents.end = live;
+      scanLivenessThroughActions(curr->contents.actions, live);
+      // liveness is now calculated at the start. if something
+      // changed, all predecessor blocks need recomputation
+      if (curr->contents.start == live) continue;
+#ifdef CFG_DEBUG
+      std::cout << "change noticed at start of " << debugIds[curr] << " from " << curr->contents.start.size() << " to " << live.size() << ", more work to do\n";
+#endif
+      assert(curr->contents.start.size() < live.size());
+      curr->contents.start = live;
+      for (auto* in : curr->in) {
+        queue.push_back(in);
+#ifdef CFG_DEBUG_ORDER
+        // alternative code for changing the flow order; results must be the same, as we detect a property of the graph.
+        if (queue.empty()) {
+          queue.push_back(in);
+        } else {
+          //abort();
+          auto* front = queue[0];
+          queue[0] = in;
+          queue.push_back(front);
+        }
+#endif
+      }
+    }
+    // live locals at the entry block include params, obviously, but also
+    // vars, in which case the 0-init value is actually used.
+#ifdef CFG_DEBUG
+    std::hash<std::vector<bool>> hasher;
+    std::cout << getFunction()->name << ": interference hash: " << hasher(*(std::vector<bool>*)&interferences) << "\n";
+    for (size_t i = 0; i < numLocals; i++) {
+      std::cout << "int for " << getFunction()->getLocalName(i) << " [" << i << "]: ";
+      for (size_t j = 0; j < numLocals; j++) {
+        assert(interferences[i * numLocals + j] == interferences[j * numLocals + i]);
+        if (interferences[i * numLocals + j]) std::cout << getFunction()->getLocalName(j) << " ";
+      }
+      std::cout << "\n";
+    }
+#endif
+  }
+
+  // merge starts of a list of blocks, adding new interferences as necessary. return
+  // whether anything changed vs an old state (which indicates further processing is necessary).
+  bool mergeStartsAndCheckChange(std::vector<BasicBlock*>& blocks, LocalSet& old, LocalSet& ret) {
+    // merge all the live locals, and add interferences that show up from the merging.
+    // we can assume that locals live together already are already known to be in conflict.
+    if (blocks.size() == 0) return false;
+    ret = blocks[0]->contents.start;
+    if (blocks.size() == 1) {
+      // new interferences are impossible, they would have already been in conflict in the single predecessor.
+      return old != ret;
+    }
+    // more than one, so we must merge
+    for (size_t i = 1; i < blocks.size(); i++) {
+      ret = ret.merge(blocks[i]->contents.start);
+    }
+    // If there is no change in the merged result, then no new conflicts are possible.
+    // Assume the opposite, that we would be missing a conflict between x and y. Then
+    // since the merged result has not changed, x and y were present before as well.
+    // If they were present in the same origin block, then they were already in
+    // conflict there, and it is not a new conflict. If not, then they each arrive
+    // from different origins, with x arriving from X = { b_0..b_i } and y arriving from
+    // Y = { b_i+1..b_j }, where all those blocks are unique (since x and y never arrive from
+    // the same block, by assumption). Livenesses are monotonic, i.e., flowing only adds
+    // locals, never removes, so compared to the past state, we only added to X and Y.
+    // Mark the past arrivals X' and Y', and note that those two cannot be empty, since
+    // by assumption the merged result has not changed - x and y were already present
+    // before. But that means that x and y were already in conflict in the past, so
+    // this is not a new conflict.
+    if (ret == old) return false;
+    // add conflicts
+    for (auto i : ret) {
+      for (auto j : ret) {
+        interfere(i, j);
+      }
+    }
+    return true;
+  }
+
+  void scanLivenessThroughActions(std::vector<Action>& actions, LocalSet& live) {
+    // move towards the front
+    for (int i = int(actions.size()) - 1; i >= 0; i--) {
+      auto& action = actions[i];
+      if (action.isGet()) {
+        // new live local, interferes with all the rest
+        for (auto i : live) {
+          interfere(i, action.index);
+        }
+        live.insert(action.index);
+        assert(live.size() <= numLocals);
+      } else {
+        if (live.erase(action.index)) {
+          action.effective = true;
+        }
+      }
+    }
+  }
+
+  // Indices decision making
+
+  std::vector<Index> pickIndices() { // returns a vector of oldIndex => newIndex
+    // simple greedy coloring
+    // TODO: multiple orderings
+    // TODO: take into account eliminated copies
+    // TODO: take into account distribution (99-1 is better than 50-50 with two registers, for gzip)
+    std::vector<Index> indices; // old => new
+    std::vector<WasmType> types;
+    std::vector<bool> newInterferences; // new index * numLocals => list of all interferences of locals merged to it
+    indices.resize(numLocals);
+    types.resize(numLocals);
+    newInterferences.resize(numLocals * numLocals);
+    std::fill(newInterferences.begin(), newInterferences.end(), 0);
+    Index nextFree = 0;
+    // we can't reorder parameters, they are fixed in order, and cannot coalesce
+    auto numParams = getFunction()->getNumParams();
+    Index i = 0;
+    for (; i < numParams; i++) {
+      indices[i] = i;
+      types[i] = getFunction()->getLocalType(i);
+      std::copy(interferences.begin() + numLocals * i, interferences.begin() + numLocals * (i + 1), newInterferences.begin() + numLocals * i);
+      nextFree++;
+    }
+    for (; i < numLocals; i++) {
+      Index found = -1;
+      for (Index j = 0; j < nextFree; j++) {
+        if (!newInterferences[j * numLocals + i] && getFunction()->getLocalType(i) == types[j]) {
+          indices[i] = found = j;
+          break;
+        }
+      }
+      if (found == Index(-1)) {
+        indices[i] = found = nextFree;
+        types[found] = getFunction()->getLocalType(i);
+        nextFree++;
+      }
+      // merge new interferences for the new index
+      for (size_t j = 0; j < numLocals; j++) {
+        newInterferences[found * numLocals + j] = newInterferences[found * numLocals + j] | interferences[i * numLocals + j];
+      }
+    }
+    return indices;
+  }
+
+  void applyIndices(std::vector<Index>& indices, Expression* root) {
+    assert(indices.size() == numLocals);
+    for (auto& curr : basicBlocks) {
+      auto& actions = curr->contents.actions;
+      for (auto& action : actions) {
+        if (action.isGet()) {
+          auto* get = (*action.origin)->cast<GetLocal>();
+          get->index = indices[get->index];
+        } else {
+          auto* set = (*action.origin)->cast<SetLocal>();
+          set->index = indices[set->index];
+          // in addition, we can optimize out redundant copies and ineffective sets
+          GetLocal* get;
+          if ((get = set->value->dynCast<GetLocal>()) && get->index == set->index) {
+            *action.origin = get; // further optimizations may get rid of the get, if this is in a place where the output does not matter
+          } else if (!action.effective) {
+            *action.origin = set->value; // value may have no side effects, further optimizations can eliminate it
+          }
+        }
+      }
+    }
+    // update type list
+    auto numParams = getFunction()->getNumParams();
+    Index newNumLocals = 0;
+    for (auto index : indices) {
+      newNumLocals = std::max(newNumLocals, index + 1);
+    }
+    auto oldVars = getFunction()->vars;
+    getFunction()->vars.resize(newNumLocals - numParams);
+    for (size_t index = numParams; index < numLocals; index++) {
+      Index newIndex = indices[index];
+      if (newIndex >= numParams) {
+        getFunction()->vars[newIndex - numParams] = oldVars[index - numParams];
+      }
+    }
+    // names are gone
+    getFunction()->localNames.clear();
+    getFunction()->localIndices.clear();
+  }
+};
+
+static RegisterPass<CoalesceLocals> registerPass("coalesce-locals", "reduce # of locals by coalescing");
+
+} // namespace wasm