| /* |
| * 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. |
| */ |
| |
| // |
| // Convert the AST to a CFG, while traversing it. |
| // |
| // Note that this is not the same as the relooper CFG. The relooper is |
| // designed for compilation to an AST, this is for processing. There is |
| // no built-in support for transforming this CFG into the AST back |
| // again, it is just metadata on the side for computation purposes. |
| // |
| // Usage: As the traversal proceeds, you can note information and add it to |
| // the current basic block using currBasicBlock, on the contents |
| // property, whose type is user-defined. |
| // |
| |
| #ifndef liveness_traversal_h |
| #define liveness_traversal_h |
| |
| #include "support/sorted_vector.h" |
| #include "wasm.h" |
| #include "wasm-builder.h" |
| #include "wasm-traversal.h" |
| #include "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. |
| typedef SortedVector LocalSet; |
| |
| // A liveness-relevant action. Supports a get, a set, or an |
| // "other" which can be used for other purposes, to mark |
| // their position in a block |
| struct Action { |
| enum What { |
| Get = 0, |
| Set = 1, |
| Other = 2 |
| }; |
| 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) { |
| assert(what != Other); |
| if (what == Get) assert((*origin)->is<GetLocal>()); |
| if (what == Set) assert((*origin)->is<SetLocal>()); |
| } |
| Action(Expression** origin) : what(Other), origin(origin) {} |
| |
| bool isGet() { return what == Get; } |
| bool isSet() { return what == Set; } |
| bool isOther() { return what == Other; } |
| }; |
| |
| // 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"; |
| } |
| } |
| }; |
| |
| template<typename SubType, typename VisitorType> |
| struct LivenessWalker : public CFGWalker<SubType, VisitorType, Liveness> { |
| typedef typename CFGWalker<SubType, VisitorType, Liveness>::BasicBlock BasicBlock; |
| |
| Index numLocals; |
| std::unordered_set<BasicBlock*> liveBlocks; |
| std::vector<uint8_t> copies; // canonicalized - accesses should check (low, high) TODO: use a map for high N, as this tends to be sparse? or don't look at copies at all for big N? |
| std::vector<Index> totalCopies; // total # of copies for each local, with all others |
| |
| // cfg traversal work |
| |
| static void doVisitGetLocal(SubType* self, Expression** currp) { |
| auto* curr = (*currp)->cast<GetLocal>(); |
| // if in unreachable code, ignore |
| if (!self->currBasicBlock) { |
| *currp = Builder(*self->getModule()).replaceWithIdenticalType(curr); |
| return; |
| } |
| self->currBasicBlock->contents.actions.emplace_back(Action::Get, curr->index, currp); |
| } |
| |
| static void doVisitSetLocal(SubType* self, Expression** currp) { |
| auto* curr = (*currp)->cast<SetLocal>(); |
| // if in unreachable code, we don't need the tee (but might need the value, if it has side effects) |
| if (!self->currBasicBlock) { |
| if (curr->isTee()) { |
| *currp = curr->value; |
| } else { |
| *currp = Builder(*self->getModule()).makeDrop(curr->value); |
| } |
| return; |
| } |
| self->currBasicBlock->contents.actions.emplace_back(Action::Set, curr->index, currp); |
| // if this is a copy, note it |
| if (auto* get = self->getCopy(curr)) { |
| // add 2 units, so that backedge prioritization can decide ties, but not much more |
| self->addCopy(curr->index, get->index); |
| self->addCopy(curr->index, get->index); |
| } |
| } |
| |
| // A simple copy is a set of a get. A more interesting copy |
| // is a set of an if with a value, where one side a get. |
| // That can happen when we create an if value in simplify-locals. TODO: recurse into |
| // nested ifs, and block return values? Those cases are trickier, need to |
| // count to see if worth it. |
| // TODO: an if can have two copies |
| GetLocal* getCopy(SetLocal* set) { |
| if (auto* get = set->value->dynCast<GetLocal>()) return get; |
| if (auto* iff = set->value->dynCast<If>()) { |
| if (auto* get = iff->ifTrue->dynCast<GetLocal>()) return get; |
| if (iff->ifFalse) { |
| if (auto* get = iff->ifFalse->dynCast<GetLocal>()) return get; |
| } |
| } |
| return nullptr; |
| } |
| |
| // main entry point |
| |
| void doWalkFunction(Function* func) { |
| numLocals = func->getNumLocals(); |
| copies.resize(numLocals * numLocals); |
| std::fill(copies.begin(), copies.end(), 0); |
| totalCopies.resize(numLocals); |
| std::fill(totalCopies.begin(), totalCopies.end(), 0); |
| // create the CFG by walking the IR |
| CFGWalker<SubType, VisitorType, Liveness>::doWalkFunction(func); |
| // ignore links to dead blocks, so they don't confuse us and we can see their stores are all ineffective |
| liveBlocks = CFGWalker<SubType, VisitorType, Liveness>::findLiveBlocks(); |
| CFGWalker<SubType, VisitorType, Liveness>::unlinkDeadBlocks(liveBlocks); |
| // flow liveness across blocks |
| flowLiveness(); |
| } |
| |
| void flowLiveness() { |
| // keep working while stuff is flowing |
| std::unordered_set<BasicBlock*> queue; |
| for (auto& curr : CFGWalker<SubType, VisitorType, Liveness>::basicBlocks) { |
| if (liveBlocks.count(curr.get()) == 0) continue; // ignore dead blocks |
| queue.insert(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 through the block using that |
| while (queue.size() > 0) { |
| auto iter = queue.begin(); |
| auto* curr = *iter; |
| queue.erase(iter); |
| LocalSet live; |
| if (!mergeStartsAndCheckChange(curr->out, curr->contents.end, live)) continue; |
| 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; |
| assert(curr->contents.start.size() < live.size()); |
| curr->contents.start = live; |
| for (auto* in : curr->in) { |
| queue.insert(in); |
| } |
| } |
| } |
| |
| // merge starts of a list of blocks. return |
| // whether anything changed vs an old state (which indicates further processing is necessary). |
| bool mergeStartsAndCheckChange(std::vector<BasicBlock*>& blocks, LocalSet& old, LocalSet& ret) { |
| if (blocks.size() == 0) return false; |
| ret = blocks[0]->contents.start; |
| if (blocks.size() > 1) { |
| // more than one, so we must merge |
| for (Index i = 1; i < blocks.size(); i++) { |
| ret = ret.merge(blocks[i]->contents.start); |
| } |
| } |
| return old != ret; |
| } |
| |
| 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()) { |
| live.insert(action.index); |
| } else { |
| live.erase(action.index); |
| } |
| } |
| } |
| |
| void addCopy(Index i, Index j) { |
| auto k = std::min(i, j) * numLocals + std::max(i, j); |
| copies[k] = std::min(copies[k], uint8_t(254)) + 1; |
| totalCopies[i]++; |
| totalCopies[j]++; |
| } |
| |
| uint8_t getCopies(Index i, Index j) { |
| return copies[std::min(i, j) * numLocals + std::max(i, j)]; |
| } |
| }; |
| |
| } // namespace wasm |
| |
| #endif // liveness_traversal_h |