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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.
*/
//
// Computes liveness information for locals.
//
#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 LivenessAction {
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
LivenessAction(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>());
}
LivenessAction(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<LivenessAction> 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(LivenessAction::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(LivenessAction::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<LivenessAction>& 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
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