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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.
*/
//
// Locals-related optimizations
//
// This "sinks" set_locals, pushing them to the next get_local where possible,
// and removing the set if there are no gets remaining (the latter is
// particularly useful in ssa mode, but not only).
//
// After this pass, some locals may be completely unused. reorder-locals
// can get rid of those (the operation is trivial there after it sorts by use
// frequency).
#include <wasm.h>
#include <wasm-traversal.h>
#include <pass.h>
#include <ast_utils.h>
namespace wasm {
struct SimplifyLocals : public WalkerPass<LinearExecutionWalker<SimplifyLocals>> {
bool isFunctionParallel() { return true; }
struct SinkableInfo {
Expression** item;
EffectAnalyzer effects;
SinkableInfo(Expression** item) : item(item) {
effects.walk(*item);
}
};
// locals in current linear execution trace, which we try to sink
std::map<Index, SinkableInfo> sinkables;
bool sunk;
// local => # of get_locals for it
std::map<Index, int> numGetLocals;
// for each set_local, its origin pointer
std::map<SetLocal*, Expression**> setLocalOrigins;
void noteNonLinear() {
sinkables.clear();
}
void visitGetLocal(GetLocal *curr) {
auto found = sinkables.find(curr->index);
if (found != sinkables.end()) {
// sink it, and nop the origin TODO: clean up nops
replaceCurrent(*found->second.item);
// reuse the getlocal that is dying
*found->second.item = curr;
ExpressionManipulator::nop(curr);
sinkables.erase(found);
sunk = true;
} else {
numGetLocals[curr->index]++;
}
}
void visitSetLocal(SetLocal *curr) {
// if we are a potentially-sinkable thing, forget it - this
// write overrides the last TODO: optimizable
// TODO: if no get_locals left, can remove the set as well (== expressionizer in emscripten optimizer)
auto found = sinkables.find(curr->index);
if (found != sinkables.end()) {
sinkables.erase(found);
}
}
void checkInvalidations(EffectAnalyzer& effects) {
// TODO: this is O(bad)
std::vector<Index> invalidated;
for (auto& sinkable : sinkables) {
if (effects.invalidates(sinkable.second.effects)) {
invalidated.push_back(sinkable.first);
}
}
for (auto index : invalidated) {
sinkables.erase(index);
}
}
static void visitPre(SimplifyLocals* self, Expression** currp) {
Expression* curr = *currp;
EffectAnalyzer effects;
if (effects.checkPre(curr)) {
self->checkInvalidations(effects);
}
}
static void visitPost(SimplifyLocals* self, Expression** currp) {
Expression* curr = *currp;
EffectAnalyzer effects;
if (effects.checkPost(curr)) {
self->checkInvalidations(effects);
}
// noting origins in the post means it happens after a
// get_local was replaced by a set_local in a sinking
// operation, so we track those movements properly.
if (curr->is<SetLocal>()) {
self->setLocalOrigins[curr->cast<SetLocal>()] = currp;
}
}
static void tryMarkSinkable(SimplifyLocals* self, Expression** currp) {
auto* curr = (*currp)->dynCast<SetLocal>();
if (curr) {
Index index = curr->index;
assert(self->sinkables.count(index) == 0);
self->sinkables.emplace(std::make_pair(index, SinkableInfo(currp)));
}
}
// override scan to add a pre and a post check task to all nodes
static void scan(SimplifyLocals* self, Expression** currp) {
self->pushTask(visitPost, currp);
auto* curr = *currp;
if (curr->is<Block>()) {
// special-case blocks, by marking their children as locals.
// TODO sink from elsewhere? (need to make sure value is not used)
self->pushTask(SimplifyLocals::doNoteNonLinear, currp);
auto& list = curr->cast<Block>()->list;
int size = list.size();
// we can't sink the last element, as it might be a return value;
// and anyhow, control flow is nonlinear at the end of the block so
// it would be invalidated.
for (int i = size - 1; i >= 0; i--) {
if (i < size - 1) {
self->pushTask(tryMarkSinkable, &list[i]);
}
self->pushTask(scan, &list[i]);
}
} else {
WalkerPass<LinearExecutionWalker<SimplifyLocals>>::scan(self, currp);
}
self->pushTask(visitPre, currp);
}
void walk(Expression*& root) {
// multiple passes may be required per function, consider this:
// x = load
// y = store
// c(x, y)
// the load cannot cross the store, but y can be sunk, after which so can x
do {
sunk = false;
// main operation
WalkerPass<LinearExecutionWalker<SimplifyLocals>>::walk(root);
// after optimizing a function, we can see if we have set_locals
// for a local with no remaining gets, in which case, we can
// remove the set.
std::vector<SetLocal*> optimizables;
for (auto pair : setLocalOrigins) {
SetLocal* curr = pair.first;
if (numGetLocals[curr->index] == 0) {
// no gets, can remove the set and leave just the value
optimizables.push_back(curr);
}
}
for (auto* curr : optimizables) {
Expression** origin = setLocalOrigins[curr];
*origin = curr->value;
// nested set_values need to be handled properly.
// consider (set_local x (set_local y (..)), where both can be
// reduced to their values, and we might do it in either
// order.
if (curr->value->is<SetLocal>()) {
setLocalOrigins[curr->value->cast<SetLocal>()] = origin;
}
}
// clean up
numGetLocals.clear();
setLocalOrigins.clear();
sinkables.clear();
} while (sunk);
}
};
static RegisterPass<SimplifyLocals> registerPass("simplify-locals", "miscellaneous locals-related optimizations");
} // namespace wasm
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