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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.
*/
//
// Local CSE
//
// In each linear area of execution,
// * track each relevant (big enough) expression
// * if already seen, write to a local if not already, and reuse
// * invalidate the list as we see effects
//
// TODO: global, inter-block gvn etc.
//
#include <wasm.h>
#include <wasm-builder.h>
#include <wasm-traversal.h>
#include <pass.h>
#include <ast/effects.h>
#include <ast/hashed.h>
namespace wasm {
const Index UNUSED = -1;
struct LocalCSE : public WalkerPass<LinearExecutionWalker<LocalCSE>> {
bool isFunctionParallel() override { return true; }
Pass* create() override { return new LocalCSE(); }
// information for an expression we can reuse
struct UsableInfo {
Expression** item;
Index index; // if not UNUSED, then the local we are assigned to, use that to reuse us
EffectAnalyzer effects;
UsableInfo(Expression** item, PassOptions& passOptions) : item(item), index(UNUSED), effects(passOptions, *item) {}
};
// a list of usables in a linear execution trace
typedef HashedExpressionMap<UsableInfo> Usables;
// locals in current linear execution trace, which we try to sink
Usables usables;
static void doNoteNonLinear(LocalCSE* self, Expression** currp) {
self->usables.clear();
}
void checkInvalidations(EffectAnalyzer& effects) {
// TODO: this is O(bad)
std::vector<HashedExpression> invalidated;
for (auto& sinkable : usables) {
if (effects.invalidates(sinkable.second.effects)) {
invalidated.push_back(sinkable.first);
}
}
for (auto index : invalidated) {
usables.erase(index);
}
}
std::vector<Expression*> expressionStack;
static void visitPre(LocalCSE* self, Expression** currp) {
// pre operations
Expression* curr = *currp;
EffectAnalyzer effects(self->getPassOptions());
if (effects.checkPre(curr)) {
self->checkInvalidations(effects);
}
self->expressionStack.push_back(curr);
}
static void visitPost(LocalCSE* self, Expression** currp) {
auto* curr = *currp;
// main operations
if (self->isRelevant(curr)) {
self->handle(currp, curr);
}
// post operations
EffectAnalyzer effects(self->getPassOptions());
if (effects.checkPost(curr)) {
self->checkInvalidations(effects);
}
self->expressionStack.pop_back();
}
// override scan to add a pre and a post check task to all nodes
static void scan(LocalCSE* self, Expression** currp) {
self->pushTask(visitPost, currp);
WalkerPass<LinearExecutionWalker<LocalCSE>>::scan(self, currp);
self->pushTask(visitPre, currp);
}
bool isRelevant(Expression* curr) {
if (curr->is<GetLocal>()) {
return false; // trivial, this is what we optimize to!
}
if (!isConcreteWasmType(curr->type)) {
return false; // don't bother with unreachable etc.
}
if (EffectAnalyzer(getPassOptions(), curr).hasSideEffects()) {
return false; // we can't combine things with side effects
}
// check what we care about TODO: use optimize/shrink levels?
return Measurer::measure(curr) > 1;
}
void handle(Expression** currp, Expression* curr) {
HashedExpression hashed(curr);
auto iter = usables.find(hashed);
if (iter != usables.end()) {
// already exists in the table, this is good to reuse
auto& info = iter->second;
if (info.index == UNUSED) {
// we need to assign to a local. create a new one
auto index = info.index = Builder::addVar(getFunction(), curr->type);
(*info.item) = Builder(*getModule()).makeTeeLocal(index, *info.item);
}
replaceCurrent(
Builder(*getModule()).makeGetLocal(info.index, curr->type)
);
} else {
// not in table, add this, maybe we can help others later
usables.emplace(std::make_pair(hashed, UsableInfo(currp, getPassOptions())));
}
}
};
Pass *createLocalCSEPass() {
return new LocalCSE();
}
} // namespace wasm
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