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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.
*/
//
// Removes branches that go to where they go anyhow
//
#include <wasm.h>
#include <pass.h>
namespace wasm {
struct RemoveUnusedBrs : public Pass {
// preparation: try to unify branches, as the fewer there are, the higher a chance we can remove them
// specifically for if-else, turn an if-else with branches to the same target at the end of each
// child, and with a value, to a branch to that target containing the if-else
void visitIf(If* curr) override {
if (!curr->ifFalse) return;
if (curr->type != none) return; // already has a returned value
// an if_else that indirectly returns a value by breaking to the same target can potentially remove both breaks, and break outside once
auto getLast = [](Expression *side) -> Expression* {
Block* b = side->dyn_cast<Block>();
if (!b) return nullptr;
if (b->list.size() == 0) return nullptr;
return b->list.back();
};
auto process = [&](Expression *side, bool doIt) {
Expression* last = getLast(side);
if (!last) return Name();
Block* b = side->cast<Block>();
Break* br = last->dyn_cast<Break>();
if (!br) return Name();
if (br->condition) return Name();
if (!br->value) return Name();
if (doIt) {
b->list[b->list.size()-1] = br->value;
}
return br->name;
};
// do both, or none
if (process(curr->ifTrue, false).is() && process(curr->ifTrue, false) == process(curr->ifFalse, false)) {
auto br = getLast(curr->ifTrue)->cast<Break>(); // we are about to discard this, so why not reuse it!
process(curr->ifTrue, true);
process(curr->ifFalse, true);
curr->type = br->value->type; // if_else now returns a value
br->value = curr;
// no need to change anything else in the br - target is correct already
replaceCurrent(br);
}
}
// main portion
void visitBlock(Block *curr) override {
if (curr->name.isNull()) return;
if (curr->list.size() == 0) return;
// preparation - remove all code after a break, since it can't execute, and it might confuse us (we look at the last)
for (size_t i = 0; i < curr->list.size()-1; i++) {
if (curr->list[i]->is<Break>()) {
curr->list.resize(i+1);
break;
}
}
Expression* last = curr->list.back();
if (Break* br = last->dyn_cast<Break>()) {
if (br->condition) return;
if (br->name == curr->name) {
if (!br->value) {
curr->list.pop_back();
} else {
curr->list[curr->list.size()-1] = br->value; // can replace with the value
}
}
}
}
};
static RegisterPass<RemoveUnusedBrs> registerPass("remove-unused-brs", "removes breaks from locations that are never branched to");
} // namespace wasm
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