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/*
* Copyright 2016 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 dead, i.e. unreachable, code.
//
// We keep a record of when control flow is reachable. When it isn't, we
// kill (turn into unreachable). We then fold away entire unreachable
// expressions.
//
// When dead code causes an operation to not happen, like a store, a call
// or an add, we replace with a block with a list of what does happen.
// That isn't necessarily smaller, but blocks are friendlier to other
// optimizations: blocks can be merged and eliminated, and they clearly
// have no side effects.
//
#include <wasm.h>
#include <pass.h>
#include <wasm-builder.h>
#include <wasm-interpreter.h>
namespace wasm {
// Execute an expression by itself. Errors if we hit anything we need anything not in the expression itself standalone.
class StandaloneExpressionRunner : public ExpressionRunner<StandaloneExpressionRunner> {
public:
struct NonstandaloneException {}; // TODO: use a flow with a special name, as this is likely very slow
Flow visitLoop(Loop* curr) {
// loops might be infinite, so must be careful
// but we can't tell if non-infinite, since we don't have state, so loops are just impossible to optimize for now
throw NonstandaloneException();
}
Flow visitCall(Call* curr) {
throw NonstandaloneException();
}
Flow visitCallImport(CallImport* curr) {
throw NonstandaloneException();
}
Flow visitCallIndirect(CallIndirect* curr) {
throw NonstandaloneException();
}
Flow visitGetLocal(GetLocal *curr) {
throw NonstandaloneException();
}
Flow visitSetLocal(SetLocal *curr) {
throw NonstandaloneException();
}
Flow visitLoad(Load *curr) {
throw NonstandaloneException();
}
Flow visitStore(Store *curr) {
throw NonstandaloneException();
}
Flow visitHost(Host *curr) {
throw NonstandaloneException();
}
void trap(const char* why) override {
throw NonstandaloneException();
}
};
struct Precompute : public WalkerPass<PostWalker<Precompute, UnifiedExpressionVisitor<Precompute>>> {
bool isFunctionParallel() override { return true; }
Pass* create() override { return new Precompute; }
void visitExpression(Expression* curr) {
if (curr->is<Const>()) return;
// try to evaluate this into a const
Flow flow;
try {
flow = StandaloneExpressionRunner().visit(curr);
} catch (StandaloneExpressionRunner::NonstandaloneException& e) {
return;
}
if (flow.breaking()) return; // TODO: can create a break as a replacement
if (isConcreteWasmType(flow.value.type)) {
replaceCurrent(Builder(*getModule()).makeConst(flow.value));
}
}
};
Pass *createPrecomputePass() {
return new Precompute();
}
} // namespace wasm
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