forked from OSchip/llvm-project
96 lines
2.8 KiB
C++
96 lines
2.8 KiB
C++
//===- SimplifyCFG.cpp - CFG Simplification Pass --------------------------===//
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//
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// This file implements dead code elimination and basic block merging.
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//
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// Specifically, this:
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// * removes basic blocks with no predecessors
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// * merges a basic block into its predecessor if there is only one and the
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// predecessor only has one successor.
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// * Eliminates PHI nodes for basic blocks with a single predecessor
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// * Eliminates a basic block that only contains an unconditional branch
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/Transforms/Scalar.h"
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#include "llvm/Transforms/Utils/Local.h"
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#include "llvm/Module.h"
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#include "llvm/Support/CFG.h"
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#include "llvm/Pass.h"
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#include "Support/Statistic.h"
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#include <set>
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namespace {
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Statistic<> NumSimpl("cfgsimplify", "Number of blocks simplified");
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struct CFGSimplifyPass : public FunctionPass {
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virtual bool runOnFunction(Function &F);
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};
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RegisterOpt<CFGSimplifyPass> X("simplifycfg", "Simplify the CFG");
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}
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Pass *createCFGSimplificationPass() {
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return new CFGSimplifyPass();
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}
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static bool MarkAliveBlocks(BasicBlock *BB, std::set<BasicBlock*> &Reachable) {
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if (Reachable.count(BB)) return false;
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Reachable.insert(BB);
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bool Changed = ConstantFoldTerminator(BB);
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for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE; ++SI)
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MarkAliveBlocks(*SI, Reachable);
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return Changed;
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}
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// It is possible that we may require multiple passes over the code to fully
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// simplify the CFG.
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//
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bool CFGSimplifyPass::runOnFunction(Function &F) {
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std::set<BasicBlock*> Reachable;
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bool Changed = MarkAliveBlocks(F.begin(), Reachable);
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// If there are unreachable blocks in the CFG...
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if (Reachable.size() != F.size()) {
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assert(Reachable.size() < F.size());
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NumSimpl += F.size()-Reachable.size();
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// Loop over all of the basic blocks that are not reachable, dropping all of
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// their internal references...
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for (Function::iterator BB = ++F.begin(), E = F.end(); BB != E; ++BB)
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if (!Reachable.count(BB)) {
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for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI!=SE; ++SI)
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if (Reachable.count(*SI))
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(*SI)->removePredecessor(BB);
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BB->dropAllReferences();
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}
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for (Function::iterator I = ++F.begin(); I != F.end();)
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if (!Reachable.count(I))
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I = F.getBasicBlockList().erase(I);
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else
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++I;
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Changed = true;
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}
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bool LocalChange = true;
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while (LocalChange) {
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LocalChange = false;
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// Loop over all of the basic blocks (except the first one) and remove them
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// if they are unneeded...
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//
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for (Function::iterator BBIt = ++F.begin(); BBIt != F.end(); ) {
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if (SimplifyCFG(BBIt++)) {
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LocalChange = true;
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++NumSimpl;
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}
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}
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Changed |= LocalChange;
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}
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return Changed;
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}
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