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