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//===- SimplifyCFGPass.cpp - CFG Simplification Pass ----------------------===//
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// The LLVM Compiler Infrastructure
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// This file is distributed under the University of Illinois Open Source
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// License. See LICENSE.TXT for details.
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//===----------------------------------------------------------------------===//
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// This file implements dead code elimination and basic block merging, along
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// with a collection of other peephole control flow optimizations. For example:
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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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// * Changes invoke instructions to nounwind functions to be calls.
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// * Change things like "if (x) if (y)" into "if (x&y)".
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//===----------------------------------------------------------------------===//
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#define DEBUG_TYPE "simplifycfg"
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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/Constants.h"
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#include "llvm/Instructions.h"
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#include "llvm/Module.h"
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#include "llvm/Attributes.h"
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#include "llvm/Support/CFG.h"
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#include "llvm/Pass.h"
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#include "llvm/Target/TargetData.h"
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#include "llvm/ADT/SmallVector.h"
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#include "llvm/ADT/SmallPtrSet.h"
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#include "llvm/ADT/Statistic.h"
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STATISTIC(NumSimpl, "Number of blocks simplified");
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struct CFGSimplifyPass : public FunctionPass {
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static char ID; // Pass identification, replacement for typeid
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CFGSimplifyPass() : FunctionPass(&ID) {}
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virtual bool runOnFunction(Function &F);
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char CFGSimplifyPass::ID = 0;
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static RegisterPass<CFGSimplifyPass> X("simplifycfg", "Simplify the CFG");
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// Public interface to the CFGSimplification pass
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FunctionPass *llvm::createCFGSimplificationPass() {
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return new CFGSimplifyPass();
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/// ChangeToUnreachable - Insert an unreachable instruction before the specified
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/// instruction, making it and the rest of the code in the block dead.
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static void ChangeToUnreachable(Instruction *I) {
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BasicBlock *BB = I->getParent();
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// Loop over all of the successors, removing BB's entry from any PHI
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for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE; ++SI)
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(*SI)->removePredecessor(BB);
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new UnreachableInst(I->getContext(), I);
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// All instructions after this are dead.
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BasicBlock::iterator BBI = I, BBE = BB->end();
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if (!BBI->use_empty())
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BBI->replaceAllUsesWith(UndefValue::get(BBI->getType()));
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BB->getInstList().erase(BBI++);
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/// ChangeToCall - Convert the specified invoke into a normal call.
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static void ChangeToCall(InvokeInst *II) {
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BasicBlock *BB = II->getParent();
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SmallVector<Value*, 8> Args(II->op_begin()+3, II->op_end());
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CallInst *NewCall = CallInst::Create(II->getCalledValue(), Args.begin(),
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NewCall->takeName(II);
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NewCall->setCallingConv(II->getCallingConv());
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NewCall->setAttributes(II->getAttributes());
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II->replaceAllUsesWith(NewCall);
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// Follow the call by a branch to the normal destination.
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BranchInst::Create(II->getNormalDest(), II);
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// Update PHI nodes in the unwind destination
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II->getUnwindDest()->removePredecessor(BB);
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BB->getInstList().erase(II);
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static bool MarkAliveBlocks(BasicBlock *BB,
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SmallPtrSet<BasicBlock*, 128> &Reachable) {
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SmallVector<BasicBlock*, 128> Worklist;
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Worklist.push_back(BB);
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bool Changed = false;
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BB = Worklist.pop_back_val();
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if (!Reachable.insert(BB))
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// Do a quick scan of the basic block, turning any obviously unreachable
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// instructions into LLVM unreachable insts. The instruction combining pass
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// canonicalizes unreachable insts into stores to null or undef.
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for (BasicBlock::iterator BBI = BB->begin(), E = BB->end(); BBI != E;++BBI){
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if (CallInst *CI = dyn_cast<CallInst>(BBI)) {
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if (CI->doesNotReturn()) {
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// If we found a call to a no-return function, insert an unreachable
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// instruction after it. Make sure there isn't *already* one there
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if (!isa<UnreachableInst>(BBI)) {
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ChangeToUnreachable(BBI);
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// Store to undef and store to null are undefined and used to signal that
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// they should be changed to unreachable by passes that can't modify the
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if (StoreInst *SI = dyn_cast<StoreInst>(BBI)) {
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Value *Ptr = SI->getOperand(1);
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if (isa<UndefValue>(Ptr) ||
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(isa<ConstantPointerNull>(Ptr) &&
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SI->getPointerAddressSpace() == 0)) {
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ChangeToUnreachable(SI);
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// Turn invokes that call 'nounwind' functions into ordinary calls.
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if (InvokeInst *II = dyn_cast<InvokeInst>(BB->getTerminator()))
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if (II->doesNotThrow()) {
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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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Worklist.push_back(*SI);
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} while (!Worklist.empty());
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/// RemoveUnreachableBlocksFromFn - Remove blocks that are not reachable, even
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/// if they are in a dead cycle. Return true if a change was made, false
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static bool RemoveUnreachableBlocksFromFn(Function &F) {
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SmallPtrSet<BasicBlock*, 128> 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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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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/// MergeEmptyReturnBlocks - If we have more than one empty (other than phi
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/// node) return blocks, merge them together to promote recursive block merging.
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static bool MergeEmptyReturnBlocks(Function &F) {
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bool Changed = false;
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BasicBlock *RetBlock = 0;
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// Scan all the blocks in the function, looking for empty return blocks.
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for (Function::iterator BBI = F.begin(), E = F.end(); BBI != E; ) {
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BasicBlock &BB = *BBI++;
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// Only look at return blocks.
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ReturnInst *Ret = dyn_cast<ReturnInst>(BB.getTerminator());
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if (Ret == 0) continue;
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// Only look at the block if it is empty or the only other thing in it is a
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// single PHI node that is the operand to the return.
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if (Ret != &BB.front()) {
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// Check for something else in the block.
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BasicBlock::iterator I = Ret;
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if (!isa<PHINode>(I) || I != BB.begin() ||
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Ret->getNumOperands() == 0 ||
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Ret->getOperand(0) != I)
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// If this is the first returning block, remember it and keep going.
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// Otherwise, we found a duplicate return block. Merge the two.
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// Case when there is no input to the return or when the returned values
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// agree is trivial. Note that they can't agree if there are phis in the
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if (Ret->getNumOperands() == 0 ||
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Ret->getOperand(0) ==
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cast<ReturnInst>(RetBlock->getTerminator())->getOperand(0)) {
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BB.replaceAllUsesWith(RetBlock);
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BB.eraseFromParent();
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// If the canonical return block has no PHI node, create one now.
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PHINode *RetBlockPHI = dyn_cast<PHINode>(RetBlock->begin());
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if (RetBlockPHI == 0) {
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Value *InVal = cast<ReturnInst>(RetBlock->begin())->getOperand(0);
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RetBlockPHI = PHINode::Create(Ret->getOperand(0)->getType(), "merge",
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for (pred_iterator PI = pred_begin(RetBlock), E = pred_end(RetBlock);
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RetBlockPHI->addIncoming(InVal, *PI);
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RetBlock->getTerminator()->setOperand(0, RetBlockPHI);
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// Turn BB into a block that just unconditionally branches to the return
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// block. This handles the case when the two return blocks have a common
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// predecessor but that return different things.
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RetBlockPHI->addIncoming(Ret->getOperand(0), &BB);
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BB.getTerminator()->eraseFromParent();
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BranchInst::Create(RetBlock, &BB);
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/// IterativeSimplifyCFG - Call SimplifyCFG on all the blocks in the function,
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/// iterating until no more changes are made.
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static bool IterativeSimplifyCFG(Function &F, const TargetData *TD) {
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bool Changed = false;
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bool LocalChange = true;
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while (LocalChange) {
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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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for (Function::iterator BBIt = ++F.begin(); BBIt != F.end(); ) {
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if (SimplifyCFG(BBIt++, TD)) {
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Changed |= LocalChange;
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// It is possible that we may require multiple passes over the code to fully
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bool CFGSimplifyPass::runOnFunction(Function &F) {
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const TargetData *TD = getAnalysisIfAvailable<TargetData>();
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bool EverChanged = RemoveUnreachableBlocksFromFn(F);
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EverChanged |= MergeEmptyReturnBlocks(F);
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EverChanged |= IterativeSimplifyCFG(F, TD);
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// If neither pass changed anything, we're done.
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if (!EverChanged) return false;
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// IterativeSimplifyCFG can (rarely) make some loops dead. If this happens,
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// RemoveUnreachableBlocksFromFn is needed to nuke them, which means we should
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// iterate between the two optimizations. We structure the code like this to
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// avoid reruning IterativeSimplifyCFG if the second pass of
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// RemoveUnreachableBlocksFromFn doesn't do anything.
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if (!RemoveUnreachableBlocksFromFn(F))
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EverChanged = IterativeSimplifyCFG(F, TD);
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EverChanged |= RemoveUnreachableBlocksFromFn(F);
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} while (EverChanged);