| //===- JumpThreading.cpp - Thread control through conditional blocks ------===// |
| // |
| // The LLVM Compiler Infrastructure |
| // |
| // This file is distributed under the University of Illinois Open Source |
| // License. See LICENSE.TXT for details. |
| // |
| //===----------------------------------------------------------------------===// |
| // |
| // This file implements the Jump Threading pass. |
| // |
| //===----------------------------------------------------------------------===// |
| |
| #define DEBUG_TYPE "jump-threading" |
| #include "llvm/Transforms/Scalar.h" |
| #include "llvm/IntrinsicInst.h" |
| #include "llvm/Pass.h" |
| #include "llvm/ADT/DenseMap.h" |
| #include "llvm/ADT/Statistic.h" |
| #include "llvm/ADT/STLExtras.h" |
| #include "llvm/Analysis/ConstantFolding.h" |
| #include "llvm/Transforms/Utils/BasicBlockUtils.h" |
| #include "llvm/Transforms/Utils/Local.h" |
| #include "llvm/Target/TargetData.h" |
| #include "llvm/Support/CommandLine.h" |
| #include "llvm/Support/Compiler.h" |
| #include "llvm/Support/Debug.h" |
| #include "llvm/ADT/SmallPtrSet.h" |
| using namespace llvm; |
| |
| STATISTIC(NumThreads, "Number of jumps threaded"); |
| STATISTIC(NumFolds, "Number of terminators folded"); |
| |
| static cl::opt<unsigned> |
| Threshold("jump-threading-threshold", |
| cl::desc("Max block size to duplicate for jump threading"), |
| cl::init(6), cl::Hidden); |
| |
| namespace { |
| /// This pass performs 'jump threading', which looks at blocks that have |
| /// multiple predecessors and multiple successors. If one or more of the |
| /// predecessors of the block can be proven to always jump to one of the |
| /// successors, we forward the edge from the predecessor to the successor by |
| /// duplicating the contents of this block. |
| /// |
| /// An example of when this can occur is code like this: |
| /// |
| /// if () { ... |
| /// X = 4; |
| /// } |
| /// if (X < 3) { |
| /// |
| /// In this case, the unconditional branch at the end of the first if can be |
| /// revectored to the false side of the second if. |
| /// |
| class VISIBILITY_HIDDEN JumpThreading : public FunctionPass { |
| TargetData *TD; |
| public: |
| static char ID; // Pass identification |
| JumpThreading() : FunctionPass(&ID) {} |
| |
| virtual void getAnalysisUsage(AnalysisUsage &AU) const { |
| AU.addRequired<TargetData>(); |
| } |
| |
| bool runOnFunction(Function &F); |
| bool ProcessBlock(BasicBlock *BB); |
| void ThreadEdge(BasicBlock *BB, BasicBlock *PredBB, BasicBlock *SuccBB); |
| BasicBlock *FactorCommonPHIPreds(PHINode *PN, Constant *CstVal); |
| bool ProcessBranchOnDuplicateCond(BasicBlock *PredBB, BasicBlock *DestBB); |
| bool ProcessSwitchOnDuplicateCond(BasicBlock *PredBB, BasicBlock *DestBB); |
| |
| bool ProcessJumpOnPHI(PHINode *PN); |
| bool ProcessBranchOnLogical(Value *V, BasicBlock *BB, bool isAnd); |
| bool ProcessBranchOnCompare(CmpInst *Cmp, BasicBlock *BB); |
| |
| bool SimplifyPartiallyRedundantLoad(LoadInst *LI); |
| }; |
| } |
| |
| char JumpThreading::ID = 0; |
| static RegisterPass<JumpThreading> |
| X("jump-threading", "Jump Threading"); |
| |
| // Public interface to the Jump Threading pass |
| FunctionPass *llvm::createJumpThreadingPass() { return new JumpThreading(); } |
| |
| /// runOnFunction - Top level algorithm. |
| /// |
| bool JumpThreading::runOnFunction(Function &F) { |
| DOUT << "Jump threading on function '" << F.getNameStart() << "'\n"; |
| TD = &getAnalysis<TargetData>(); |
| |
| bool AnotherIteration = true, EverChanged = false; |
| while (AnotherIteration) { |
| AnotherIteration = false; |
| bool Changed = false; |
| for (Function::iterator I = F.begin(), E = F.end(); I != E;) { |
| BasicBlock *BB = I; |
| while (ProcessBlock(BB)) |
| Changed = true; |
| |
| ++I; |
| |
| // If the block is trivially dead, zap it. This eliminates the successor |
| // edges which simplifies the CFG. |
| if (pred_begin(BB) == pred_end(BB) && |
| BB != &BB->getParent()->getEntryBlock()) { |
| DOUT << " JT: Deleting dead block '" << BB->getNameStart() |
| << "' with terminator: " << *BB->getTerminator(); |
| DeleteDeadBlock(BB); |
| Changed = true; |
| } |
| } |
| AnotherIteration = Changed; |
| EverChanged |= Changed; |
| } |
| return EverChanged; |
| } |
| |
| /// FactorCommonPHIPreds - If there are multiple preds with the same incoming |
| /// value for the PHI, factor them together so we get one block to thread for |
| /// the whole group. |
| /// This is important for things like "phi i1 [true, true, false, true, x]" |
| /// where we only need to clone the block for the true blocks once. |
| /// |
| BasicBlock *JumpThreading::FactorCommonPHIPreds(PHINode *PN, Constant *CstVal) { |
| SmallVector<BasicBlock*, 16> CommonPreds; |
| for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) |
| if (PN->getIncomingValue(i) == CstVal) |
| CommonPreds.push_back(PN->getIncomingBlock(i)); |
| |
| if (CommonPreds.size() == 1) |
| return CommonPreds[0]; |
| |
| DOUT << " Factoring out " << CommonPreds.size() |
| << " common predecessors.\n"; |
| return SplitBlockPredecessors(PN->getParent(), |
| &CommonPreds[0], CommonPreds.size(), |
| ".thr_comm", this); |
| } |
| |
| |
| /// getJumpThreadDuplicationCost - Return the cost of duplicating this block to |
| /// thread across it. |
| static unsigned getJumpThreadDuplicationCost(const BasicBlock *BB) { |
| /// Ignore PHI nodes, these will be flattened when duplication happens. |
| BasicBlock::const_iterator I = BB->getFirstNonPHI(); |
| |
| // Sum up the cost of each instruction until we get to the terminator. Don't |
| // include the terminator because the copy won't include it. |
| unsigned Size = 0; |
| for (; !isa<TerminatorInst>(I); ++I) { |
| // Debugger intrinsics don't incur code size. |
| if (isa<DbgInfoIntrinsic>(I)) continue; |
| |
| // If this is a pointer->pointer bitcast, it is free. |
| if (isa<BitCastInst>(I) && isa<PointerType>(I->getType())) |
| continue; |
| |
| // All other instructions count for at least one unit. |
| ++Size; |
| |
| // Calls are more expensive. If they are non-intrinsic calls, we model them |
| // as having cost of 4. If they are a non-vector intrinsic, we model them |
| // as having cost of 2 total, and if they are a vector intrinsic, we model |
| // them as having cost 1. |
| if (const CallInst *CI = dyn_cast<CallInst>(I)) { |
| if (!isa<IntrinsicInst>(CI)) |
| Size += 3; |
| else if (isa<VectorType>(CI->getType())) |
| Size += 1; |
| } |
| } |
| |
| // Threading through a switch statement is particularly profitable. If this |
| // block ends in a switch, decrease its cost to make it more likely to happen. |
| if (isa<SwitchInst>(I)) |
| Size = Size > 6 ? Size-6 : 0; |
| |
| return Size; |
| } |
| |
| /// ProcessBlock - If there are any predecessors whose control can be threaded |
| /// through to a successor, transform them now. |
| bool JumpThreading::ProcessBlock(BasicBlock *BB) { |
| // If this block has a single predecessor, and if that pred has a single |
| // successor, merge the blocks. This encourages recursive jump threading |
| // because now the condition in this block can be threaded through |
| // predecessors of our predecessor block. |
| if (BasicBlock *SinglePred = BB->getSinglePredecessor()) |
| if (SinglePred->getTerminator()->getNumSuccessors() == 1 && |
| SinglePred != BB) { |
| // Remember if SinglePred was the entry block of the function. If so, we |
| // will need to move BB back to the entry position. |
| bool isEntry = SinglePred == &SinglePred->getParent()->getEntryBlock(); |
| MergeBasicBlockIntoOnlyPred(BB); |
| |
| if (isEntry && BB != &BB->getParent()->getEntryBlock()) |
| BB->moveBefore(&BB->getParent()->getEntryBlock()); |
| return true; |
| } |
| |
| // See if this block ends with a branch or switch. If so, see if the |
| // condition is a phi node. If so, and if an entry of the phi node is a |
| // constant, we can thread the block. |
| Value *Condition; |
| if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) { |
| // Can't thread an unconditional jump. |
| if (BI->isUnconditional()) return false; |
| Condition = BI->getCondition(); |
| } else if (SwitchInst *SI = dyn_cast<SwitchInst>(BB->getTerminator())) |
| Condition = SI->getCondition(); |
| else |
| return false; // Must be an invoke. |
| |
| // If the terminator of this block is branching on a constant, simplify the |
| // terminator to an unconditional branch. This can occur due to threading in |
| // other blocks. |
| if (isa<ConstantInt>(Condition)) { |
| DOUT << " In block '" << BB->getNameStart() |
| << "' folding terminator: " << *BB->getTerminator(); |
| ++NumFolds; |
| ConstantFoldTerminator(BB); |
| return true; |
| } |
| |
| // If the terminator is branching on an undef, we can pick any of the |
| // successors to branch to. Since this is arbitrary, we pick the successor |
| // with the fewest predecessors. This should reduce the in-degree of the |
| // others. |
| if (isa<UndefValue>(Condition)) { |
| TerminatorInst *BBTerm = BB->getTerminator(); |
| unsigned MinSucc = 0; |
| BasicBlock *TestBB = BBTerm->getSuccessor(MinSucc); |
| // Compute the successor with the minimum number of predecessors. |
| unsigned MinNumPreds = std::distance(pred_begin(TestBB), pred_end(TestBB)); |
| for (unsigned i = 1, e = BBTerm->getNumSuccessors(); i != e; ++i) { |
| TestBB = BBTerm->getSuccessor(i); |
| unsigned NumPreds = std::distance(pred_begin(TestBB), pred_end(TestBB)); |
| if (NumPreds < MinNumPreds) |
| MinSucc = i; |
| } |
| |
| // Fold the branch/switch. |
| for (unsigned i = 0, e = BBTerm->getNumSuccessors(); i != e; ++i) { |
| if (i == MinSucc) continue; |
| BBTerm->getSuccessor(i)->removePredecessor(BB); |
| } |
| |
| DOUT << " In block '" << BB->getNameStart() |
| << "' folding undef terminator: " << *BBTerm; |
| BranchInst::Create(BBTerm->getSuccessor(MinSucc), BBTerm); |
| BBTerm->eraseFromParent(); |
| return true; |
| } |
| |
| Instruction *CondInst = dyn_cast<Instruction>(Condition); |
| |
| // If the condition is an instruction defined in another block, see if a |
| // predecessor has the same condition: |
| // br COND, BBX, BBY |
| // BBX: |
| // br COND, BBZ, BBW |
| if (!Condition->hasOneUse() && // Multiple uses. |
| (CondInst == 0 || CondInst->getParent() != BB)) { // Non-local definition. |
| pred_iterator PI = pred_begin(BB), E = pred_end(BB); |
| if (isa<BranchInst>(BB->getTerminator())) { |
| for (; PI != E; ++PI) |
| if (BranchInst *PBI = dyn_cast<BranchInst>((*PI)->getTerminator())) |
| if (PBI->isConditional() && PBI->getCondition() == Condition && |
| ProcessBranchOnDuplicateCond(*PI, BB)) |
| return true; |
| } else { |
| assert(isa<SwitchInst>(BB->getTerminator()) && "Unknown jump terminator"); |
| for (; PI != E; ++PI) |
| if (SwitchInst *PSI = dyn_cast<SwitchInst>((*PI)->getTerminator())) |
| if (PSI->getCondition() == Condition && |
| ProcessSwitchOnDuplicateCond(*PI, BB)) |
| return true; |
| } |
| } |
| |
| // If there is only a single predecessor of this block, nothing to fold. |
| if (BB->getSinglePredecessor()) |
| return false; |
| |
| // All the rest of our checks depend on the condition being an instruction. |
| if (CondInst == 0) |
| return false; |
| |
| // See if this is a phi node in the current block. |
| if (PHINode *PN = dyn_cast<PHINode>(CondInst)) |
| if (PN->getParent() == BB) |
| return ProcessJumpOnPHI(PN); |
| |
| // If this is a conditional branch whose condition is and/or of a phi, try to |
| // simplify it. |
| if ((CondInst->getOpcode() == Instruction::And || |
| CondInst->getOpcode() == Instruction::Or) && |
| isa<BranchInst>(BB->getTerminator()) && |
| ProcessBranchOnLogical(CondInst, BB, |
| CondInst->getOpcode() == Instruction::And)) |
| return true; |
| |
| // If we have "br (phi != 42)" and the phi node has any constant values as |
| // operands, we can thread through this block. |
| if (CmpInst *CondCmp = dyn_cast<CmpInst>(CondInst)) |
| if (isa<PHINode>(CondCmp->getOperand(0)) && |
| isa<Constant>(CondCmp->getOperand(1)) && |
| ProcessBranchOnCompare(CondCmp, BB)) |
| return true; |
| |
| // Check for some cases that are worth simplifying. Right now we want to look |
| // for loads that are used by a switch or by the condition for the branch. If |
| // we see one, check to see if it's partially redundant. If so, insert a PHI |
| // which can then be used to thread the values. |
| // |
| // This is particularly important because reg2mem inserts loads and stores all |
| // over the place, and this blocks jump threading if we don't zap them. |
| Value *SimplifyValue = CondInst; |
| if (CmpInst *CondCmp = dyn_cast<CmpInst>(SimplifyValue)) |
| if (isa<Constant>(CondCmp->getOperand(1))) |
| SimplifyValue = CondCmp->getOperand(0); |
| |
| if (LoadInst *LI = dyn_cast<LoadInst>(SimplifyValue)) |
| if (SimplifyPartiallyRedundantLoad(LI)) |
| return true; |
| |
| // TODO: If we have: "br (X > 0)" and we have a predecessor where we know |
| // "(X == 4)" thread through this block. |
| |
| return false; |
| } |
| |
| /// ProcessBranchOnDuplicateCond - We found a block and a predecessor of that |
| /// block that jump on exactly the same condition. This means that we almost |
| /// always know the direction of the edge in the DESTBB: |
| /// PREDBB: |
| /// br COND, DESTBB, BBY |
| /// DESTBB: |
| /// br COND, BBZ, BBW |
| /// |
| /// If DESTBB has multiple predecessors, we can't just constant fold the branch |
| /// in DESTBB, we have to thread over it. |
| bool JumpThreading::ProcessBranchOnDuplicateCond(BasicBlock *PredBB, |
| BasicBlock *BB) { |
| BranchInst *PredBI = cast<BranchInst>(PredBB->getTerminator()); |
| |
| // If both successors of PredBB go to DESTBB, we don't know anything. We can |
| // fold the branch to an unconditional one, which allows other recursive |
| // simplifications. |
| bool BranchDir; |
| if (PredBI->getSuccessor(1) != BB) |
| BranchDir = true; |
| else if (PredBI->getSuccessor(0) != BB) |
| BranchDir = false; |
| else { |
| DOUT << " In block '" << PredBB->getNameStart() |
| << "' folding terminator: " << *PredBB->getTerminator(); |
| ++NumFolds; |
| ConstantFoldTerminator(PredBB); |
| return true; |
| } |
| |
| BranchInst *DestBI = cast<BranchInst>(BB->getTerminator()); |
| |
| // If the dest block has one predecessor, just fix the branch condition to a |
| // constant and fold it. |
| if (BB->getSinglePredecessor()) { |
| DOUT << " In block '" << BB->getNameStart() |
| << "' folding condition to '" << BranchDir << "': " |
| << *BB->getTerminator(); |
| ++NumFolds; |
| DestBI->setCondition(ConstantInt::get(Type::Int1Ty, BranchDir)); |
| ConstantFoldTerminator(BB); |
| return true; |
| } |
| |
| // Otherwise we need to thread from PredBB to DestBB's successor which |
| // involves code duplication. Check to see if it is worth it. |
| unsigned JumpThreadCost = getJumpThreadDuplicationCost(BB); |
| if (JumpThreadCost > Threshold) { |
| DOUT << " Not threading BB '" << BB->getNameStart() |
| << "' - Cost is too high: " << JumpThreadCost << "\n"; |
| return false; |
| } |
| |
| // Next, figure out which successor we are threading to. |
| BasicBlock *SuccBB = DestBI->getSuccessor(!BranchDir); |
| |
| // If threading to the same block as we come from, we would infinite loop. |
| if (SuccBB == BB) { |
| DOUT << " Not threading BB '" << BB->getNameStart() |
| << "' - would thread to self!\n"; |
| return false; |
| } |
| |
| // And finally, do it! |
| DOUT << " Threading edge from '" << PredBB->getNameStart() << "' to '" |
| << SuccBB->getNameStart() << "' with cost: " << JumpThreadCost |
| << ", across block:\n " |
| << *BB << "\n"; |
| |
| ThreadEdge(BB, PredBB, SuccBB); |
| ++NumThreads; |
| return true; |
| } |
| |
| /// ProcessSwitchOnDuplicateCond - We found a block and a predecessor of that |
| /// block that switch on exactly the same condition. This means that we almost |
| /// always know the direction of the edge in the DESTBB: |
| /// PREDBB: |
| /// switch COND [... DESTBB, BBY ... ] |
| /// DESTBB: |
| /// switch COND [... BBZ, BBW ] |
| /// |
| /// Optimizing switches like this is very important, because simplifycfg builds |
| /// switches out of repeated 'if' conditions. |
| bool JumpThreading::ProcessSwitchOnDuplicateCond(BasicBlock *PredBB, |
| BasicBlock *DestBB) { |
| // Can't thread edge to self. |
| if (PredBB == DestBB) |
| return false; |
| |
| |
| SwitchInst *PredSI = cast<SwitchInst>(PredBB->getTerminator()); |
| SwitchInst *DestSI = cast<SwitchInst>(DestBB->getTerminator()); |
| |
| // There are a variety of optimizations that we can potentially do on these |
| // blocks: we order them from most to least preferable. |
| |
| // If DESTBB *just* contains the switch, then we can forward edges from PREDBB |
| // directly to their destination. This does not introduce *any* code size |
| // growth. |
| |
| // FIXME: Thread if it just contains a PHI. |
| if (isa<SwitchInst>(DestBB->begin())) { |
| bool MadeChange = false; |
| // Ignore the default edge for now. |
| for (unsigned i = 1, e = DestSI->getNumSuccessors(); i != e; ++i) { |
| ConstantInt *DestVal = DestSI->getCaseValue(i); |
| BasicBlock *DestSucc = DestSI->getSuccessor(i); |
| |
| // Okay, DestSI has a case for 'DestVal' that goes to 'DestSucc'. See if |
| // PredSI has an explicit case for it. If so, forward. If it is covered |
| // by the default case, we can't update PredSI. |
| unsigned PredCase = PredSI->findCaseValue(DestVal); |
| if (PredCase == 0) continue; |
| |
| // If PredSI doesn't go to DestBB on this value, then it won't reach the |
| // case on this condition. |
| if (PredSI->getSuccessor(PredCase) != DestBB && |
| DestSI->getSuccessor(i) != DestBB) |
| continue; |
| |
| // Otherwise, we're safe to make the change. Make sure that the edge from |
| // DestSI to DestSucc is not critical and has no PHI nodes. |
| DOUT << "FORWARDING EDGE " << *DestVal << " FROM: " << *PredSI; |
| DOUT << "THROUGH: " << *DestSI; |
| |
| // If the destination has PHI nodes, just split the edge for updating |
| // simplicity. |
| if (isa<PHINode>(DestSucc->begin()) && !DestSucc->getSinglePredecessor()){ |
| SplitCriticalEdge(DestSI, i, this); |
| DestSucc = DestSI->getSuccessor(i); |
| } |
| FoldSingleEntryPHINodes(DestSucc); |
| PredSI->setSuccessor(PredCase, DestSucc); |
| MadeChange = true; |
| } |
| |
| if (MadeChange) |
| return true; |
| } |
| |
| return false; |
| } |
| |
| |
| /// SimplifyPartiallyRedundantLoad - If LI is an obviously partially redundant |
| /// load instruction, eliminate it by replacing it with a PHI node. This is an |
| /// important optimization that encourages jump threading, and needs to be run |
| /// interlaced with other jump threading tasks. |
| bool JumpThreading::SimplifyPartiallyRedundantLoad(LoadInst *LI) { |
| // Don't hack volatile loads. |
| if (LI->isVolatile()) return false; |
| |
| // If the load is defined in a block with exactly one predecessor, it can't be |
| // partially redundant. |
| BasicBlock *LoadBB = LI->getParent(); |
| if (LoadBB->getSinglePredecessor()) |
| return false; |
| |
| Value *LoadedPtr = LI->getOperand(0); |
| |
| // If the loaded operand is defined in the LoadBB, it can't be available. |
| // FIXME: Could do PHI translation, that would be fun :) |
| if (Instruction *PtrOp = dyn_cast<Instruction>(LoadedPtr)) |
| if (PtrOp->getParent() == LoadBB) |
| return false; |
| |
| // Scan a few instructions up from the load, to see if it is obviously live at |
| // the entry to its block. |
| BasicBlock::iterator BBIt = LI; |
| |
| if (Value *AvailableVal = FindAvailableLoadedValue(LoadedPtr, LoadBB, |
| BBIt, 6)) { |
| // If the value if the load is locally available within the block, just use |
| // it. This frequently occurs for reg2mem'd allocas. |
| //cerr << "LOAD ELIMINATED:\n" << *BBIt << *LI << "\n"; |
| |
| // If the returned value is the load itself, replace with an undef. This can |
| // only happen in dead loops. |
| if (AvailableVal == LI) AvailableVal = UndefValue::get(LI->getType()); |
| LI->replaceAllUsesWith(AvailableVal); |
| LI->eraseFromParent(); |
| return true; |
| } |
| |
| // Otherwise, if we scanned the whole block and got to the top of the block, |
| // we know the block is locally transparent to the load. If not, something |
| // might clobber its value. |
| if (BBIt != LoadBB->begin()) |
| return false; |
| |
| |
| SmallPtrSet<BasicBlock*, 8> PredsScanned; |
| typedef SmallVector<std::pair<BasicBlock*, Value*>, 8> AvailablePredsTy; |
| AvailablePredsTy AvailablePreds; |
| BasicBlock *OneUnavailablePred = 0; |
| |
| // If we got here, the loaded value is transparent through to the start of the |
| // block. Check to see if it is available in any of the predecessor blocks. |
| for (pred_iterator PI = pred_begin(LoadBB), PE = pred_end(LoadBB); |
| PI != PE; ++PI) { |
| BasicBlock *PredBB = *PI; |
| |
| // If we already scanned this predecessor, skip it. |
| if (!PredsScanned.insert(PredBB)) |
| continue; |
| |
| // Scan the predecessor to see if the value is available in the pred. |
| BBIt = PredBB->end(); |
| Value *PredAvailable = FindAvailableLoadedValue(LoadedPtr, PredBB, BBIt, 6); |
| if (!PredAvailable) { |
| OneUnavailablePred = PredBB; |
| continue; |
| } |
| |
| // If so, this load is partially redundant. Remember this info so that we |
| // can create a PHI node. |
| AvailablePreds.push_back(std::make_pair(PredBB, PredAvailable)); |
| } |
| |
| // If the loaded value isn't available in any predecessor, it isn't partially |
| // redundant. |
| if (AvailablePreds.empty()) return false; |
| |
| // Okay, the loaded value is available in at least one (and maybe all!) |
| // predecessors. If the value is unavailable in more than one unique |
| // predecessor, we want to insert a merge block for those common predecessors. |
| // This ensures that we only have to insert one reload, thus not increasing |
| // code size. |
| BasicBlock *UnavailablePred = 0; |
| |
| // If there is exactly one predecessor where the value is unavailable, the |
| // already computed 'OneUnavailablePred' block is it. If it ends in an |
| // unconditional branch, we know that it isn't a critical edge. |
| if (PredsScanned.size() == AvailablePreds.size()+1 && |
| OneUnavailablePred->getTerminator()->getNumSuccessors() == 1) { |
| UnavailablePred = OneUnavailablePred; |
| } else if (PredsScanned.size() != AvailablePreds.size()) { |
| // Otherwise, we had multiple unavailable predecessors or we had a critical |
| // edge from the one. |
| SmallVector<BasicBlock*, 8> PredsToSplit; |
| SmallPtrSet<BasicBlock*, 8> AvailablePredSet; |
| |
| for (unsigned i = 0, e = AvailablePreds.size(); i != e; ++i) |
| AvailablePredSet.insert(AvailablePreds[i].first); |
| |
| // Add all the unavailable predecessors to the PredsToSplit list. |
| for (pred_iterator PI = pred_begin(LoadBB), PE = pred_end(LoadBB); |
| PI != PE; ++PI) |
| if (!AvailablePredSet.count(*PI)) |
| PredsToSplit.push_back(*PI); |
| |
| // Split them out to their own block. |
| UnavailablePred = |
| SplitBlockPredecessors(LoadBB, &PredsToSplit[0], PredsToSplit.size(), |
| "thread-split", this); |
| } |
| |
| // If the value isn't available in all predecessors, then there will be |
| // exactly one where it isn't available. Insert a load on that edge and add |
| // it to the AvailablePreds list. |
| if (UnavailablePred) { |
| assert(UnavailablePred->getTerminator()->getNumSuccessors() == 1 && |
| "Can't handle critical edge here!"); |
| Value *NewVal = new LoadInst(LoadedPtr, LI->getName()+".pr", |
| UnavailablePred->getTerminator()); |
| AvailablePreds.push_back(std::make_pair(UnavailablePred, NewVal)); |
| } |
| |
| // Now we know that each predecessor of this block has a value in |
| // AvailablePreds, sort them for efficient access as we're walking the preds. |
| array_pod_sort(AvailablePreds.begin(), AvailablePreds.end()); |
| |
| // Create a PHI node at the start of the block for the PRE'd load value. |
| PHINode *PN = PHINode::Create(LI->getType(), "", LoadBB->begin()); |
| PN->takeName(LI); |
| |
| // Insert new entries into the PHI for each predecessor. A single block may |
| // have multiple entries here. |
| for (pred_iterator PI = pred_begin(LoadBB), E = pred_end(LoadBB); PI != E; |
| ++PI) { |
| AvailablePredsTy::iterator I = |
| std::lower_bound(AvailablePreds.begin(), AvailablePreds.end(), |
| std::make_pair(*PI, (Value*)0)); |
| |
| assert(I != AvailablePreds.end() && I->first == *PI && |
| "Didn't find entry for predecessor!"); |
| |
| PN->addIncoming(I->second, I->first); |
| } |
| |
| //cerr << "PRE: " << *LI << *PN << "\n"; |
| |
| LI->replaceAllUsesWith(PN); |
| LI->eraseFromParent(); |
| |
| return true; |
| } |
| |
| |
| /// ProcessJumpOnPHI - We have a conditional branch of switch on a PHI node in |
| /// the current block. See if there are any simplifications we can do based on |
| /// inputs to the phi node. |
| /// |
| bool JumpThreading::ProcessJumpOnPHI(PHINode *PN) { |
| // See if the phi node has any constant values. If so, we can determine where |
| // the corresponding predecessor will branch. |
| ConstantInt *PredCst = 0; |
| for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) |
| if ((PredCst = dyn_cast<ConstantInt>(PN->getIncomingValue(i)))) |
| break; |
| |
| // If no incoming value has a constant, we don't know the destination of any |
| // predecessors. |
| if (PredCst == 0) |
| return false; |
| |
| // See if the cost of duplicating this block is low enough. |
| BasicBlock *BB = PN->getParent(); |
| unsigned JumpThreadCost = getJumpThreadDuplicationCost(BB); |
| if (JumpThreadCost > Threshold) { |
| DOUT << " Not threading BB '" << BB->getNameStart() |
| << "' - Cost is too high: " << JumpThreadCost << "\n"; |
| return false; |
| } |
| |
| // If so, we can actually do this threading. Merge any common predecessors |
| // that will act the same. |
| BasicBlock *PredBB = FactorCommonPHIPreds(PN, PredCst); |
| |
| // Next, figure out which successor we are threading to. |
| BasicBlock *SuccBB; |
| if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) |
| SuccBB = BI->getSuccessor(PredCst == ConstantInt::getFalse()); |
| else { |
| SwitchInst *SI = cast<SwitchInst>(BB->getTerminator()); |
| SuccBB = SI->getSuccessor(SI->findCaseValue(PredCst)); |
| } |
| |
| // If threading to the same block as we come from, we would infinite loop. |
| if (SuccBB == BB) { |
| DOUT << " Not threading BB '" << BB->getNameStart() |
| << "' - would thread to self!\n"; |
| return false; |
| } |
| |
| // And finally, do it! |
| DOUT << " Threading edge from '" << PredBB->getNameStart() << "' to '" |
| << SuccBB->getNameStart() << "' with cost: " << JumpThreadCost |
| << ", across block:\n " |
| << *BB << "\n"; |
| |
| ThreadEdge(BB, PredBB, SuccBB); |
| ++NumThreads; |
| return true; |
| } |
| |
| /// ProcessJumpOnLogicalPHI - PN's basic block contains a conditional branch |
| /// whose condition is an AND/OR where one side is PN. If PN has constant |
| /// operands that permit us to evaluate the condition for some operand, thread |
| /// through the block. For example with: |
| /// br (and X, phi(Y, Z, false)) |
| /// the predecessor corresponding to the 'false' will always jump to the false |
| /// destination of the branch. |
| /// |
| bool JumpThreading::ProcessBranchOnLogical(Value *V, BasicBlock *BB, |
| bool isAnd) { |
| // If this is a binary operator tree of the same AND/OR opcode, check the |
| // LHS/RHS. |
| if (BinaryOperator *BO = dyn_cast<BinaryOperator>(V)) |
| if ((isAnd && BO->getOpcode() == Instruction::And) || |
| (!isAnd && BO->getOpcode() == Instruction::Or)) { |
| if (ProcessBranchOnLogical(BO->getOperand(0), BB, isAnd)) |
| return true; |
| if (ProcessBranchOnLogical(BO->getOperand(1), BB, isAnd)) |
| return true; |
| } |
| |
| // If this isn't a PHI node, we can't handle it. |
| PHINode *PN = dyn_cast<PHINode>(V); |
| if (!PN || PN->getParent() != BB) return false; |
| |
| // We can only do the simplification for phi nodes of 'false' with AND or |
| // 'true' with OR. See if we have any entries in the phi for this. |
| unsigned PredNo = ~0U; |
| ConstantInt *PredCst = ConstantInt::get(Type::Int1Ty, !isAnd); |
| for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) { |
| if (PN->getIncomingValue(i) == PredCst) { |
| PredNo = i; |
| break; |
| } |
| } |
| |
| // If no match, bail out. |
| if (PredNo == ~0U) |
| return false; |
| |
| // See if the cost of duplicating this block is low enough. |
| unsigned JumpThreadCost = getJumpThreadDuplicationCost(BB); |
| if (JumpThreadCost > Threshold) { |
| DOUT << " Not threading BB '" << BB->getNameStart() |
| << "' - Cost is too high: " << JumpThreadCost << "\n"; |
| return false; |
| } |
| |
| // If so, we can actually do this threading. Merge any common predecessors |
| // that will act the same. |
| BasicBlock *PredBB = FactorCommonPHIPreds(PN, PredCst); |
| |
| // Next, figure out which successor we are threading to. If this was an AND, |
| // the constant must be FALSE, and we must be targeting the 'false' block. |
| // If this is an OR, the constant must be TRUE, and we must be targeting the |
| // 'true' block. |
| BasicBlock *SuccBB = BB->getTerminator()->getSuccessor(isAnd); |
| |
| // If threading to the same block as we come from, we would infinite loop. |
| if (SuccBB == BB) { |
| DOUT << " Not threading BB '" << BB->getNameStart() |
| << "' - would thread to self!\n"; |
| return false; |
| } |
| |
| // And finally, do it! |
| DOUT << " Threading edge through bool from '" << PredBB->getNameStart() |
| << "' to '" << SuccBB->getNameStart() << "' with cost: " |
| << JumpThreadCost << ", across block:\n " |
| << *BB << "\n"; |
| |
| ThreadEdge(BB, PredBB, SuccBB); |
| ++NumThreads; |
| return true; |
| } |
| |
| /// ProcessBranchOnCompare - We found a branch on a comparison between a phi |
| /// node and a constant. If the PHI node contains any constants as inputs, we |
| /// can fold the compare for that edge and thread through it. |
| bool JumpThreading::ProcessBranchOnCompare(CmpInst *Cmp, BasicBlock *BB) { |
| PHINode *PN = cast<PHINode>(Cmp->getOperand(0)); |
| Constant *RHS = cast<Constant>(Cmp->getOperand(1)); |
| |
| // If the phi isn't in the current block, an incoming edge to this block |
| // doesn't control the destination. |
| if (PN->getParent() != BB) |
| return false; |
| |
| // We can do this simplification if any comparisons fold to true or false. |
| // See if any do. |
| Constant *PredCst = 0; |
| bool TrueDirection = false; |
| for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) { |
| PredCst = dyn_cast<Constant>(PN->getIncomingValue(i)); |
| if (PredCst == 0) continue; |
| |
| Constant *Res; |
| if (ICmpInst *ICI = dyn_cast<ICmpInst>(Cmp)) |
| Res = ConstantExpr::getICmp(ICI->getPredicate(), PredCst, RHS); |
| else |
| Res = ConstantExpr::getFCmp(cast<FCmpInst>(Cmp)->getPredicate(), |
| PredCst, RHS); |
| // If this folded to a constant expr, we can't do anything. |
| if (ConstantInt *ResC = dyn_cast<ConstantInt>(Res)) { |
| TrueDirection = ResC->getZExtValue(); |
| break; |
| } |
| // If this folded to undef, just go the false way. |
| if (isa<UndefValue>(Res)) { |
| TrueDirection = false; |
| break; |
| } |
| |
| // Otherwise, we can't fold this input. |
| PredCst = 0; |
| } |
| |
| // If no match, bail out. |
| if (PredCst == 0) |
| return false; |
| |
| // See if the cost of duplicating this block is low enough. |
| unsigned JumpThreadCost = getJumpThreadDuplicationCost(BB); |
| if (JumpThreadCost > Threshold) { |
| DOUT << " Not threading BB '" << BB->getNameStart() |
| << "' - Cost is too high: " << JumpThreadCost << "\n"; |
| return false; |
| } |
| |
| // If so, we can actually do this threading. Merge any common predecessors |
| // that will act the same. |
| BasicBlock *PredBB = FactorCommonPHIPreds(PN, PredCst); |
| |
| // Next, get our successor. |
| BasicBlock *SuccBB = BB->getTerminator()->getSuccessor(!TrueDirection); |
| |
| // If threading to the same block as we come from, we would infinite loop. |
| if (SuccBB == BB) { |
| DOUT << " Not threading BB '" << BB->getNameStart() |
| << "' - would thread to self!\n"; |
| return false; |
| } |
| |
| |
| // And finally, do it! |
| DOUT << " Threading edge through bool from '" << PredBB->getNameStart() |
| << "' to '" << SuccBB->getNameStart() << "' with cost: " |
| << JumpThreadCost << ", across block:\n " |
| << *BB << "\n"; |
| |
| ThreadEdge(BB, PredBB, SuccBB); |
| ++NumThreads; |
| return true; |
| } |
| |
| |
| /// ThreadEdge - We have decided that it is safe and profitable to thread an |
| /// edge from PredBB to SuccBB across BB. Transform the IR to reflect this |
| /// change. |
| void JumpThreading::ThreadEdge(BasicBlock *BB, BasicBlock *PredBB, |
| BasicBlock *SuccBB) { |
| |
| // Jump Threading can not update SSA properties correctly if the values |
| // defined in the duplicated block are used outside of the block itself. For |
| // this reason, we spill all values that are used outside of BB to the stack. |
| for (BasicBlock::iterator I = BB->begin(); I != BB->end(); ++I) { |
| if (!I->isUsedOutsideOfBlock(BB)) |
| continue; |
| |
| // We found a use of I outside of BB. Create a new stack slot to |
| // break this inter-block usage pattern. |
| DemoteRegToStack(*I); |
| } |
| |
| // We are going to have to map operands from the original BB block to the new |
| // copy of the block 'NewBB'. If there are PHI nodes in BB, evaluate them to |
| // account for entry from PredBB. |
| DenseMap<Instruction*, Value*> ValueMapping; |
| |
| BasicBlock *NewBB = |
| BasicBlock::Create(BB->getName()+".thread", BB->getParent(), BB); |
| NewBB->moveAfter(PredBB); |
| |
| BasicBlock::iterator BI = BB->begin(); |
| for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI) |
| ValueMapping[PN] = PN->getIncomingValueForBlock(PredBB); |
| |
| // Clone the non-phi instructions of BB into NewBB, keeping track of the |
| // mapping and using it to remap operands in the cloned instructions. |
| for (; !isa<TerminatorInst>(BI); ++BI) { |
| Instruction *New = BI->clone(); |
| New->setName(BI->getNameStart()); |
| NewBB->getInstList().push_back(New); |
| ValueMapping[BI] = New; |
| |
| // Remap operands to patch up intra-block references. |
| for (unsigned i = 0, e = New->getNumOperands(); i != e; ++i) |
| if (Instruction *Inst = dyn_cast<Instruction>(New->getOperand(i))) |
| if (Value *Remapped = ValueMapping[Inst]) |
| New->setOperand(i, Remapped); |
| } |
| |
| // We didn't copy the terminator from BB over to NewBB, because there is now |
| // an unconditional jump to SuccBB. Insert the unconditional jump. |
| BranchInst::Create(SuccBB, NewBB); |
| |
| // Check to see if SuccBB has PHI nodes. If so, we need to add entries to the |
| // PHI nodes for NewBB now. |
| for (BasicBlock::iterator PNI = SuccBB->begin(); isa<PHINode>(PNI); ++PNI) { |
| PHINode *PN = cast<PHINode>(PNI); |
| // Ok, we have a PHI node. Figure out what the incoming value was for the |
| // DestBlock. |
| Value *IV = PN->getIncomingValueForBlock(BB); |
| |
| // Remap the value if necessary. |
| if (Instruction *Inst = dyn_cast<Instruction>(IV)) |
| if (Value *MappedIV = ValueMapping[Inst]) |
| IV = MappedIV; |
| PN->addIncoming(IV, NewBB); |
| } |
| |
| // Ok, NewBB is good to go. Update the terminator of PredBB to jump to |
| // NewBB instead of BB. This eliminates predecessors from BB, which requires |
| // us to simplify any PHI nodes in BB. |
| TerminatorInst *PredTerm = PredBB->getTerminator(); |
| for (unsigned i = 0, e = PredTerm->getNumSuccessors(); i != e; ++i) |
| if (PredTerm->getSuccessor(i) == BB) { |
| BB->removePredecessor(PredBB); |
| PredTerm->setSuccessor(i, NewBB); |
| } |
| |
| // At this point, the IR is fully up to date and consistent. Do a quick scan |
| // over the new instructions and zap any that are constants or dead. This |
| // frequently happens because of phi translation. |
| BI = NewBB->begin(); |
| for (BasicBlock::iterator E = NewBB->end(); BI != E; ) { |
| Instruction *Inst = BI++; |
| if (Constant *C = ConstantFoldInstruction(Inst, TD)) { |
| Inst->replaceAllUsesWith(C); |
| Inst->eraseFromParent(); |
| continue; |
| } |
| |
| RecursivelyDeleteTriviallyDeadInstructions(Inst); |
| } |
| } |