| //===- 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/Transforms/Utils/BasicBlockUtils.h" |
| #include "llvm/Transforms/Utils/Local.h" |
| #include "llvm/Support/CommandLine.h" |
| #include "llvm/Support/Compiler.h" |
| #include "llvm/Support/Debug.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 { |
| public: |
| static char ID; // Pass identification |
| JumpThreading() : FunctionPass((intptr_t)&ID) {} |
| |
| bool runOnFunction(Function &F); |
| bool ThreadBlock(BasicBlock *BB); |
| void ThreadEdge(BasicBlock *BB, BasicBlock *PredBB, BasicBlock *SuccBB); |
| BasicBlock *FactorCommonPHIPreds(PHINode *PN, Constant *CstVal); |
| |
| bool ProcessJumpOnPHI(PHINode *PN); |
| bool ProcessBranchOnLogical(Value *V, BasicBlock *BB, bool isAnd); |
| bool ProcessBranchOnCompare(CmpInst *Cmp, BasicBlock *BB); |
| }; |
| } |
| |
| 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"; |
| |
| bool AnotherIteration = true, EverChanged = false; |
| while (AnotherIteration) { |
| AnotherIteration = false; |
| bool Changed = false; |
| for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I) |
| while (ThreadBlock(I)) |
| 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; |
| } |
| |
| |
| /// ThreadBlock - If there are any predecessors whose control can be threaded |
| /// through to a successor, transform them now. |
| bool JumpThreading::ThreadBlock(BasicBlock *BB) { |
| // 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 there is only a single predecessor of this block, nothing to fold. |
| if (BB->getSinglePredecessor()) |
| return false; |
| |
| // See if this is a phi node in the current block. |
| PHINode *PN = dyn_cast<PHINode>(Condition); |
| if (PN && 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 (BinaryOperator *CondI = dyn_cast<BinaryOperator>(Condition)) { |
| if ((CondI->getOpcode() == Instruction::And || |
| CondI->getOpcode() == Instruction::Or) && |
| isa<BranchInst>(BB->getTerminator()) && |
| ProcessBranchOnLogical(CondI, BB, |
| CondI->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>(Condition)) |
| if (isa<PHINode>(CondCmp->getOperand(0)) && |
| isa<Constant>(CondCmp->getOperand(1)) && |
| ProcessBranchOnCompare(CondCmp, BB)) |
| return true; |
| |
| return false; |
| } |
| |
| /// 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. |
| if (!isa<StructType>(I->getType())) { |
| DemoteRegToStack(*I); |
| continue; |
| } |
| |
| // Alternatively, I must be a call or invoke that returns multiple retvals. |
| // We can't use 'DemoteRegToStack' because that will create loads and |
| // stores of aggregates which is not valid yet. If I is a call, we can just |
| // pull all the getresult instructions up to this block. If I is an invoke, |
| // we are out of luck. |
| BasicBlock::iterator IP = I; ++IP; |
| for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); |
| UI != E; ++UI) |
| cast<GetResultInst>(UI)->moveBefore(IP); |
| } |
| |
| // 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); |
| } |
| |
| // Finally, 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); |
| } |
| } |