| //===-- LoopUnswitch.cpp - Hoist loop-invariant conditionals in loop ------===// |
| // |
| // The LLVM Compiler Infrastructure |
| // |
| // This file is distributed under the University of Illinois Open Source |
| // License. See LICENSE.TXT for details. |
| // |
| //===----------------------------------------------------------------------===// |
| // |
| // This pass transforms loops that contain branches on loop-invariant conditions |
| // to have multiple loops. For example, it turns the left into the right code: |
| // |
| // for (...) if (lic) |
| // A for (...) |
| // if (lic) A; B; C |
| // B else |
| // C for (...) |
| // A; C |
| // |
| // This can increase the size of the code exponentially (doubling it every time |
| // a loop is unswitched) so we only unswitch if the resultant code will be |
| // smaller than a threshold. |
| // |
| // This pass expects LICM to be run before it to hoist invariant conditions out |
| // of the loop, to make the unswitching opportunity obvious. |
| // |
| //===----------------------------------------------------------------------===// |
| |
| #define DEBUG_TYPE "loop-unswitch" |
| #include "llvm/Transforms/Scalar.h" |
| #include "llvm/Constants.h" |
| #include "llvm/DerivedTypes.h" |
| #include "llvm/Function.h" |
| #include "llvm/Instructions.h" |
| #include "llvm/Analysis/ConstantFolding.h" |
| #include "llvm/Analysis/LoopInfo.h" |
| #include "llvm/Analysis/LoopPass.h" |
| #include "llvm/Analysis/Dominators.h" |
| #include "llvm/Transforms/Utils/Cloning.h" |
| #include "llvm/Transforms/Utils/Local.h" |
| #include "llvm/Transforms/Utils/BasicBlockUtils.h" |
| #include "llvm/ADT/Statistic.h" |
| #include "llvm/ADT/SmallPtrSet.h" |
| #include "llvm/ADT/STLExtras.h" |
| #include "llvm/Support/CommandLine.h" |
| #include "llvm/Support/Compiler.h" |
| #include "llvm/Support/Debug.h" |
| #include <algorithm> |
| #include <set> |
| using namespace llvm; |
| |
| STATISTIC(NumBranches, "Number of branches unswitched"); |
| STATISTIC(NumSwitches, "Number of switches unswitched"); |
| STATISTIC(NumSelects , "Number of selects unswitched"); |
| STATISTIC(NumTrivial , "Number of unswitches that are trivial"); |
| STATISTIC(NumSimplify, "Number of simplifications of unswitched code"); |
| |
| static cl::opt<unsigned> |
| Threshold("loop-unswitch-threshold", cl::desc("Max loop size to unswitch"), |
| cl::init(10), cl::Hidden); |
| |
| namespace { |
| class VISIBILITY_HIDDEN LoopUnswitch : public LoopPass { |
| LoopInfo *LI; // Loop information |
| LPPassManager *LPM; |
| |
| // LoopProcessWorklist - Used to check if second loop needs processing |
| // after RewriteLoopBodyWithConditionConstant rewrites first loop. |
| std::vector<Loop*> LoopProcessWorklist; |
| SmallPtrSet<Value *,8> UnswitchedVals; |
| |
| bool OptimizeForSize; |
| bool redoLoop; |
| |
| Loop *currentLoop; |
| DominanceFrontier *DF; |
| DominatorTree *DT; |
| BasicBlock *loopHeader; |
| BasicBlock *loopPreheader; |
| |
| // LoopBlocks contains all of the basic blocks of the loop, including the |
| // preheader of the loop, the body of the loop, and the exit blocks of the |
| // loop, in that order. |
| std::vector<BasicBlock*> LoopBlocks; |
| // NewBlocks contained cloned copy of basic blocks from LoopBlocks. |
| std::vector<BasicBlock*> NewBlocks; |
| |
| public: |
| static char ID; // Pass ID, replacement for typeid |
| explicit LoopUnswitch(bool Os = false) : |
| LoopPass(&ID), OptimizeForSize(Os), redoLoop(false), |
| currentLoop(NULL), DF(NULL), DT(NULL), loopHeader(NULL), |
| loopPreheader(NULL) {} |
| |
| bool runOnLoop(Loop *L, LPPassManager &LPM); |
| bool processCurrentLoop(); |
| |
| /// This transformation requires natural loop information & requires that |
| /// loop preheaders be inserted into the CFG... |
| /// |
| virtual void getAnalysisUsage(AnalysisUsage &AU) const { |
| AU.addRequiredID(LoopSimplifyID); |
| AU.addPreservedID(LoopSimplifyID); |
| AU.addRequired<LoopInfo>(); |
| AU.addPreserved<LoopInfo>(); |
| AU.addRequiredID(LCSSAID); |
| AU.addPreservedID(LCSSAID); |
| AU.addPreserved<DominatorTree>(); |
| AU.addPreserved<DominanceFrontier>(); |
| } |
| |
| private: |
| |
| /// RemoveLoopFromWorklist - If the specified loop is on the loop worklist, |
| /// remove it. |
| void RemoveLoopFromWorklist(Loop *L) { |
| std::vector<Loop*>::iterator I = std::find(LoopProcessWorklist.begin(), |
| LoopProcessWorklist.end(), L); |
| if (I != LoopProcessWorklist.end()) |
| LoopProcessWorklist.erase(I); |
| } |
| |
| void initLoopData() { |
| loopHeader = currentLoop->getHeader(); |
| loopPreheader = currentLoop->getLoopPreheader(); |
| } |
| |
| /// Split all of the edges from inside the loop to their exit blocks. |
| /// Update the appropriate Phi nodes as we do so. |
| void SplitExitEdges(Loop *L, const SmallVector<BasicBlock *, 8> &ExitBlocks); |
| |
| bool UnswitchIfProfitable(Value *LoopCond, Constant *Val); |
| unsigned getLoopUnswitchCost(Value *LIC); |
| void UnswitchTrivialCondition(Loop *L, Value *Cond, Constant *Val, |
| BasicBlock *ExitBlock); |
| void UnswitchNontrivialCondition(Value *LIC, Constant *OnVal, Loop *L); |
| |
| void RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC, |
| Constant *Val, bool isEqual); |
| |
| void EmitPreheaderBranchOnCondition(Value *LIC, Constant *Val, |
| BasicBlock *TrueDest, |
| BasicBlock *FalseDest, |
| Instruction *InsertPt); |
| |
| void SimplifyCode(std::vector<Instruction*> &Worklist, Loop *L); |
| void RemoveBlockIfDead(BasicBlock *BB, |
| std::vector<Instruction*> &Worklist, Loop *l); |
| void RemoveLoopFromHierarchy(Loop *L); |
| bool IsTrivialUnswitchCondition(Value *Cond, Constant **Val = 0, |
| BasicBlock **LoopExit = 0); |
| |
| }; |
| } |
| char LoopUnswitch::ID = 0; |
| static RegisterPass<LoopUnswitch> X("loop-unswitch", "Unswitch loops"); |
| |
| Pass *llvm::createLoopUnswitchPass(bool Os) { |
| return new LoopUnswitch(Os); |
| } |
| |
| /// FindLIVLoopCondition - Cond is a condition that occurs in L. If it is |
| /// invariant in the loop, or has an invariant piece, return the invariant. |
| /// Otherwise, return null. |
| static Value *FindLIVLoopCondition(Value *Cond, Loop *L, bool &Changed) { |
| // Constants should be folded, not unswitched on! |
| if (isa<Constant>(Cond)) return 0; |
| |
| // TODO: Handle: br (VARIANT|INVARIANT). |
| // TODO: Hoist simple expressions out of loops. |
| if (L->isLoopInvariant(Cond)) return Cond; |
| |
| if (BinaryOperator *BO = dyn_cast<BinaryOperator>(Cond)) |
| if (BO->getOpcode() == Instruction::And || |
| BO->getOpcode() == Instruction::Or) { |
| // If either the left or right side is invariant, we can unswitch on this, |
| // which will cause the branch to go away in one loop and the condition to |
| // simplify in the other one. |
| if (Value *LHS = FindLIVLoopCondition(BO->getOperand(0), L, Changed)) |
| return LHS; |
| if (Value *RHS = FindLIVLoopCondition(BO->getOperand(1), L, Changed)) |
| return RHS; |
| } |
| |
| return 0; |
| } |
| |
| bool LoopUnswitch::runOnLoop(Loop *L, LPPassManager &LPM_Ref) { |
| LI = &getAnalysis<LoopInfo>(); |
| LPM = &LPM_Ref; |
| DF = getAnalysisIfAvailable<DominanceFrontier>(); |
| DT = getAnalysisIfAvailable<DominatorTree>(); |
| currentLoop = L; |
| Function *F = currentLoop->getHeader()->getParent(); |
| bool Changed = false; |
| do { |
| assert(currentLoop->isLCSSAForm()); |
| redoLoop = false; |
| Changed |= processCurrentLoop(); |
| } while(redoLoop); |
| |
| if (Changed) { |
| // FIXME: Reconstruct dom info, because it is not preserved properly. |
| if (DT) |
| DT->runOnFunction(*F); |
| if (DF) |
| DF->runOnFunction(*F); |
| } |
| return Changed; |
| } |
| |
| /// processCurrentLoop - Do actual work and unswitch loop if possible |
| /// and profitable. |
| bool LoopUnswitch::processCurrentLoop() { |
| bool Changed = false; |
| |
| // Loop over all of the basic blocks in the loop. If we find an interior |
| // block that is branching on a loop-invariant condition, we can unswitch this |
| // loop. |
| for (Loop::block_iterator I = currentLoop->block_begin(), |
| E = currentLoop->block_end(); |
| I != E; ++I) { |
| TerminatorInst *TI = (*I)->getTerminator(); |
| if (BranchInst *BI = dyn_cast<BranchInst>(TI)) { |
| // If this isn't branching on an invariant condition, we can't unswitch |
| // it. |
| if (BI->isConditional()) { |
| // See if this, or some part of it, is loop invariant. If so, we can |
| // unswitch on it if we desire. |
| Value *LoopCond = FindLIVLoopCondition(BI->getCondition(), |
| currentLoop, Changed); |
| if (LoopCond && UnswitchIfProfitable(LoopCond, |
| ConstantInt::getTrue())) { |
| ++NumBranches; |
| return true; |
| } |
| } |
| } else if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) { |
| Value *LoopCond = FindLIVLoopCondition(SI->getCondition(), |
| currentLoop, Changed); |
| if (LoopCond && SI->getNumCases() > 1) { |
| // Find a value to unswitch on: |
| // FIXME: this should chose the most expensive case! |
| Constant *UnswitchVal = SI->getCaseValue(1); |
| // Do not process same value again and again. |
| if (!UnswitchedVals.insert(UnswitchVal)) |
| continue; |
| |
| if (UnswitchIfProfitable(LoopCond, UnswitchVal)) { |
| ++NumSwitches; |
| return true; |
| } |
| } |
| } |
| |
| // Scan the instructions to check for unswitchable values. |
| for (BasicBlock::iterator BBI = (*I)->begin(), E = (*I)->end(); |
| BBI != E; ++BBI) |
| if (SelectInst *SI = dyn_cast<SelectInst>(BBI)) { |
| Value *LoopCond = FindLIVLoopCondition(SI->getCondition(), |
| currentLoop, Changed); |
| if (LoopCond && UnswitchIfProfitable(LoopCond, |
| ConstantInt::getTrue())) { |
| ++NumSelects; |
| return true; |
| } |
| } |
| } |
| return Changed; |
| } |
| |
| /// isTrivialLoopExitBlock - Check to see if all paths from BB either: |
| /// 1. Exit the loop with no side effects. |
| /// 2. Branch to the latch block with no side-effects. |
| /// |
| /// If these conditions are true, we return true and set ExitBB to the block we |
| /// exit through. |
| /// |
| static bool isTrivialLoopExitBlockHelper(Loop *L, BasicBlock *BB, |
| BasicBlock *&ExitBB, |
| std::set<BasicBlock*> &Visited) { |
| if (!Visited.insert(BB).second) { |
| // Already visited and Ok, end of recursion. |
| return true; |
| } else if (!L->contains(BB)) { |
| // Otherwise, this is a loop exit, this is fine so long as this is the |
| // first exit. |
| if (ExitBB != 0) return false; |
| ExitBB = BB; |
| return true; |
| } |
| |
| // Otherwise, this is an unvisited intra-loop node. Check all successors. |
| for (succ_iterator SI = succ_begin(BB), E = succ_end(BB); SI != E; ++SI) { |
| // Check to see if the successor is a trivial loop exit. |
| if (!isTrivialLoopExitBlockHelper(L, *SI, ExitBB, Visited)) |
| return false; |
| } |
| |
| // Okay, everything after this looks good, check to make sure that this block |
| // doesn't include any side effects. |
| for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) |
| if (I->mayHaveSideEffects()) |
| return false; |
| |
| return true; |
| } |
| |
| /// isTrivialLoopExitBlock - Return true if the specified block unconditionally |
| /// leads to an exit from the specified loop, and has no side-effects in the |
| /// process. If so, return the block that is exited to, otherwise return null. |
| static BasicBlock *isTrivialLoopExitBlock(Loop *L, BasicBlock *BB) { |
| std::set<BasicBlock*> Visited; |
| Visited.insert(L->getHeader()); // Branches to header are ok. |
| BasicBlock *ExitBB = 0; |
| if (isTrivialLoopExitBlockHelper(L, BB, ExitBB, Visited)) |
| return ExitBB; |
| return 0; |
| } |
| |
| /// IsTrivialUnswitchCondition - Check to see if this unswitch condition is |
| /// trivial: that is, that the condition controls whether or not the loop does |
| /// anything at all. If this is a trivial condition, unswitching produces no |
| /// code duplications (equivalently, it produces a simpler loop and a new empty |
| /// loop, which gets deleted). |
| /// |
| /// If this is a trivial condition, return true, otherwise return false. When |
| /// returning true, this sets Cond and Val to the condition that controls the |
| /// trivial condition: when Cond dynamically equals Val, the loop is known to |
| /// exit. Finally, this sets LoopExit to the BB that the loop exits to when |
| /// Cond == Val. |
| /// |
| bool LoopUnswitch::IsTrivialUnswitchCondition(Value *Cond, Constant **Val, |
| BasicBlock **LoopExit) { |
| BasicBlock *Header = currentLoop->getHeader(); |
| TerminatorInst *HeaderTerm = Header->getTerminator(); |
| |
| BasicBlock *LoopExitBB = 0; |
| if (BranchInst *BI = dyn_cast<BranchInst>(HeaderTerm)) { |
| // If the header block doesn't end with a conditional branch on Cond, we |
| // can't handle it. |
| if (!BI->isConditional() || BI->getCondition() != Cond) |
| return false; |
| |
| // Check to see if a successor of the branch is guaranteed to go to the |
| // latch block or exit through a one exit block without having any |
| // side-effects. If so, determine the value of Cond that causes it to do |
| // this. |
| if ((LoopExitBB = isTrivialLoopExitBlock(currentLoop, |
| BI->getSuccessor(0)))) { |
| if (Val) *Val = ConstantInt::getTrue(); |
| } else if ((LoopExitBB = isTrivialLoopExitBlock(currentLoop, |
| BI->getSuccessor(1)))) { |
| if (Val) *Val = ConstantInt::getFalse(); |
| } |
| } else if (SwitchInst *SI = dyn_cast<SwitchInst>(HeaderTerm)) { |
| // If this isn't a switch on Cond, we can't handle it. |
| if (SI->getCondition() != Cond) return false; |
| |
| // Check to see if a successor of the switch is guaranteed to go to the |
| // latch block or exit through a one exit block without having any |
| // side-effects. If so, determine the value of Cond that causes it to do |
| // this. Note that we can't trivially unswitch on the default case. |
| for (unsigned i = 1, e = SI->getNumSuccessors(); i != e; ++i) |
| if ((LoopExitBB = isTrivialLoopExitBlock(currentLoop, |
| SI->getSuccessor(i)))) { |
| // Okay, we found a trivial case, remember the value that is trivial. |
| if (Val) *Val = SI->getCaseValue(i); |
| break; |
| } |
| } |
| |
| // If we didn't find a single unique LoopExit block, or if the loop exit block |
| // contains phi nodes, this isn't trivial. |
| if (!LoopExitBB || isa<PHINode>(LoopExitBB->begin())) |
| return false; // Can't handle this. |
| |
| if (LoopExit) *LoopExit = LoopExitBB; |
| |
| // We already know that nothing uses any scalar values defined inside of this |
| // loop. As such, we just have to check to see if this loop will execute any |
| // side-effecting instructions (e.g. stores, calls, volatile loads) in the |
| // part of the loop that the code *would* execute. We already checked the |
| // tail, check the header now. |
| for (BasicBlock::iterator I = Header->begin(), E = Header->end(); I != E; ++I) |
| if (I->mayHaveSideEffects()) |
| return false; |
| return true; |
| } |
| |
| /// getLoopUnswitchCost - Return the cost (code size growth) that will happen if |
| /// we choose to unswitch current loop on the specified value. |
| /// |
| unsigned LoopUnswitch::getLoopUnswitchCost(Value *LIC) { |
| // If the condition is trivial, always unswitch. There is no code growth for |
| // this case. |
| if (IsTrivialUnswitchCondition(LIC)) |
| return 0; |
| |
| // FIXME: This is really overly conservative. However, more liberal |
| // estimations have thus far resulted in excessive unswitching, which is bad |
| // both in compile time and in code size. This should be replaced once |
| // someone figures out how a good estimation. |
| return currentLoop->getBlocks().size(); |
| |
| unsigned Cost = 0; |
| // FIXME: this is brain dead. It should take into consideration code |
| // shrinkage. |
| for (Loop::block_iterator I = currentLoop->block_begin(), |
| E = currentLoop->block_end(); |
| I != E; ++I) { |
| BasicBlock *BB = *I; |
| // Do not include empty blocks in the cost calculation. This happen due to |
| // loop canonicalization and will be removed. |
| if (BB->begin() == BasicBlock::iterator(BB->getTerminator())) |
| continue; |
| |
| // Count basic blocks. |
| ++Cost; |
| } |
| |
| return Cost; |
| } |
| |
| /// UnswitchIfProfitable - We have found that we can unswitch currentLoop when |
| /// LoopCond == Val to simplify the loop. If we decide that this is profitable, |
| /// unswitch the loop, reprocess the pieces, then return true. |
| bool LoopUnswitch::UnswitchIfProfitable(Value *LoopCond, Constant *Val){ |
| |
| initLoopData(); |
| Function *F = loopHeader->getParent(); |
| |
| |
| // Check to see if it would be profitable to unswitch current loop. |
| unsigned Cost = getLoopUnswitchCost(LoopCond); |
| |
| // Do not do non-trivial unswitch while optimizing for size. |
| if (Cost && OptimizeForSize) |
| return false; |
| if (Cost && !F->isDeclaration() && F->hasFnAttr(Attribute::OptimizeForSize)) |
| return false; |
| |
| if (Cost > Threshold) { |
| // FIXME: this should estimate growth by the amount of code shared by the |
| // resultant unswitched loops. |
| // |
| DOUT << "NOT unswitching loop %" |
| << currentLoop->getHeader()->getName() << ", cost too high: " |
| << currentLoop->getBlocks().size() << "\n"; |
| return false; |
| } |
| |
| Constant *CondVal; |
| BasicBlock *ExitBlock; |
| if (IsTrivialUnswitchCondition(LoopCond, &CondVal, &ExitBlock)) { |
| UnswitchTrivialCondition(currentLoop, LoopCond, CondVal, ExitBlock); |
| } else { |
| UnswitchNontrivialCondition(LoopCond, Val, currentLoop); |
| } |
| |
| return true; |
| } |
| |
| // RemapInstruction - Convert the instruction operands from referencing the |
| // current values into those specified by ValueMap. |
| // |
| static inline void RemapInstruction(Instruction *I, |
| DenseMap<const Value *, Value*> &ValueMap) { |
| for (unsigned op = 0, E = I->getNumOperands(); op != E; ++op) { |
| Value *Op = I->getOperand(op); |
| DenseMap<const Value *, Value*>::iterator It = ValueMap.find(Op); |
| if (It != ValueMap.end()) Op = It->second; |
| I->setOperand(op, Op); |
| } |
| } |
| |
| /// CloneLoop - Recursively clone the specified loop and all of its children, |
| /// mapping the blocks with the specified map. |
| static Loop *CloneLoop(Loop *L, Loop *PL, DenseMap<const Value*, Value*> &VM, |
| LoopInfo *LI, LPPassManager *LPM) { |
| Loop *New = new Loop(); |
| |
| LPM->insertLoop(New, PL); |
| |
| // Add all of the blocks in L to the new loop. |
| for (Loop::block_iterator I = L->block_begin(), E = L->block_end(); |
| I != E; ++I) |
| if (LI->getLoopFor(*I) == L) |
| New->addBasicBlockToLoop(cast<BasicBlock>(VM[*I]), LI->getBase()); |
| |
| // Add all of the subloops to the new loop. |
| for (Loop::iterator I = L->begin(), E = L->end(); I != E; ++I) |
| CloneLoop(*I, New, VM, LI, LPM); |
| |
| return New; |
| } |
| |
| /// EmitPreheaderBranchOnCondition - Emit a conditional branch on two values |
| /// if LIC == Val, branch to TrueDst, otherwise branch to FalseDest. Insert the |
| /// code immediately before InsertPt. |
| void LoopUnswitch::EmitPreheaderBranchOnCondition(Value *LIC, Constant *Val, |
| BasicBlock *TrueDest, |
| BasicBlock *FalseDest, |
| Instruction *InsertPt) { |
| // Insert a conditional branch on LIC to the two preheaders. The original |
| // code is the true version and the new code is the false version. |
| Value *BranchVal = LIC; |
| if (!isa<ConstantInt>(Val) || Val->getType() != Type::Int1Ty) |
| BranchVal = new ICmpInst(ICmpInst::ICMP_EQ, LIC, Val, "tmp", InsertPt); |
| else if (Val != ConstantInt::getTrue()) |
| // We want to enter the new loop when the condition is true. |
| std::swap(TrueDest, FalseDest); |
| |
| // Insert the new branch. |
| BranchInst::Create(TrueDest, FalseDest, BranchVal, InsertPt); |
| } |
| |
| /// UnswitchTrivialCondition - Given a loop that has a trivial unswitchable |
| /// condition in it (a cond branch from its header block to its latch block, |
| /// where the path through the loop that doesn't execute its body has no |
| /// side-effects), unswitch it. This doesn't involve any code duplication, just |
| /// moving the conditional branch outside of the loop and updating loop info. |
| void LoopUnswitch::UnswitchTrivialCondition(Loop *L, Value *Cond, |
| Constant *Val, |
| BasicBlock *ExitBlock) { |
| DOUT << "loop-unswitch: Trivial-Unswitch loop %" |
| << loopHeader->getName() << " [" << L->getBlocks().size() |
| << " blocks] in Function " << L->getHeader()->getParent()->getName() |
| << " on cond: " << *Val << " == " << *Cond << "\n"; |
| |
| // First step, split the preheader, so that we know that there is a safe place |
| // to insert the conditional branch. We will change loopPreheader to have a |
| // conditional branch on Cond. |
| BasicBlock *NewPH = SplitEdge(loopPreheader, loopHeader, this); |
| |
| // Now that we have a place to insert the conditional branch, create a place |
| // to branch to: this is the exit block out of the loop that we should |
| // short-circuit to. |
| |
| // Split this block now, so that the loop maintains its exit block, and so |
| // that the jump from the preheader can execute the contents of the exit block |
| // without actually branching to it (the exit block should be dominated by the |
| // loop header, not the preheader). |
| assert(!L->contains(ExitBlock) && "Exit block is in the loop?"); |
| BasicBlock *NewExit = SplitBlock(ExitBlock, ExitBlock->begin(), this); |
| |
| // Okay, now we have a position to branch from and a position to branch to, |
| // insert the new conditional branch. |
| EmitPreheaderBranchOnCondition(Cond, Val, NewExit, NewPH, |
| loopPreheader->getTerminator()); |
| LPM->deleteSimpleAnalysisValue(loopPreheader->getTerminator(), L); |
| loopPreheader->getTerminator()->eraseFromParent(); |
| |
| // We need to reprocess this loop, it could be unswitched again. |
| redoLoop = true; |
| |
| // Now that we know that the loop is never entered when this condition is a |
| // particular value, rewrite the loop with this info. We know that this will |
| // at least eliminate the old branch. |
| RewriteLoopBodyWithConditionConstant(L, Cond, Val, false); |
| ++NumTrivial; |
| } |
| |
| /// SplitExitEdges - Split all of the edges from inside the loop to their exit |
| /// blocks. Update the appropriate Phi nodes as we do so. |
| void LoopUnswitch::SplitExitEdges(Loop *L, |
| const SmallVector<BasicBlock *, 8> &ExitBlocks) |
| { |
| |
| for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) { |
| BasicBlock *ExitBlock = ExitBlocks[i]; |
| std::vector<BasicBlock*> Preds(pred_begin(ExitBlock), pred_end(ExitBlock)); |
| |
| for (unsigned j = 0, e = Preds.size(); j != e; ++j) { |
| BasicBlock* NewExitBlock = SplitEdge(Preds[j], ExitBlock, this); |
| BasicBlock* StartBlock = Preds[j]; |
| BasicBlock* EndBlock; |
| if (NewExitBlock->getSinglePredecessor() == ExitBlock) { |
| EndBlock = NewExitBlock; |
| NewExitBlock = EndBlock->getSinglePredecessor(); |
| } else { |
| EndBlock = ExitBlock; |
| } |
| |
| std::set<PHINode*> InsertedPHIs; |
| PHINode* OldLCSSA = 0; |
| for (BasicBlock::iterator I = EndBlock->begin(); |
| (OldLCSSA = dyn_cast<PHINode>(I)); ++I) { |
| Value* OldValue = OldLCSSA->getIncomingValueForBlock(NewExitBlock); |
| PHINode* NewLCSSA = PHINode::Create(OldLCSSA->getType(), |
| OldLCSSA->getName() + ".us-lcssa", |
| NewExitBlock->getTerminator()); |
| NewLCSSA->addIncoming(OldValue, StartBlock); |
| OldLCSSA->setIncomingValue(OldLCSSA->getBasicBlockIndex(NewExitBlock), |
| NewLCSSA); |
| InsertedPHIs.insert(NewLCSSA); |
| } |
| |
| BasicBlock::iterator InsertPt = EndBlock->getFirstNonPHI(); |
| for (BasicBlock::iterator I = NewExitBlock->begin(); |
| (OldLCSSA = dyn_cast<PHINode>(I)) && InsertedPHIs.count(OldLCSSA) == 0; |
| ++I) { |
| PHINode *NewLCSSA = PHINode::Create(OldLCSSA->getType(), |
| OldLCSSA->getName() + ".us-lcssa", |
| InsertPt); |
| OldLCSSA->replaceAllUsesWith(NewLCSSA); |
| NewLCSSA->addIncoming(OldLCSSA, NewExitBlock); |
| } |
| |
| } |
| } |
| |
| } |
| |
| /// UnswitchNontrivialCondition - We determined that the loop is profitable |
| /// to unswitch when LIC equal Val. Split it into loop versions and test the |
| /// condition outside of either loop. Return the loops created as Out1/Out2. |
| void LoopUnswitch::UnswitchNontrivialCondition(Value *LIC, Constant *Val, |
| Loop *L) { |
| Function *F = loopHeader->getParent(); |
| DOUT << "loop-unswitch: Unswitching loop %" |
| << loopHeader->getName() << " [" << L->getBlocks().size() |
| << " blocks] in Function " << F->getName() |
| << " when '" << *Val << "' == " << *LIC << "\n"; |
| |
| LoopBlocks.clear(); |
| NewBlocks.clear(); |
| |
| // First step, split the preheader and exit blocks, and add these blocks to |
| // the LoopBlocks list. |
| BasicBlock *NewPreheader = SplitEdge(loopPreheader, loopHeader, this); |
| LoopBlocks.push_back(NewPreheader); |
| |
| // We want the loop to come after the preheader, but before the exit blocks. |
| LoopBlocks.insert(LoopBlocks.end(), L->block_begin(), L->block_end()); |
| |
| SmallVector<BasicBlock*, 8> ExitBlocks; |
| L->getUniqueExitBlocks(ExitBlocks); |
| |
| // Split all of the edges from inside the loop to their exit blocks. Update |
| // the appropriate Phi nodes as we do so. |
| SplitExitEdges(L, ExitBlocks); |
| |
| // The exit blocks may have been changed due to edge splitting, recompute. |
| ExitBlocks.clear(); |
| L->getUniqueExitBlocks(ExitBlocks); |
| |
| // Add exit blocks to the loop blocks. |
| LoopBlocks.insert(LoopBlocks.end(), ExitBlocks.begin(), ExitBlocks.end()); |
| |
| // Next step, clone all of the basic blocks that make up the loop (including |
| // the loop preheader and exit blocks), keeping track of the mapping between |
| // the instructions and blocks. |
| NewBlocks.reserve(LoopBlocks.size()); |
| DenseMap<const Value*, Value*> ValueMap; |
| for (unsigned i = 0, e = LoopBlocks.size(); i != e; ++i) { |
| BasicBlock *New = CloneBasicBlock(LoopBlocks[i], ValueMap, ".us", F); |
| NewBlocks.push_back(New); |
| ValueMap[LoopBlocks[i]] = New; // Keep the BB mapping. |
| LPM->cloneBasicBlockSimpleAnalysis(LoopBlocks[i], New, L); |
| } |
| |
| // Splice the newly inserted blocks into the function right before the |
| // original preheader. |
| F->getBasicBlockList().splice(LoopBlocks[0], F->getBasicBlockList(), |
| NewBlocks[0], F->end()); |
| |
| // Now we create the new Loop object for the versioned loop. |
| Loop *NewLoop = CloneLoop(L, L->getParentLoop(), ValueMap, LI, LPM); |
| Loop *ParentLoop = L->getParentLoop(); |
| if (ParentLoop) { |
| // Make sure to add the cloned preheader and exit blocks to the parent loop |
| // as well. |
| ParentLoop->addBasicBlockToLoop(NewBlocks[0], LI->getBase()); |
| } |
| |
| for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) { |
| BasicBlock *NewExit = cast<BasicBlock>(ValueMap[ExitBlocks[i]]); |
| // The new exit block should be in the same loop as the old one. |
| if (Loop *ExitBBLoop = LI->getLoopFor(ExitBlocks[i])) |
| ExitBBLoop->addBasicBlockToLoop(NewExit, LI->getBase()); |
| |
| assert(NewExit->getTerminator()->getNumSuccessors() == 1 && |
| "Exit block should have been split to have one successor!"); |
| BasicBlock *ExitSucc = NewExit->getTerminator()->getSuccessor(0); |
| |
| // If the successor of the exit block had PHI nodes, add an entry for |
| // NewExit. |
| PHINode *PN; |
| for (BasicBlock::iterator I = ExitSucc->begin(); |
| (PN = dyn_cast<PHINode>(I)); ++I) { |
| Value *V = PN->getIncomingValueForBlock(ExitBlocks[i]); |
| DenseMap<const Value *, Value*>::iterator It = ValueMap.find(V); |
| if (It != ValueMap.end()) V = It->second; |
| PN->addIncoming(V, NewExit); |
| } |
| } |
| |
| // Rewrite the code to refer to itself. |
| for (unsigned i = 0, e = NewBlocks.size(); i != e; ++i) |
| for (BasicBlock::iterator I = NewBlocks[i]->begin(), |
| E = NewBlocks[i]->end(); I != E; ++I) |
| RemapInstruction(I, ValueMap); |
| |
| // Rewrite the original preheader to select between versions of the loop. |
| BranchInst *OldBR = cast<BranchInst>(loopPreheader->getTerminator()); |
| assert(OldBR->isUnconditional() && OldBR->getSuccessor(0) == LoopBlocks[0] && |
| "Preheader splitting did not work correctly!"); |
| |
| // Emit the new branch that selects between the two versions of this loop. |
| EmitPreheaderBranchOnCondition(LIC, Val, NewBlocks[0], LoopBlocks[0], OldBR); |
| LPM->deleteSimpleAnalysisValue(OldBR, L); |
| OldBR->eraseFromParent(); |
| |
| LoopProcessWorklist.push_back(NewLoop); |
| redoLoop = true; |
| |
| // Now we rewrite the original code to know that the condition is true and the |
| // new code to know that the condition is false. |
| RewriteLoopBodyWithConditionConstant(L , LIC, Val, false); |
| |
| // It's possible that simplifying one loop could cause the other to be |
| // deleted. If so, don't simplify it. |
| if (!LoopProcessWorklist.empty() && LoopProcessWorklist.back() == NewLoop) |
| RewriteLoopBodyWithConditionConstant(NewLoop, LIC, Val, true); |
| |
| } |
| |
| /// RemoveFromWorklist - Remove all instances of I from the worklist vector |
| /// specified. |
| static void RemoveFromWorklist(Instruction *I, |
| std::vector<Instruction*> &Worklist) { |
| std::vector<Instruction*>::iterator WI = std::find(Worklist.begin(), |
| Worklist.end(), I); |
| while (WI != Worklist.end()) { |
| unsigned Offset = WI-Worklist.begin(); |
| Worklist.erase(WI); |
| WI = std::find(Worklist.begin()+Offset, Worklist.end(), I); |
| } |
| } |
| |
| /// ReplaceUsesOfWith - When we find that I really equals V, remove I from the |
| /// program, replacing all uses with V and update the worklist. |
| static void ReplaceUsesOfWith(Instruction *I, Value *V, |
| std::vector<Instruction*> &Worklist, |
| Loop *L, LPPassManager *LPM) { |
| DOUT << "Replace with '" << *V << "': " << *I; |
| |
| // Add uses to the worklist, which may be dead now. |
| for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) |
| if (Instruction *Use = dyn_cast<Instruction>(I->getOperand(i))) |
| Worklist.push_back(Use); |
| |
| // Add users to the worklist which may be simplified now. |
| for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); |
| UI != E; ++UI) |
| Worklist.push_back(cast<Instruction>(*UI)); |
| LPM->deleteSimpleAnalysisValue(I, L); |
| RemoveFromWorklist(I, Worklist); |
| I->replaceAllUsesWith(V); |
| I->eraseFromParent(); |
| ++NumSimplify; |
| } |
| |
| /// RemoveBlockIfDead - If the specified block is dead, remove it, update loop |
| /// information, and remove any dead successors it has. |
| /// |
| void LoopUnswitch::RemoveBlockIfDead(BasicBlock *BB, |
| std::vector<Instruction*> &Worklist, |
| Loop *L) { |
| if (pred_begin(BB) != pred_end(BB)) { |
| // This block isn't dead, since an edge to BB was just removed, see if there |
| // are any easy simplifications we can do now. |
| if (BasicBlock *Pred = BB->getSinglePredecessor()) { |
| // If it has one pred, fold phi nodes in BB. |
| while (isa<PHINode>(BB->begin())) |
| ReplaceUsesOfWith(BB->begin(), |
| cast<PHINode>(BB->begin())->getIncomingValue(0), |
| Worklist, L, LPM); |
| |
| // If this is the header of a loop and the only pred is the latch, we now |
| // have an unreachable loop. |
| if (Loop *L = LI->getLoopFor(BB)) |
| if (loopHeader == BB && L->contains(Pred)) { |
| // Remove the branch from the latch to the header block, this makes |
| // the header dead, which will make the latch dead (because the header |
| // dominates the latch). |
| LPM->deleteSimpleAnalysisValue(Pred->getTerminator(), L); |
| Pred->getTerminator()->eraseFromParent(); |
| new UnreachableInst(Pred); |
| |
| // The loop is now broken, remove it from LI. |
| RemoveLoopFromHierarchy(L); |
| |
| // Reprocess the header, which now IS dead. |
| RemoveBlockIfDead(BB, Worklist, L); |
| return; |
| } |
| |
| // If pred ends in a uncond branch, add uncond branch to worklist so that |
| // the two blocks will get merged. |
| if (BranchInst *BI = dyn_cast<BranchInst>(Pred->getTerminator())) |
| if (BI->isUnconditional()) |
| Worklist.push_back(BI); |
| } |
| return; |
| } |
| |
| DOUT << "Nuking dead block: " << *BB; |
| |
| // Remove the instructions in the basic block from the worklist. |
| for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) { |
| RemoveFromWorklist(I, Worklist); |
| |
| // Anything that uses the instructions in this basic block should have their |
| // uses replaced with undefs. |
| if (!I->use_empty()) |
| I->replaceAllUsesWith(UndefValue::get(I->getType())); |
| } |
| |
| // If this is the edge to the header block for a loop, remove the loop and |
| // promote all subloops. |
| if (Loop *BBLoop = LI->getLoopFor(BB)) { |
| if (BBLoop->getLoopLatch() == BB) |
| RemoveLoopFromHierarchy(BBLoop); |
| } |
| |
| // Remove the block from the loop info, which removes it from any loops it |
| // was in. |
| LI->removeBlock(BB); |
| |
| |
| // Remove phi node entries in successors for this block. |
| TerminatorInst *TI = BB->getTerminator(); |
| SmallVector<BasicBlock*, 4> Succs; |
| for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i) { |
| Succs.push_back(TI->getSuccessor(i)); |
| TI->getSuccessor(i)->removePredecessor(BB); |
| } |
| |
| // Unique the successors, remove anything with multiple uses. |
| array_pod_sort(Succs.begin(), Succs.end()); |
| Succs.erase(std::unique(Succs.begin(), Succs.end()), Succs.end()); |
| |
| // Remove the basic block, including all of the instructions contained in it. |
| LPM->deleteSimpleAnalysisValue(BB, L); |
| BB->eraseFromParent(); |
| // Remove successor blocks here that are not dead, so that we know we only |
| // have dead blocks in this list. Nondead blocks have a way of becoming dead, |
| // then getting removed before we revisit them, which is badness. |
| // |
| for (unsigned i = 0; i != Succs.size(); ++i) |
| if (pred_begin(Succs[i]) != pred_end(Succs[i])) { |
| // One exception is loop headers. If this block was the preheader for a |
| // loop, then we DO want to visit the loop so the loop gets deleted. |
| // We know that if the successor is a loop header, that this loop had to |
| // be the preheader: the case where this was the latch block was handled |
| // above and headers can only have two predecessors. |
| if (!LI->isLoopHeader(Succs[i])) { |
| Succs.erase(Succs.begin()+i); |
| --i; |
| } |
| } |
| |
| for (unsigned i = 0, e = Succs.size(); i != e; ++i) |
| RemoveBlockIfDead(Succs[i], Worklist, L); |
| } |
| |
| /// RemoveLoopFromHierarchy - We have discovered that the specified loop has |
| /// become unwrapped, either because the backedge was deleted, or because the |
| /// edge into the header was removed. If the edge into the header from the |
| /// latch block was removed, the loop is unwrapped but subloops are still alive, |
| /// so they just reparent loops. If the loops are actually dead, they will be |
| /// removed later. |
| void LoopUnswitch::RemoveLoopFromHierarchy(Loop *L) { |
| LPM->deleteLoopFromQueue(L); |
| RemoveLoopFromWorklist(L); |
| } |
| |
| // RewriteLoopBodyWithConditionConstant - We know either that the value LIC has |
| // the value specified by Val in the specified loop, or we know it does NOT have |
| // that value. Rewrite any uses of LIC or of properties correlated to it. |
| void LoopUnswitch::RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC, |
| Constant *Val, |
| bool IsEqual) { |
| assert(!isa<Constant>(LIC) && "Why are we unswitching on a constant?"); |
| |
| // FIXME: Support correlated properties, like: |
| // for (...) |
| // if (li1 < li2) |
| // ... |
| // if (li1 > li2) |
| // ... |
| |
| // FOLD boolean conditions (X|LIC), (X&LIC). Fold conditional branches, |
| // selects, switches. |
| std::vector<User*> Users(LIC->use_begin(), LIC->use_end()); |
| std::vector<Instruction*> Worklist; |
| |
| // If we know that LIC == Val, or that LIC == NotVal, just replace uses of LIC |
| // in the loop with the appropriate one directly. |
| if (IsEqual || (isa<ConstantInt>(Val) && Val->getType() == Type::Int1Ty)) { |
| Value *Replacement; |
| if (IsEqual) |
| Replacement = Val; |
| else |
| Replacement = ConstantInt::get(Type::Int1Ty, |
| !cast<ConstantInt>(Val)->getZExtValue()); |
| |
| for (unsigned i = 0, e = Users.size(); i != e; ++i) |
| if (Instruction *U = cast<Instruction>(Users[i])) { |
| if (!L->contains(U->getParent())) |
| continue; |
| U->replaceUsesOfWith(LIC, Replacement); |
| Worklist.push_back(U); |
| } |
| } else { |
| // Otherwise, we don't know the precise value of LIC, but we do know that it |
| // is certainly NOT "Val". As such, simplify any uses in the loop that we |
| // can. This case occurs when we unswitch switch statements. |
| for (unsigned i = 0, e = Users.size(); i != e; ++i) |
| if (Instruction *U = cast<Instruction>(Users[i])) { |
| if (!L->contains(U->getParent())) |
| continue; |
| |
| Worklist.push_back(U); |
| |
| // If we know that LIC is not Val, use this info to simplify code. |
| if (SwitchInst *SI = dyn_cast<SwitchInst>(U)) { |
| for (unsigned i = 1, e = SI->getNumCases(); i != e; ++i) { |
| if (SI->getCaseValue(i) == Val) { |
| // Found a dead case value. Don't remove PHI nodes in the |
| // successor if they become single-entry, those PHI nodes may |
| // be in the Users list. |
| |
| // FIXME: This is a hack. We need to keep the successor around |
| // and hooked up so as to preserve the loop structure, because |
| // trying to update it is complicated. So instead we preserve the |
| // loop structure and put the block on an dead code path. |
| |
| BasicBlock *SISucc = SI->getSuccessor(i); |
| BasicBlock* Old = SI->getParent(); |
| BasicBlock* Split = SplitBlock(Old, SI, this); |
| |
| Instruction* OldTerm = Old->getTerminator(); |
| BranchInst::Create(Split, SISucc, |
| ConstantInt::getTrue(), OldTerm); |
| |
| LPM->deleteSimpleAnalysisValue(Old->getTerminator(), L); |
| Old->getTerminator()->eraseFromParent(); |
| |
| PHINode *PN; |
| for (BasicBlock::iterator II = SISucc->begin(); |
| (PN = dyn_cast<PHINode>(II)); ++II) { |
| Value *InVal = PN->removeIncomingValue(Split, false); |
| PN->addIncoming(InVal, Old); |
| } |
| |
| SI->removeCase(i); |
| break; |
| } |
| } |
| } |
| |
| // TODO: We could do other simplifications, for example, turning |
| // LIC == Val -> false. |
| } |
| } |
| |
| SimplifyCode(Worklist, L); |
| } |
| |
| /// SimplifyCode - Okay, now that we have simplified some instructions in the |
| /// loop, walk over it and constant prop, dce, and fold control flow where |
| /// possible. Note that this is effectively a very simple loop-structure-aware |
| /// optimizer. During processing of this loop, L could very well be deleted, so |
| /// it must not be used. |
| /// |
| /// FIXME: When the loop optimizer is more mature, separate this out to a new |
| /// pass. |
| /// |
| void LoopUnswitch::SimplifyCode(std::vector<Instruction*> &Worklist, Loop *L) { |
| while (!Worklist.empty()) { |
| Instruction *I = Worklist.back(); |
| Worklist.pop_back(); |
| |
| // Simple constant folding. |
| if (Constant *C = ConstantFoldInstruction(I)) { |
| ReplaceUsesOfWith(I, C, Worklist, L, LPM); |
| continue; |
| } |
| |
| // Simple DCE. |
| if (isInstructionTriviallyDead(I)) { |
| DOUT << "Remove dead instruction '" << *I; |
| |
| // Add uses to the worklist, which may be dead now. |
| for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) |
| if (Instruction *Use = dyn_cast<Instruction>(I->getOperand(i))) |
| Worklist.push_back(Use); |
| LPM->deleteSimpleAnalysisValue(I, L); |
| RemoveFromWorklist(I, Worklist); |
| I->eraseFromParent(); |
| ++NumSimplify; |
| continue; |
| } |
| |
| // Special case hacks that appear commonly in unswitched code. |
| switch (I->getOpcode()) { |
| case Instruction::Select: |
| if (ConstantInt *CB = dyn_cast<ConstantInt>(I->getOperand(0))) { |
| ReplaceUsesOfWith(I, I->getOperand(!CB->getZExtValue()+1), Worklist, L, |
| LPM); |
| continue; |
| } |
| break; |
| case Instruction::And: |
| if (isa<ConstantInt>(I->getOperand(0)) && |
| I->getOperand(0)->getType() == Type::Int1Ty) // constant -> RHS |
| cast<BinaryOperator>(I)->swapOperands(); |
| if (ConstantInt *CB = dyn_cast<ConstantInt>(I->getOperand(1))) |
| if (CB->getType() == Type::Int1Ty) { |
| if (CB->isOne()) // X & 1 -> X |
| ReplaceUsesOfWith(I, I->getOperand(0), Worklist, L, LPM); |
| else // X & 0 -> 0 |
| ReplaceUsesOfWith(I, I->getOperand(1), Worklist, L, LPM); |
| continue; |
| } |
| break; |
| case Instruction::Or: |
| if (isa<ConstantInt>(I->getOperand(0)) && |
| I->getOperand(0)->getType() == Type::Int1Ty) // constant -> RHS |
| cast<BinaryOperator>(I)->swapOperands(); |
| if (ConstantInt *CB = dyn_cast<ConstantInt>(I->getOperand(1))) |
| if (CB->getType() == Type::Int1Ty) { |
| if (CB->isOne()) // X | 1 -> 1 |
| ReplaceUsesOfWith(I, I->getOperand(1), Worklist, L, LPM); |
| else // X | 0 -> X |
| ReplaceUsesOfWith(I, I->getOperand(0), Worklist, L, LPM); |
| continue; |
| } |
| break; |
| case Instruction::Br: { |
| BranchInst *BI = cast<BranchInst>(I); |
| if (BI->isUnconditional()) { |
| // If BI's parent is the only pred of the successor, fold the two blocks |
| // together. |
| BasicBlock *Pred = BI->getParent(); |
| BasicBlock *Succ = BI->getSuccessor(0); |
| BasicBlock *SinglePred = Succ->getSinglePredecessor(); |
| if (!SinglePred) continue; // Nothing to do. |
| assert(SinglePred == Pred && "CFG broken"); |
| |
| DOUT << "Merging blocks: " << Pred->getName() << " <- " |
| << Succ->getName() << "\n"; |
| |
| // Resolve any single entry PHI nodes in Succ. |
| while (PHINode *PN = dyn_cast<PHINode>(Succ->begin())) |
| ReplaceUsesOfWith(PN, PN->getIncomingValue(0), Worklist, L, LPM); |
| |
| // Move all of the successor contents from Succ to Pred. |
| Pred->getInstList().splice(BI, Succ->getInstList(), Succ->begin(), |
| Succ->end()); |
| LPM->deleteSimpleAnalysisValue(BI, L); |
| BI->eraseFromParent(); |
| RemoveFromWorklist(BI, Worklist); |
| |
| // If Succ has any successors with PHI nodes, update them to have |
| // entries coming from Pred instead of Succ. |
| Succ->replaceAllUsesWith(Pred); |
| |
| // Remove Succ from the loop tree. |
| LI->removeBlock(Succ); |
| LPM->deleteSimpleAnalysisValue(Succ, L); |
| Succ->eraseFromParent(); |
| ++NumSimplify; |
| } else if (ConstantInt *CB = dyn_cast<ConstantInt>(BI->getCondition())){ |
| // Conditional branch. Turn it into an unconditional branch, then |
| // remove dead blocks. |
| break; // FIXME: Enable. |
| |
| DOUT << "Folded branch: " << *BI; |
| BasicBlock *DeadSucc = BI->getSuccessor(CB->getZExtValue()); |
| BasicBlock *LiveSucc = BI->getSuccessor(!CB->getZExtValue()); |
| DeadSucc->removePredecessor(BI->getParent(), true); |
| Worklist.push_back(BranchInst::Create(LiveSucc, BI)); |
| LPM->deleteSimpleAnalysisValue(BI, L); |
| BI->eraseFromParent(); |
| RemoveFromWorklist(BI, Worklist); |
| ++NumSimplify; |
| |
| RemoveBlockIfDead(DeadSucc, Worklist, L); |
| } |
| break; |
| } |
| } |
| } |
| } |