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Diffstat (limited to 'contrib/llvm/lib/Transforms/Scalar/LoopUnswitch.cpp')
| -rw-r--r-- | contrib/llvm/lib/Transforms/Scalar/LoopUnswitch.cpp | 1057 |
1 files changed, 1057 insertions, 0 deletions
diff --git a/contrib/llvm/lib/Transforms/Scalar/LoopUnswitch.cpp b/contrib/llvm/lib/Transforms/Scalar/LoopUnswitch.cpp new file mode 100644 index 0000000..ae7bf40 --- /dev/null +++ b/contrib/llvm/lib/Transforms/Scalar/LoopUnswitch.cpp @@ -0,0 +1,1057 @@ +//===-- 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/InlineCost.h" +#include "llvm/Analysis/InstructionSimplify.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/Debug.h" +#include "llvm/Support/raw_ostream.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"); + +// The specific value of 50 here was chosen based only on intuition and a +// few specific examples. +static cl::opt<unsigned> +Threshold("loop-unswitch-threshold", cl::desc("Max loop size to unswitch"), + cl::init(50), cl::Hidden); + +namespace { + class 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: + + virtual void releaseMemory() { + UnswitchedVals.clear(); + } + + /// 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); + 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) { + // We can never unswitch on vector conditions. + if (Cond->getType()->isVectorTy()) + return 0; + + // Constants should be folded, not unswitched on! + if (isa<Constant>(Cond)) return 0; + + // TODO: Handle: br (VARIANT|INVARIANT). + + // Hoist simple values out. + if (L->makeLoopInvariant(Cond, Changed)) + 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(*DT)); + 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; + LLVMContext &Context = currentLoop->getHeader()->getContext(); + + // 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(Context))) { + ++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(Context))) { + ++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(); + LLVMContext &Context = Header->getContext(); + + 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(Context); + } else if ((LoopExitBB = isTrivialLoopExitBlock(currentLoop, + BI->getSuccessor(1)))) { + if (Val) *Val = ConstantInt::getFalse(Context); + } + } 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; +} + +/// 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(); + + // If LoopSimplify was unable to form a preheader, don't do any unswitching. + if (!loopPreheader) + return false; + + Function *F = loopHeader->getParent(); + + Constant *CondVal = 0; + BasicBlock *ExitBlock = 0; + if (IsTrivialUnswitchCondition(LoopCond, &CondVal, &ExitBlock)) { + // If the condition is trivial, always unswitch. There is no code growth + // for this case. + UnswitchTrivialCondition(currentLoop, LoopCond, CondVal, ExitBlock); + return true; + } + + // Check to see if it would be profitable to unswitch current loop. + + // Do not do non-trivial unswitch while optimizing for size. + if (OptimizeForSize || F->hasFnAttr(Attribute::OptimizeForSize)) + return false; + + // FIXME: This is overly conservative because it does not take into + // consideration code simplification opportunities and code that can + // be shared by the resultant unswitched loops. + CodeMetrics Metrics; + for (Loop::block_iterator I = currentLoop->block_begin(), + E = currentLoop->block_end(); + I != E; ++I) + Metrics.analyzeBasicBlock(*I); + + // Limit the number of instructions to avoid causing significant code + // expansion, and the number of basic blocks, to avoid loops with + // large numbers of branches which cause loop unswitching to go crazy. + // This is a very ad-hoc heuristic. + if (Metrics.NumInsts > Threshold || + Metrics.NumBlocks * 5 > Threshold || + Metrics.NeverInline) { + DEBUG(dbgs() << "NOT unswitching loop %" + << currentLoop->getHeader()->getName() << ", cost too high: " + << currentLoop->getBlocks().size() << "\n"); + return false; + } + + 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::getInt1Ty(LIC->getContext())) + BranchVal = new ICmpInst(InsertPt, ICmpInst::ICMP_EQ, LIC, Val, "tmp"); + else if (Val != ConstantInt::getTrue(Val->getContext())) + // We want to enter the new loop when the condition is true. + std::swap(TrueDest, FalseDest); + + // Insert the new branch. + BranchInst *BI = BranchInst::Create(TrueDest, FalseDest, BranchVal, InsertPt); + + // If either edge is critical, split it. This helps preserve LoopSimplify + // form for enclosing loops. + SplitCriticalEdge(BI, 0, this); + SplitCriticalEdge(BI, 1, this); +} + +/// 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) { + DEBUG(dbgs() << "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]; + SmallVector<BasicBlock *, 4> Preds(pred_begin(ExitBlock), + pred_end(ExitBlock)); + SplitBlockPredecessors(ExitBlock, Preds.data(), Preds.size(), + ".us-lcssa", this); + } +} + +/// 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(); + DEBUG(dbgs() << "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 *NewBB = CloneBasicBlock(LoopBlocks[i], ValueMap, ".us", F); + NewBlocks.push_back(NewBB); + ValueMap[LoopBlocks[i]] = NewBB; // Keep the BB mapping. + LPM->cloneBasicBlockSimpleAnalysis(LoopBlocks[i], NewBB, L); + } + + // Splice the newly inserted blocks into the function right before the + // original preheader. + F->getBasicBlockList().splice(NewPreheader, 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(); isa<PHINode>(I); ++I) { + PN = cast<PHINode>(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; + + // Keep a WeakVH holding onto LIC. If the first call to RewriteLoopBody + // deletes the instruction (for example by simplifying a PHI that feeds into + // the condition that we're unswitching on), we don't rewrite the second + // iteration. + WeakVH LICHandle(LIC); + + // 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 + // changed to another value or a constant. If its a constant, don't simplify + // it. + if (!LoopProcessWorklist.empty() && LoopProcessWorklist.back() == NewLoop && + LICHandle && !isa<Constant>(LICHandle)) + RewriteLoopBodyWithConditionConstant(NewLoop, LICHandle, 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) { + DEBUG(dbgs() << "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(BB->getContext(), 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; + } + + DEBUG(dbgs() << "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 is not void type then replaceAllUsesWith undef. + // This allows ValueHandlers and custom metadata to adjust itself. + if (!I->getType()->isVoidTy()) + 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; + LLVMContext &Context = Val->getContext(); + + + // 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()->isIntegerTy(1))) { + Value *Replacement; + if (IsEqual) + Replacement = Val; + else + Replacement = ConstantInt::get(Type::getInt1Ty(Val->getContext()), + !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)) + continue; + U->replaceUsesOfWith(LIC, Replacement); + Worklist.push_back(U); + } + SimplifyCode(Worklist, L); + return; + } + + // 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) { + Instruction *U = cast<Instruction>(Users[i]); + if (!L->contains(U)) + continue; + + Worklist.push_back(U); + + // TODO: We could do other simplifications, for example, turning + // 'icmp eq LIC, Val' -> false. + + // If we know that LIC is not Val, use this info to simplify code. + SwitchInst *SI = dyn_cast<SwitchInst>(U); + if (SI == 0 || !isa<ConstantInt>(Val)) continue; + + unsigned DeadCase = SI->findCaseValue(cast<ConstantInt>(Val)); + if (DeadCase == 0) continue; // Default case is live for multiple values. + + // 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 a dead code path. + BasicBlock *Switch = SI->getParent(); + SplitEdge(Switch, SI->getSuccessor(DeadCase), this); + // Compute the successors instead of relying on the return value + // of SplitEdge, since it may have split the switch successor + // after PHI nodes. + BasicBlock *NewSISucc = SI->getSuccessor(DeadCase); + BasicBlock *OldSISucc = *succ_begin(NewSISucc); + // Create an "unreachable" destination. + BasicBlock *Abort = BasicBlock::Create(Context, "us-unreachable", + Switch->getParent(), + OldSISucc); + new UnreachableInst(Context, Abort); + // Force the new case destination to branch to the "unreachable" + // block while maintaining a (dead) CFG edge to the old block. + NewSISucc->getTerminator()->eraseFromParent(); + BranchInst::Create(Abort, OldSISucc, + ConstantInt::getTrue(Context), NewSISucc); + // Release the PHI operands for this edge. + for (BasicBlock::iterator II = NewSISucc->begin(); + PHINode *PN = dyn_cast<PHINode>(II); ++II) + PN->setIncomingValue(PN->getBasicBlockIndex(Switch), + UndefValue::get(PN->getType())); + // Tell the domtree about the new block. We don't fully update the + // domtree here -- instead we force it to do a full recomputation + // after the pass is complete -- but we do need to inform it of + // new blocks. + if (DT) + DT->addNewBlock(Abort, NewSISucc); + } + + 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)) { + DEBUG(dbgs() << "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; + } + + // See if instruction simplification can hack this up. This is common for + // things like "select false, X, Y" after unswitching made the condition be + // 'false'. + if (Value *V = SimplifyInstruction(I)) { + ReplaceUsesOfWith(I, V, Worklist, L, LPM); + continue; + } + + // Special case hacks that appear commonly in unswitched code. + if (BranchInst *BI = dyn_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"); + + DEBUG(dbgs() << "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; + continue; + } + + if (ConstantInt *CB = dyn_cast<ConstantInt>(BI->getCondition())){ + // Conditional branch. Turn it into an unconditional branch, then + // remove dead blocks. + continue; // FIXME: Enable. + + DEBUG(dbgs() << "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); + } + continue; + } + } +} |
