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Diffstat (limited to 'lib/Transforms/Scalar/JumpThreading.cpp')
| -rw-r--r-- | lib/Transforms/Scalar/JumpThreading.cpp | 954 |
1 files changed, 954 insertions, 0 deletions
diff --git a/lib/Transforms/Scalar/JumpThreading.cpp b/lib/Transforms/Scalar/JumpThreading.cpp new file mode 100644 index 0000000..c0ca2df --- /dev/null +++ b/lib/Transforms/Scalar/JumpThreading.cpp @@ -0,0 +1,954 @@ +//===- 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/Analysis/ConstantFolding.h" +#include "llvm/Transforms/Utils/BasicBlockUtils.h" +#include "llvm/Transforms/Utils/Local.h" +#include "llvm/Target/TargetData.h" +#include "llvm/ADT/DenseMap.h" +#include "llvm/ADT/Statistic.h" +#include "llvm/ADT/STLExtras.h" +#include "llvm/ADT/SmallPtrSet.h" +#include "llvm/ADT/SmallSet.h" +#include "llvm/Support/CommandLine.h" +#include "llvm/Support/Compiler.h" +#include "llvm/Support/Debug.h" +#include "llvm/Support/ValueHandle.h" +using namespace llvm; + +STATISTIC(NumThreads, "Number of jumps threaded"); +STATISTIC(NumFolds, "Number of terminators folded"); + +static cl::opt<unsigned> +Threshold("jump-threading-threshold", + cl::desc("Max block size to duplicate for jump threading"), + cl::init(6), cl::Hidden); + +namespace { + /// This pass performs 'jump threading', which looks at blocks that have + /// multiple predecessors and multiple successors. If one or more of the + /// predecessors of the block can be proven to always jump to one of the + /// successors, we forward the edge from the predecessor to the successor by + /// duplicating the contents of this block. + /// + /// An example of when this can occur is code like this: + /// + /// if () { ... + /// X = 4; + /// } + /// if (X < 3) { + /// + /// In this case, the unconditional branch at the end of the first if can be + /// revectored to the false side of the second if. + /// + class VISIBILITY_HIDDEN JumpThreading : public FunctionPass { + TargetData *TD; +#ifdef NDEBUG + SmallPtrSet<BasicBlock*, 16> LoopHeaders; +#else + SmallSet<AssertingVH<BasicBlock>, 16> LoopHeaders; +#endif + public: + static char ID; // Pass identification + JumpThreading() : FunctionPass(&ID) {} + + virtual void getAnalysisUsage(AnalysisUsage &AU) const { + AU.addRequired<TargetData>(); + } + + bool runOnFunction(Function &F); + void FindLoopHeaders(Function &F); + + bool ProcessBlock(BasicBlock *BB); + bool ThreadEdge(BasicBlock *BB, BasicBlock *PredBB, BasicBlock *SuccBB, + unsigned JumpThreadCost); + BasicBlock *FactorCommonPHIPreds(PHINode *PN, Constant *CstVal); + bool ProcessBranchOnDuplicateCond(BasicBlock *PredBB, BasicBlock *DestBB); + bool ProcessSwitchOnDuplicateCond(BasicBlock *PredBB, BasicBlock *DestBB); + + bool ProcessJumpOnPHI(PHINode *PN); + bool ProcessBranchOnLogical(Value *V, BasicBlock *BB, bool isAnd); + bool ProcessBranchOnCompare(CmpInst *Cmp, BasicBlock *BB); + + bool SimplifyPartiallyRedundantLoad(LoadInst *LI); + }; +} + +char JumpThreading::ID = 0; +static RegisterPass<JumpThreading> +X("jump-threading", "Jump Threading"); + +// Public interface to the Jump Threading pass +FunctionPass *llvm::createJumpThreadingPass() { return new JumpThreading(); } + +/// runOnFunction - Top level algorithm. +/// +bool JumpThreading::runOnFunction(Function &F) { + DOUT << "Jump threading on function '" << F.getNameStart() << "'\n"; + TD = &getAnalysis<TargetData>(); + + FindLoopHeaders(F); + + bool AnotherIteration = true, EverChanged = false; + while (AnotherIteration) { + AnotherIteration = false; + bool Changed = false; + for (Function::iterator I = F.begin(), E = F.end(); I != E;) { + BasicBlock *BB = I; + while (ProcessBlock(BB)) + Changed = true; + + ++I; + + // If the block is trivially dead, zap it. This eliminates the successor + // edges which simplifies the CFG. + if (pred_begin(BB) == pred_end(BB) && + BB != &BB->getParent()->getEntryBlock()) { + DOUT << " JT: Deleting dead block '" << BB->getNameStart() + << "' with terminator: " << *BB->getTerminator(); + LoopHeaders.erase(BB); + DeleteDeadBlock(BB); + Changed = true; + } + } + AnotherIteration = Changed; + EverChanged |= Changed; + } + + LoopHeaders.clear(); + return EverChanged; +} + +/// FindLoopHeaders - We do not want jump threading to turn proper loop +/// structures into irreducible loops. Doing this breaks up the loop nesting +/// hierarchy and pessimizes later transformations. To prevent this from +/// happening, we first have to find the loop headers. Here we approximate this +/// by finding targets of backedges in the CFG. +/// +/// Note that there definitely are cases when we want to allow threading of +/// edges across a loop header. For example, threading a jump from outside the +/// loop (the preheader) to an exit block of the loop is definitely profitable. +/// It is also almost always profitable to thread backedges from within the loop +/// to exit blocks, and is often profitable to thread backedges to other blocks +/// within the loop (forming a nested loop). This simple analysis is not rich +/// enough to track all of these properties and keep it up-to-date as the CFG +/// mutates, so we don't allow any of these transformations. +/// +void JumpThreading::FindLoopHeaders(Function &F) { + SmallVector<std::pair<const BasicBlock*,const BasicBlock*>, 32> Edges; + FindFunctionBackedges(F, Edges); + + for (unsigned i = 0, e = Edges.size(); i != e; ++i) + LoopHeaders.insert(const_cast<BasicBlock*>(Edges[i].second)); +} + + +/// FactorCommonPHIPreds - If there are multiple preds with the same incoming +/// value for the PHI, factor them together so we get one block to thread for +/// the whole group. +/// This is important for things like "phi i1 [true, true, false, true, x]" +/// where we only need to clone the block for the true blocks once. +/// +BasicBlock *JumpThreading::FactorCommonPHIPreds(PHINode *PN, Constant *CstVal) { + SmallVector<BasicBlock*, 16> CommonPreds; + for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) + if (PN->getIncomingValue(i) == CstVal) + CommonPreds.push_back(PN->getIncomingBlock(i)); + + if (CommonPreds.size() == 1) + return CommonPreds[0]; + + DOUT << " Factoring out " << CommonPreds.size() + << " common predecessors.\n"; + return SplitBlockPredecessors(PN->getParent(), + &CommonPreds[0], CommonPreds.size(), + ".thr_comm", this); +} + + +/// getJumpThreadDuplicationCost - Return the cost of duplicating this block to +/// thread across it. +static unsigned getJumpThreadDuplicationCost(const BasicBlock *BB) { + /// Ignore PHI nodes, these will be flattened when duplication happens. + BasicBlock::const_iterator I = BB->getFirstNonPHI(); + + // Sum up the cost of each instruction until we get to the terminator. Don't + // include the terminator because the copy won't include it. + unsigned Size = 0; + for (; !isa<TerminatorInst>(I); ++I) { + // Debugger intrinsics don't incur code size. + if (isa<DbgInfoIntrinsic>(I)) continue; + + // If this is a pointer->pointer bitcast, it is free. + if (isa<BitCastInst>(I) && isa<PointerType>(I->getType())) + continue; + + // All other instructions count for at least one unit. + ++Size; + + // Calls are more expensive. If they are non-intrinsic calls, we model them + // as having cost of 4. If they are a non-vector intrinsic, we model them + // as having cost of 2 total, and if they are a vector intrinsic, we model + // them as having cost 1. + if (const CallInst *CI = dyn_cast<CallInst>(I)) { + if (!isa<IntrinsicInst>(CI)) + Size += 3; + else if (isa<VectorType>(CI->getType())) + Size += 1; + } + } + + // Threading through a switch statement is particularly profitable. If this + // block ends in a switch, decrease its cost to make it more likely to happen. + if (isa<SwitchInst>(I)) + Size = Size > 6 ? Size-6 : 0; + + return Size; +} + +/// ProcessBlock - If there are any predecessors whose control can be threaded +/// through to a successor, transform them now. +bool JumpThreading::ProcessBlock(BasicBlock *BB) { + // If this block has a single predecessor, and if that pred has a single + // successor, merge the blocks. This encourages recursive jump threading + // because now the condition in this block can be threaded through + // predecessors of our predecessor block. + if (BasicBlock *SinglePred = BB->getSinglePredecessor()) + if (SinglePred->getTerminator()->getNumSuccessors() == 1 && + SinglePred != BB) { + // If SinglePred was a loop header, BB becomes one. + if (LoopHeaders.erase(SinglePred)) + LoopHeaders.insert(BB); + + // Remember if SinglePred was the entry block of the function. If so, we + // will need to move BB back to the entry position. + bool isEntry = SinglePred == &SinglePred->getParent()->getEntryBlock(); + MergeBasicBlockIntoOnlyPred(BB); + + if (isEntry && BB != &BB->getParent()->getEntryBlock()) + BB->moveBefore(&BB->getParent()->getEntryBlock()); + return true; + } + + // See if this block ends with a branch or switch. If so, see if the + // condition is a phi node. If so, and if an entry of the phi node is a + // constant, we can thread the block. + Value *Condition; + if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) { + // Can't thread an unconditional jump. + if (BI->isUnconditional()) return false; + Condition = BI->getCondition(); + } else if (SwitchInst *SI = dyn_cast<SwitchInst>(BB->getTerminator())) + Condition = SI->getCondition(); + else + return false; // Must be an invoke. + + // If the terminator of this block is branching on a constant, simplify the + // terminator to an unconditional branch. This can occur due to threading in + // other blocks. + if (isa<ConstantInt>(Condition)) { + DOUT << " In block '" << BB->getNameStart() + << "' folding terminator: " << *BB->getTerminator(); + ++NumFolds; + ConstantFoldTerminator(BB); + return true; + } + + // If the terminator is branching on an undef, we can pick any of the + // successors to branch to. Since this is arbitrary, we pick the successor + // with the fewest predecessors. This should reduce the in-degree of the + // others. + if (isa<UndefValue>(Condition)) { + TerminatorInst *BBTerm = BB->getTerminator(); + unsigned MinSucc = 0; + BasicBlock *TestBB = BBTerm->getSuccessor(MinSucc); + // Compute the successor with the minimum number of predecessors. + unsigned MinNumPreds = std::distance(pred_begin(TestBB), pred_end(TestBB)); + for (unsigned i = 1, e = BBTerm->getNumSuccessors(); i != e; ++i) { + TestBB = BBTerm->getSuccessor(i); + unsigned NumPreds = std::distance(pred_begin(TestBB), pred_end(TestBB)); + if (NumPreds < MinNumPreds) + MinSucc = i; + } + + // Fold the branch/switch. + for (unsigned i = 0, e = BBTerm->getNumSuccessors(); i != e; ++i) { + if (i == MinSucc) continue; + BBTerm->getSuccessor(i)->removePredecessor(BB); + } + + DOUT << " In block '" << BB->getNameStart() + << "' folding undef terminator: " << *BBTerm; + BranchInst::Create(BBTerm->getSuccessor(MinSucc), BBTerm); + BBTerm->eraseFromParent(); + return true; + } + + Instruction *CondInst = dyn_cast<Instruction>(Condition); + + // If the condition is an instruction defined in another block, see if a + // predecessor has the same condition: + // br COND, BBX, BBY + // BBX: + // br COND, BBZ, BBW + if (!Condition->hasOneUse() && // Multiple uses. + (CondInst == 0 || CondInst->getParent() != BB)) { // Non-local definition. + pred_iterator PI = pred_begin(BB), E = pred_end(BB); + if (isa<BranchInst>(BB->getTerminator())) { + for (; PI != E; ++PI) + if (BranchInst *PBI = dyn_cast<BranchInst>((*PI)->getTerminator())) + if (PBI->isConditional() && PBI->getCondition() == Condition && + ProcessBranchOnDuplicateCond(*PI, BB)) + return true; + } else { + assert(isa<SwitchInst>(BB->getTerminator()) && "Unknown jump terminator"); + for (; PI != E; ++PI) + if (SwitchInst *PSI = dyn_cast<SwitchInst>((*PI)->getTerminator())) + if (PSI->getCondition() == Condition && + ProcessSwitchOnDuplicateCond(*PI, BB)) + return true; + } + } + + // If there is only a single predecessor of this block, nothing to fold. + if (BB->getSinglePredecessor()) + return false; + + // All the rest of our checks depend on the condition being an instruction. + if (CondInst == 0) + return false; + + // See if this is a phi node in the current block. + if (PHINode *PN = dyn_cast<PHINode>(CondInst)) + if (PN->getParent() == BB) + return ProcessJumpOnPHI(PN); + + // If this is a conditional branch whose condition is and/or of a phi, try to + // simplify it. + if ((CondInst->getOpcode() == Instruction::And || + CondInst->getOpcode() == Instruction::Or) && + isa<BranchInst>(BB->getTerminator()) && + ProcessBranchOnLogical(CondInst, BB, + CondInst->getOpcode() == Instruction::And)) + return true; + + // If we have "br (phi != 42)" and the phi node has any constant values as + // operands, we can thread through this block. + if (CmpInst *CondCmp = dyn_cast<CmpInst>(CondInst)) + if (isa<PHINode>(CondCmp->getOperand(0)) && + isa<Constant>(CondCmp->getOperand(1)) && + ProcessBranchOnCompare(CondCmp, BB)) + return true; + + // Check for some cases that are worth simplifying. Right now we want to look + // for loads that are used by a switch or by the condition for the branch. If + // we see one, check to see if it's partially redundant. If so, insert a PHI + // which can then be used to thread the values. + // + // This is particularly important because reg2mem inserts loads and stores all + // over the place, and this blocks jump threading if we don't zap them. + Value *SimplifyValue = CondInst; + if (CmpInst *CondCmp = dyn_cast<CmpInst>(SimplifyValue)) + if (isa<Constant>(CondCmp->getOperand(1))) + SimplifyValue = CondCmp->getOperand(0); + + if (LoadInst *LI = dyn_cast<LoadInst>(SimplifyValue)) + if (SimplifyPartiallyRedundantLoad(LI)) + return true; + + // TODO: If we have: "br (X > 0)" and we have a predecessor where we know + // "(X == 4)" thread through this block. + + return false; +} + +/// ProcessBranchOnDuplicateCond - We found a block and a predecessor of that +/// block that jump on exactly the same condition. This means that we almost +/// always know the direction of the edge in the DESTBB: +/// PREDBB: +/// br COND, DESTBB, BBY +/// DESTBB: +/// br COND, BBZ, BBW +/// +/// If DESTBB has multiple predecessors, we can't just constant fold the branch +/// in DESTBB, we have to thread over it. +bool JumpThreading::ProcessBranchOnDuplicateCond(BasicBlock *PredBB, + BasicBlock *BB) { + BranchInst *PredBI = cast<BranchInst>(PredBB->getTerminator()); + + // If both successors of PredBB go to DESTBB, we don't know anything. We can + // fold the branch to an unconditional one, which allows other recursive + // simplifications. + bool BranchDir; + if (PredBI->getSuccessor(1) != BB) + BranchDir = true; + else if (PredBI->getSuccessor(0) != BB) + BranchDir = false; + else { + DOUT << " In block '" << PredBB->getNameStart() + << "' folding terminator: " << *PredBB->getTerminator(); + ++NumFolds; + ConstantFoldTerminator(PredBB); + return true; + } + + BranchInst *DestBI = cast<BranchInst>(BB->getTerminator()); + + // If the dest block has one predecessor, just fix the branch condition to a + // constant and fold it. + if (BB->getSinglePredecessor()) { + DOUT << " In block '" << BB->getNameStart() + << "' folding condition to '" << BranchDir << "': " + << *BB->getTerminator(); + ++NumFolds; + DestBI->setCondition(ConstantInt::get(Type::Int1Ty, BranchDir)); + ConstantFoldTerminator(BB); + return true; + } + + // Otherwise we need to thread from PredBB to DestBB's successor which + // involves code duplication. Check to see if it is worth it. + unsigned JumpThreadCost = getJumpThreadDuplicationCost(BB); + if (JumpThreadCost > Threshold) { + DOUT << " Not threading BB '" << BB->getNameStart() + << "' - Cost is too high: " << JumpThreadCost << "\n"; + return false; + } + + // Next, figure out which successor we are threading to. + BasicBlock *SuccBB = DestBI->getSuccessor(!BranchDir); + + // Ok, try to thread it! + return ThreadEdge(BB, PredBB, SuccBB, JumpThreadCost); +} + +/// ProcessSwitchOnDuplicateCond - We found a block and a predecessor of that +/// block that switch on exactly the same condition. This means that we almost +/// always know the direction of the edge in the DESTBB: +/// PREDBB: +/// switch COND [... DESTBB, BBY ... ] +/// DESTBB: +/// switch COND [... BBZ, BBW ] +/// +/// Optimizing switches like this is very important, because simplifycfg builds +/// switches out of repeated 'if' conditions. +bool JumpThreading::ProcessSwitchOnDuplicateCond(BasicBlock *PredBB, + BasicBlock *DestBB) { + // Can't thread edge to self. + if (PredBB == DestBB) + return false; + + + SwitchInst *PredSI = cast<SwitchInst>(PredBB->getTerminator()); + SwitchInst *DestSI = cast<SwitchInst>(DestBB->getTerminator()); + + // There are a variety of optimizations that we can potentially do on these + // blocks: we order them from most to least preferable. + + // If DESTBB *just* contains the switch, then we can forward edges from PREDBB + // directly to their destination. This does not introduce *any* code size + // growth. Skip debug info first. + BasicBlock::iterator BBI = DestBB->begin(); + while (isa<DbgInfoIntrinsic>(BBI)) + BBI++; + + // FIXME: Thread if it just contains a PHI. + if (isa<SwitchInst>(BBI)) { + bool MadeChange = false; + // Ignore the default edge for now. + for (unsigned i = 1, e = DestSI->getNumSuccessors(); i != e; ++i) { + ConstantInt *DestVal = DestSI->getCaseValue(i); + BasicBlock *DestSucc = DestSI->getSuccessor(i); + + // Okay, DestSI has a case for 'DestVal' that goes to 'DestSucc'. See if + // PredSI has an explicit case for it. If so, forward. If it is covered + // by the default case, we can't update PredSI. + unsigned PredCase = PredSI->findCaseValue(DestVal); + if (PredCase == 0) continue; + + // If PredSI doesn't go to DestBB on this value, then it won't reach the + // case on this condition. + if (PredSI->getSuccessor(PredCase) != DestBB && + DestSI->getSuccessor(i) != DestBB) + continue; + + // Otherwise, we're safe to make the change. Make sure that the edge from + // DestSI to DestSucc is not critical and has no PHI nodes. + DOUT << "FORWARDING EDGE " << *DestVal << " FROM: " << *PredSI; + DOUT << "THROUGH: " << *DestSI; + + // If the destination has PHI nodes, just split the edge for updating + // simplicity. + if (isa<PHINode>(DestSucc->begin()) && !DestSucc->getSinglePredecessor()){ + SplitCriticalEdge(DestSI, i, this); + DestSucc = DestSI->getSuccessor(i); + } + FoldSingleEntryPHINodes(DestSucc); + PredSI->setSuccessor(PredCase, DestSucc); + MadeChange = true; + } + + if (MadeChange) + return true; + } + + return false; +} + + +/// SimplifyPartiallyRedundantLoad - If LI is an obviously partially redundant +/// load instruction, eliminate it by replacing it with a PHI node. This is an +/// important optimization that encourages jump threading, and needs to be run +/// interlaced with other jump threading tasks. +bool JumpThreading::SimplifyPartiallyRedundantLoad(LoadInst *LI) { + // Don't hack volatile loads. + if (LI->isVolatile()) return false; + + // If the load is defined in a block with exactly one predecessor, it can't be + // partially redundant. + BasicBlock *LoadBB = LI->getParent(); + if (LoadBB->getSinglePredecessor()) + return false; + + Value *LoadedPtr = LI->getOperand(0); + + // If the loaded operand is defined in the LoadBB, it can't be available. + // FIXME: Could do PHI translation, that would be fun :) + if (Instruction *PtrOp = dyn_cast<Instruction>(LoadedPtr)) + if (PtrOp->getParent() == LoadBB) + return false; + + // Scan a few instructions up from the load, to see if it is obviously live at + // the entry to its block. + BasicBlock::iterator BBIt = LI; + + if (Value *AvailableVal = FindAvailableLoadedValue(LoadedPtr, LoadBB, + BBIt, 6)) { + // If the value if the load is locally available within the block, just use + // it. This frequently occurs for reg2mem'd allocas. + //cerr << "LOAD ELIMINATED:\n" << *BBIt << *LI << "\n"; + + // If the returned value is the load itself, replace with an undef. This can + // only happen in dead loops. + if (AvailableVal == LI) AvailableVal = UndefValue::get(LI->getType()); + LI->replaceAllUsesWith(AvailableVal); + LI->eraseFromParent(); + return true; + } + + // Otherwise, if we scanned the whole block and got to the top of the block, + // we know the block is locally transparent to the load. If not, something + // might clobber its value. + if (BBIt != LoadBB->begin()) + return false; + + + SmallPtrSet<BasicBlock*, 8> PredsScanned; + typedef SmallVector<std::pair<BasicBlock*, Value*>, 8> AvailablePredsTy; + AvailablePredsTy AvailablePreds; + BasicBlock *OneUnavailablePred = 0; + + // If we got here, the loaded value is transparent through to the start of the + // block. Check to see if it is available in any of the predecessor blocks. + for (pred_iterator PI = pred_begin(LoadBB), PE = pred_end(LoadBB); + PI != PE; ++PI) { + BasicBlock *PredBB = *PI; + + // If we already scanned this predecessor, skip it. + if (!PredsScanned.insert(PredBB)) + continue; + + // Scan the predecessor to see if the value is available in the pred. + BBIt = PredBB->end(); + Value *PredAvailable = FindAvailableLoadedValue(LoadedPtr, PredBB, BBIt, 6); + if (!PredAvailable) { + OneUnavailablePred = PredBB; + continue; + } + + // If so, this load is partially redundant. Remember this info so that we + // can create a PHI node. + AvailablePreds.push_back(std::make_pair(PredBB, PredAvailable)); + } + + // If the loaded value isn't available in any predecessor, it isn't partially + // redundant. + if (AvailablePreds.empty()) return false; + + // Okay, the loaded value is available in at least one (and maybe all!) + // predecessors. If the value is unavailable in more than one unique + // predecessor, we want to insert a merge block for those common predecessors. + // This ensures that we only have to insert one reload, thus not increasing + // code size. + BasicBlock *UnavailablePred = 0; + + // If there is exactly one predecessor where the value is unavailable, the + // already computed 'OneUnavailablePred' block is it. If it ends in an + // unconditional branch, we know that it isn't a critical edge. + if (PredsScanned.size() == AvailablePreds.size()+1 && + OneUnavailablePred->getTerminator()->getNumSuccessors() == 1) { + UnavailablePred = OneUnavailablePred; + } else if (PredsScanned.size() != AvailablePreds.size()) { + // Otherwise, we had multiple unavailable predecessors or we had a critical + // edge from the one. + SmallVector<BasicBlock*, 8> PredsToSplit; + SmallPtrSet<BasicBlock*, 8> AvailablePredSet; + + for (unsigned i = 0, e = AvailablePreds.size(); i != e; ++i) + AvailablePredSet.insert(AvailablePreds[i].first); + + // Add all the unavailable predecessors to the PredsToSplit list. + for (pred_iterator PI = pred_begin(LoadBB), PE = pred_end(LoadBB); + PI != PE; ++PI) + if (!AvailablePredSet.count(*PI)) + PredsToSplit.push_back(*PI); + + // Split them out to their own block. + UnavailablePred = + SplitBlockPredecessors(LoadBB, &PredsToSplit[0], PredsToSplit.size(), + "thread-split", this); + } + + // If the value isn't available in all predecessors, then there will be + // exactly one where it isn't available. Insert a load on that edge and add + // it to the AvailablePreds list. + if (UnavailablePred) { + assert(UnavailablePred->getTerminator()->getNumSuccessors() == 1 && + "Can't handle critical edge here!"); + Value *NewVal = new LoadInst(LoadedPtr, LI->getName()+".pr", + UnavailablePred->getTerminator()); + AvailablePreds.push_back(std::make_pair(UnavailablePred, NewVal)); + } + + // Now we know that each predecessor of this block has a value in + // AvailablePreds, sort them for efficient access as we're walking the preds. + array_pod_sort(AvailablePreds.begin(), AvailablePreds.end()); + + // Create a PHI node at the start of the block for the PRE'd load value. + PHINode *PN = PHINode::Create(LI->getType(), "", LoadBB->begin()); + PN->takeName(LI); + + // Insert new entries into the PHI for each predecessor. A single block may + // have multiple entries here. + for (pred_iterator PI = pred_begin(LoadBB), E = pred_end(LoadBB); PI != E; + ++PI) { + AvailablePredsTy::iterator I = + std::lower_bound(AvailablePreds.begin(), AvailablePreds.end(), + std::make_pair(*PI, (Value*)0)); + + assert(I != AvailablePreds.end() && I->first == *PI && + "Didn't find entry for predecessor!"); + + PN->addIncoming(I->second, I->first); + } + + //cerr << "PRE: " << *LI << *PN << "\n"; + + LI->replaceAllUsesWith(PN); + LI->eraseFromParent(); + + return true; +} + + +/// ProcessJumpOnPHI - We have a conditional branch of switch on a PHI node in +/// the current block. See if there are any simplifications we can do based on +/// inputs to the phi node. +/// +bool JumpThreading::ProcessJumpOnPHI(PHINode *PN) { + // See if the phi node has any constant values. If so, we can determine where + // the corresponding predecessor will branch. + ConstantInt *PredCst = 0; + for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) + if ((PredCst = dyn_cast<ConstantInt>(PN->getIncomingValue(i)))) + break; + + // If no incoming value has a constant, we don't know the destination of any + // predecessors. + if (PredCst == 0) + return false; + + // See if the cost of duplicating this block is low enough. + BasicBlock *BB = PN->getParent(); + unsigned JumpThreadCost = getJumpThreadDuplicationCost(BB); + if (JumpThreadCost > Threshold) { + DOUT << " Not threading BB '" << BB->getNameStart() + << "' - Cost is too high: " << JumpThreadCost << "\n"; + return false; + } + + // If so, we can actually do this threading. Merge any common predecessors + // that will act the same. + BasicBlock *PredBB = FactorCommonPHIPreds(PN, PredCst); + + // Next, figure out which successor we are threading to. + BasicBlock *SuccBB; + if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) + SuccBB = BI->getSuccessor(PredCst == ConstantInt::getFalse()); + else { + SwitchInst *SI = cast<SwitchInst>(BB->getTerminator()); + SuccBB = SI->getSuccessor(SI->findCaseValue(PredCst)); + } + + // Ok, try to thread it! + return ThreadEdge(BB, PredBB, SuccBB, JumpThreadCost); +} + +/// 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); + + // Ok, try to thread it! + return ThreadEdge(BB, PredBB, SuccBB, JumpThreadCost); +} + +/// 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); + + // Ok, try to thread it! + return ThreadEdge(BB, PredBB, SuccBB, JumpThreadCost); +} + + +/// 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. +bool JumpThreading::ThreadEdge(BasicBlock *BB, BasicBlock *PredBB, + BasicBlock *SuccBB, unsigned JumpThreadCost) { + + // If threading to the same block as we come from, we would infinite loop. + if (SuccBB == BB) { + DOUT << " Not threading across BB '" << BB->getNameStart() + << "' - would thread to self!\n"; + return false; + } + + // If threading this would thread across a loop header, don't thread the edge. + // See the comments above FindLoopHeaders for justifications and caveats. + if (LoopHeaders.count(BB)) { + DOUT << " Not threading from '" << PredBB->getNameStart() + << "' across loop header BB '" << BB->getNameStart() + << "' to dest BB '" << SuccBB->getNameStart() + << "' - it might create an irreducible loop!\n"; + return false; + } + + // And finally, do it! + DOUT << " Threading edge from '" << PredBB->getNameStart() << "' to '" + << SuccBB->getNameStart() << "' with cost: " << JumpThreadCost + << ", across block:\n " + << *BB << "\n"; + + // Jump Threading can not update SSA properties correctly if the values + // defined in the duplicated block are used outside of the block itself. For + // this reason, we spill all values that are used outside of BB to the stack. + for (BasicBlock::iterator I = BB->begin(); I != BB->end(); ++I) { + if (!I->isUsedOutsideOfBlock(BB)) + continue; + + // We found a use of I outside of BB. Create a new stack slot to + // break this inter-block usage pattern. + DemoteRegToStack(*I); + } + + // We are going to have to map operands from the original BB block to the new + // copy of the block 'NewBB'. If there are PHI nodes in BB, evaluate them to + // account for entry from PredBB. + DenseMap<Instruction*, Value*> ValueMapping; + + BasicBlock *NewBB = + BasicBlock::Create(BB->getName()+".thread", BB->getParent(), BB); + NewBB->moveAfter(PredBB); + + BasicBlock::iterator BI = BB->begin(); + for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI) + ValueMapping[PN] = PN->getIncomingValueForBlock(PredBB); + + // Clone the non-phi instructions of BB into NewBB, keeping track of the + // mapping and using it to remap operands in the cloned instructions. + for (; !isa<TerminatorInst>(BI); ++BI) { + Instruction *New = BI->clone(); + New->setName(BI->getNameStart()); + NewBB->getInstList().push_back(New); + ValueMapping[BI] = New; + + // Remap operands to patch up intra-block references. + for (unsigned i = 0, e = New->getNumOperands(); i != e; ++i) + if (Instruction *Inst = dyn_cast<Instruction>(New->getOperand(i))) + if (Value *Remapped = ValueMapping[Inst]) + New->setOperand(i, Remapped); + } + + // We didn't copy the terminator from BB over to NewBB, because there is now + // an unconditional jump to SuccBB. Insert the unconditional jump. + BranchInst::Create(SuccBB, NewBB); + + // Check to see if SuccBB has PHI nodes. If so, we need to add entries to the + // PHI nodes for NewBB now. + for (BasicBlock::iterator PNI = SuccBB->begin(); isa<PHINode>(PNI); ++PNI) { + PHINode *PN = cast<PHINode>(PNI); + // Ok, we have a PHI node. Figure out what the incoming value was for the + // DestBlock. + Value *IV = PN->getIncomingValueForBlock(BB); + + // Remap the value if necessary. + if (Instruction *Inst = dyn_cast<Instruction>(IV)) + if (Value *MappedIV = ValueMapping[Inst]) + IV = MappedIV; + PN->addIncoming(IV, NewBB); + } + + // Ok, NewBB is good to go. Update the terminator of PredBB to jump to + // NewBB instead of BB. This eliminates predecessors from BB, which requires + // us to simplify any PHI nodes in BB. + TerminatorInst *PredTerm = PredBB->getTerminator(); + for (unsigned i = 0, e = PredTerm->getNumSuccessors(); i != e; ++i) + if (PredTerm->getSuccessor(i) == BB) { + BB->removePredecessor(PredBB); + PredTerm->setSuccessor(i, NewBB); + } + + // At this point, the IR is fully up to date and consistent. Do a quick scan + // over the new instructions and zap any that are constants or dead. This + // frequently happens because of phi translation. + BI = NewBB->begin(); + for (BasicBlock::iterator E = NewBB->end(); BI != E; ) { + Instruction *Inst = BI++; + if (Constant *C = ConstantFoldInstruction(Inst, TD)) { + Inst->replaceAllUsesWith(C); + Inst->eraseFromParent(); + continue; + } + + RecursivelyDeleteTriviallyDeadInstructions(Inst); + } + + // Threaded an edge! + ++NumThreads; + return true; +} |
