| //===-- UnrollLoop.cpp - Loop unrolling utilities -------------------------===// |
| // |
| // The LLVM Compiler Infrastructure |
| // |
| // This file is distributed under the University of Illinois Open Source |
| // License. See LICENSE.TXT for details. |
| // |
| //===----------------------------------------------------------------------===// |
| // |
| // This file implements some loop unrolling utilities. It does not define any |
| // actual pass or policy, but provides a single function to perform loop |
| // unrolling. |
| // |
| // It works best when loops have been canonicalized by the -indvars pass, |
| // allowing it to determine the trip counts of loops easily. |
| // |
| // The process of unrolling can produce extraneous basic blocks linked with |
| // unconditional branches. This will be corrected in the future. |
| //===----------------------------------------------------------------------===// |
| |
| #define DEBUG_TYPE "loop-unroll" |
| #include "llvm/Transforms/Utils/UnrollLoop.h" |
| #include "llvm/BasicBlock.h" |
| #include "llvm/ADT/Statistic.h" |
| #include "llvm/Analysis/ConstantFolding.h" |
| #include "llvm/Analysis/LoopPass.h" |
| #include "llvm/Support/Debug.h" |
| #include "llvm/Support/raw_ostream.h" |
| #include "llvm/Transforms/Utils/BasicBlockUtils.h" |
| #include "llvm/Transforms/Utils/Cloning.h" |
| #include "llvm/Transforms/Utils/Local.h" |
| #include <cstdio> |
| |
| using namespace llvm; |
| |
| // TODO: Should these be here or in LoopUnroll? |
| STATISTIC(NumCompletelyUnrolled, "Number of loops completely unrolled"); |
| STATISTIC(NumUnrolled, "Number of loops unrolled (completely or otherwise)"); |
| |
| /// 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); |
| } |
| } |
| |
| /// FoldBlockIntoPredecessor - Folds a basic block into its predecessor if it |
| /// only has one predecessor, and that predecessor only has one successor. |
| /// The LoopInfo Analysis that is passed will be kept consistent. |
| /// Returns the new combined block. |
| static BasicBlock *FoldBlockIntoPredecessor(BasicBlock *BB, LoopInfo* LI) { |
| // Merge basic blocks into their predecessor if there is only one distinct |
| // pred, and if there is only one distinct successor of the predecessor, and |
| // if there are no PHI nodes. |
| BasicBlock *OnlyPred = BB->getSinglePredecessor(); |
| if (!OnlyPred) return 0; |
| |
| if (OnlyPred->getTerminator()->getNumSuccessors() != 1) |
| return 0; |
| |
| DOUT << "Merging: " << *BB << "into: " << *OnlyPred; |
| |
| // Resolve any PHI nodes at the start of the block. They are all |
| // guaranteed to have exactly one entry if they exist, unless there are |
| // multiple duplicate (but guaranteed to be equal) entries for the |
| // incoming edges. This occurs when there are multiple edges from |
| // OnlyPred to OnlySucc. |
| FoldSingleEntryPHINodes(BB); |
| |
| // Delete the unconditional branch from the predecessor... |
| OnlyPred->getInstList().pop_back(); |
| |
| // Move all definitions in the successor to the predecessor... |
| OnlyPred->getInstList().splice(OnlyPred->end(), BB->getInstList()); |
| |
| // Make all PHI nodes that referred to BB now refer to Pred as their |
| // source... |
| BB->replaceAllUsesWith(OnlyPred); |
| |
| std::string OldName = BB->getName(); |
| |
| // Erase basic block from the function... |
| LI->removeBlock(BB); |
| BB->eraseFromParent(); |
| |
| // Inherit predecessor's name if it exists... |
| if (!OldName.empty() && !OnlyPred->hasName()) |
| OnlyPred->setName(OldName); |
| |
| return OnlyPred; |
| } |
| |
| /// Unroll the given loop by Count. The loop must be in LCSSA form. Returns true |
| /// if unrolling was succesful, or false if the loop was unmodified. Unrolling |
| /// can only fail when the loop's latch block is not terminated by a conditional |
| /// branch instruction. However, if the trip count (and multiple) are not known, |
| /// loop unrolling will mostly produce more code that is no faster. |
| /// |
| /// The LoopInfo Analysis that is passed will be kept consistent. |
| /// |
| /// If a LoopPassManager is passed in, and the loop is fully removed, it will be |
| /// removed from the LoopPassManager as well. LPM can also be NULL. |
| bool llvm::UnrollLoop(Loop *L, unsigned Count, LoopInfo* LI, LPPassManager* LPM) { |
| assert(L->isLCSSAForm()); |
| |
| BasicBlock *Header = L->getHeader(); |
| BasicBlock *LatchBlock = L->getLoopLatch(); |
| BranchInst *BI = dyn_cast<BranchInst>(LatchBlock->getTerminator()); |
| |
| if (!BI || BI->isUnconditional()) { |
| // The loop-rotate pass can be helpful to avoid this in many cases. |
| DOUT << " Can't unroll; loop not terminated by a conditional branch.\n"; |
| return false; |
| } |
| |
| // Find trip count |
| unsigned TripCount = L->getSmallConstantTripCount(); |
| // Find trip multiple if count is not available |
| unsigned TripMultiple = 1; |
| if (TripCount == 0) |
| TripMultiple = L->getSmallConstantTripMultiple(); |
| |
| if (TripCount != 0) |
| DOUT << " Trip Count = " << TripCount << "\n"; |
| if (TripMultiple != 1) |
| DOUT << " Trip Multiple = " << TripMultiple << "\n"; |
| |
| // Effectively "DCE" unrolled iterations that are beyond the tripcount |
| // and will never be executed. |
| if (TripCount != 0 && Count > TripCount) |
| Count = TripCount; |
| |
| assert(Count > 0); |
| assert(TripMultiple > 0); |
| assert(TripCount == 0 || TripCount % TripMultiple == 0); |
| |
| // Are we eliminating the loop control altogether? |
| bool CompletelyUnroll = Count == TripCount; |
| |
| // If we know the trip count, we know the multiple... |
| unsigned BreakoutTrip = 0; |
| if (TripCount != 0) { |
| BreakoutTrip = TripCount % Count; |
| TripMultiple = 0; |
| } else { |
| // Figure out what multiple to use. |
| BreakoutTrip = TripMultiple = |
| (unsigned)GreatestCommonDivisor64(Count, TripMultiple); |
| } |
| |
| if (CompletelyUnroll) { |
| DEBUG(errs() << "COMPLETELY UNROLLING loop %" << Header->getName() |
| << " with trip count " << TripCount << "!\n"); |
| } else { |
| DEBUG(errs() << "UNROLLING loop %" << Header->getName() |
| << " by " << Count); |
| if (TripMultiple == 0 || BreakoutTrip != TripMultiple) { |
| DOUT << " with a breakout at trip " << BreakoutTrip; |
| } else if (TripMultiple != 1) { |
| DOUT << " with " << TripMultiple << " trips per branch"; |
| } |
| DOUT << "!\n"; |
| } |
| |
| std::vector<BasicBlock*> LoopBlocks = L->getBlocks(); |
| |
| bool ContinueOnTrue = L->contains(BI->getSuccessor(0)); |
| BasicBlock *LoopExit = BI->getSuccessor(ContinueOnTrue); |
| |
| // For the first iteration of the loop, we should use the precloned values for |
| // PHI nodes. Insert associations now. |
| typedef DenseMap<const Value*, Value*> ValueMapTy; |
| ValueMapTy LastValueMap; |
| std::vector<PHINode*> OrigPHINode; |
| for (BasicBlock::iterator I = Header->begin(); isa<PHINode>(I); ++I) { |
| PHINode *PN = cast<PHINode>(I); |
| OrigPHINode.push_back(PN); |
| if (Instruction *I = |
| dyn_cast<Instruction>(PN->getIncomingValueForBlock(LatchBlock))) |
| if (L->contains(I->getParent())) |
| LastValueMap[I] = I; |
| } |
| |
| std::vector<BasicBlock*> Headers; |
| std::vector<BasicBlock*> Latches; |
| Headers.push_back(Header); |
| Latches.push_back(LatchBlock); |
| |
| for (unsigned It = 1; It != Count; ++It) { |
| char SuffixBuffer[100]; |
| sprintf(SuffixBuffer, ".%d", It); |
| |
| std::vector<BasicBlock*> NewBlocks; |
| |
| for (std::vector<BasicBlock*>::iterator BB = LoopBlocks.begin(), |
| E = LoopBlocks.end(); BB != E; ++BB) { |
| ValueMapTy ValueMap; |
| BasicBlock *New = CloneBasicBlock(*BB, ValueMap, SuffixBuffer); |
| Header->getParent()->getBasicBlockList().push_back(New); |
| |
| // Loop over all of the PHI nodes in the block, changing them to use the |
| // incoming values from the previous block. |
| if (*BB == Header) |
| for (unsigned i = 0, e = OrigPHINode.size(); i != e; ++i) { |
| PHINode *NewPHI = cast<PHINode>(ValueMap[OrigPHINode[i]]); |
| Value *InVal = NewPHI->getIncomingValueForBlock(LatchBlock); |
| if (Instruction *InValI = dyn_cast<Instruction>(InVal)) |
| if (It > 1 && L->contains(InValI->getParent())) |
| InVal = LastValueMap[InValI]; |
| ValueMap[OrigPHINode[i]] = InVal; |
| New->getInstList().erase(NewPHI); |
| } |
| |
| // Update our running map of newest clones |
| LastValueMap[*BB] = New; |
| for (ValueMapTy::iterator VI = ValueMap.begin(), VE = ValueMap.end(); |
| VI != VE; ++VI) |
| LastValueMap[VI->first] = VI->second; |
| |
| L->addBasicBlockToLoop(New, LI->getBase()); |
| |
| // Add phi entries for newly created values to all exit blocks except |
| // the successor of the latch block. The successor of the exit block will |
| // be updated specially after unrolling all the way. |
| if (*BB != LatchBlock) |
| for (Value::use_iterator UI = (*BB)->use_begin(), UE = (*BB)->use_end(); |
| UI != UE;) { |
| Instruction *UseInst = cast<Instruction>(*UI); |
| ++UI; |
| if (isa<PHINode>(UseInst) && !L->contains(UseInst->getParent())) { |
| PHINode *phi = cast<PHINode>(UseInst); |
| Value *Incoming = phi->getIncomingValueForBlock(*BB); |
| phi->addIncoming(Incoming, New); |
| } |
| } |
| |
| // Keep track of new headers and latches as we create them, so that |
| // we can insert the proper branches later. |
| if (*BB == Header) |
| Headers.push_back(New); |
| if (*BB == LatchBlock) { |
| Latches.push_back(New); |
| |
| // Also, clear out the new latch's back edge so that it doesn't look |
| // like a new loop, so that it's amenable to being merged with adjacent |
| // blocks later on. |
| TerminatorInst *Term = New->getTerminator(); |
| assert(L->contains(Term->getSuccessor(!ContinueOnTrue))); |
| assert(Term->getSuccessor(ContinueOnTrue) == LoopExit); |
| Term->setSuccessor(!ContinueOnTrue, NULL); |
| } |
| |
| NewBlocks.push_back(New); |
| } |
| |
| // Remap all instructions in the most recent iteration |
| for (unsigned i = 0; i < NewBlocks.size(); ++i) |
| for (BasicBlock::iterator I = NewBlocks[i]->begin(), |
| E = NewBlocks[i]->end(); I != E; ++I) |
| RemapInstruction(I, LastValueMap); |
| } |
| |
| // The latch block exits the loop. If there are any PHI nodes in the |
| // successor blocks, update them to use the appropriate values computed as the |
| // last iteration of the loop. |
| if (Count != 1) { |
| SmallPtrSet<PHINode*, 8> Users; |
| for (Value::use_iterator UI = LatchBlock->use_begin(), |
| UE = LatchBlock->use_end(); UI != UE; ++UI) |
| if (PHINode *phi = dyn_cast<PHINode>(*UI)) |
| Users.insert(phi); |
| |
| BasicBlock *LastIterationBB = cast<BasicBlock>(LastValueMap[LatchBlock]); |
| for (SmallPtrSet<PHINode*,8>::iterator SI = Users.begin(), SE = Users.end(); |
| SI != SE; ++SI) { |
| PHINode *PN = *SI; |
| Value *InVal = PN->removeIncomingValue(LatchBlock, false); |
| // If this value was defined in the loop, take the value defined by the |
| // last iteration of the loop. |
| if (Instruction *InValI = dyn_cast<Instruction>(InVal)) { |
| if (L->contains(InValI->getParent())) |
| InVal = LastValueMap[InVal]; |
| } |
| PN->addIncoming(InVal, LastIterationBB); |
| } |
| } |
| |
| // Now, if we're doing complete unrolling, loop over the PHI nodes in the |
| // original block, setting them to their incoming values. |
| if (CompletelyUnroll) { |
| BasicBlock *Preheader = L->getLoopPreheader(); |
| for (unsigned i = 0, e = OrigPHINode.size(); i != e; ++i) { |
| PHINode *PN = OrigPHINode[i]; |
| PN->replaceAllUsesWith(PN->getIncomingValueForBlock(Preheader)); |
| Header->getInstList().erase(PN); |
| } |
| } |
| |
| // Now that all the basic blocks for the unrolled iterations are in place, |
| // set up the branches to connect them. |
| for (unsigned i = 0, e = Latches.size(); i != e; ++i) { |
| // The original branch was replicated in each unrolled iteration. |
| BranchInst *Term = cast<BranchInst>(Latches[i]->getTerminator()); |
| |
| // The branch destination. |
| unsigned j = (i + 1) % e; |
| BasicBlock *Dest = Headers[j]; |
| bool NeedConditional = true; |
| |
| // For a complete unroll, make the last iteration end with a branch |
| // to the exit block. |
| if (CompletelyUnroll && j == 0) { |
| Dest = LoopExit; |
| NeedConditional = false; |
| } |
| |
| // If we know the trip count or a multiple of it, we can safely use an |
| // unconditional branch for some iterations. |
| if (j != BreakoutTrip && (TripMultiple == 0 || j % TripMultiple != 0)) { |
| NeedConditional = false; |
| } |
| |
| if (NeedConditional) { |
| // Update the conditional branch's successor for the following |
| // iteration. |
| Term->setSuccessor(!ContinueOnTrue, Dest); |
| } else { |
| Term->setUnconditionalDest(Dest); |
| // Merge adjacent basic blocks, if possible. |
| if (BasicBlock *Fold = FoldBlockIntoPredecessor(Dest, LI)) { |
| std::replace(Latches.begin(), Latches.end(), Dest, Fold); |
| std::replace(Headers.begin(), Headers.end(), Dest, Fold); |
| } |
| } |
| } |
| |
| // At this point, the code is well formed. We now do a quick sweep over the |
| // inserted code, doing constant propagation and dead code elimination as we |
| // go. |
| const std::vector<BasicBlock*> &NewLoopBlocks = L->getBlocks(); |
| for (std::vector<BasicBlock*>::const_iterator BB = NewLoopBlocks.begin(), |
| BBE = NewLoopBlocks.end(); BB != BBE; ++BB) |
| for (BasicBlock::iterator I = (*BB)->begin(), E = (*BB)->end(); I != E; ) { |
| Instruction *Inst = I++; |
| |
| if (isInstructionTriviallyDead(Inst)) |
| (*BB)->getInstList().erase(Inst); |
| else if (Constant *C = ConstantFoldInstruction(Inst, |
| Header->getContext())) { |
| Inst->replaceAllUsesWith(C); |
| (*BB)->getInstList().erase(Inst); |
| } |
| } |
| |
| NumCompletelyUnrolled += CompletelyUnroll; |
| ++NumUnrolled; |
| // Remove the loop from the LoopPassManager if it's completely removed. |
| if (CompletelyUnroll && LPM != NULL) |
| LPM->deleteLoopFromQueue(L); |
| |
| // If we didn't completely unroll the loop, it should still be in LCSSA form. |
| if (!CompletelyUnroll) |
| assert(L->isLCSSAForm()); |
| |
| return true; |
| } |