//===-- 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/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) { DOUT << "COMPLETELY UNROLLING loop %" << Header->getName() << " with trip count " << TripCount << "!\n"; } else { DOUT << "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)) { 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; }