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Diffstat (limited to 'contrib/llvm/lib/Transforms/Utils/LoopUnroll.cpp')
| -rw-r--r-- | contrib/llvm/lib/Transforms/Utils/LoopUnroll.cpp | 457 |
1 files changed, 457 insertions, 0 deletions
diff --git a/contrib/llvm/lib/Transforms/Utils/LoopUnroll.cpp b/contrib/llvm/lib/Transforms/Utils/LoopUnroll.cpp new file mode 100644 index 000000000000..162807d03c62 --- /dev/null +++ b/contrib/llvm/lib/Transforms/Utils/LoopUnroll.cpp @@ -0,0 +1,457 @@ +//===-- 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. +// +// 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/ADT/Statistic.h" +#include "llvm/Analysis/InstructionSimplify.h" +#include "llvm/Analysis/LoopIterator.h" +#include "llvm/Analysis/LoopPass.h" +#include "llvm/Analysis/ScalarEvolution.h" +#include "llvm/IR/BasicBlock.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 "llvm/Transforms/Utils/SimplifyIndVar.h" +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 VMap. +static inline void RemapInstruction(Instruction *I, + ValueToValueMapTy &VMap) { + for (unsigned op = 0, E = I->getNumOperands(); op != E; ++op) { + Value *Op = I->getOperand(op); + ValueToValueMapTy::iterator It = VMap.find(Op); + if (It != VMap.end()) + I->setOperand(op, It->second); + } + + if (PHINode *PN = dyn_cast<PHINode>(I)) { + for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) { + ValueToValueMapTy::iterator It = VMap.find(PN->getIncomingBlock(i)); + if (It != VMap.end()) + PN->setIncomingBlock(i, cast<BasicBlock>(It->second)); + } + } +} + +/// 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, + LPPassManager *LPM) { + // 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; + + DEBUG(dbgs() << "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(); + + // Make all PHI nodes that referred to BB now refer to Pred as their + // source... + BB->replaceAllUsesWith(OnlyPred); + + // Move all definitions in the successor to the predecessor... + OnlyPred->getInstList().splice(OnlyPred->end(), BB->getInstList()); + + // OldName will be valid until erased. + StringRef OldName = BB->getName(); + + // Erase basic block from the function... + + // ScalarEvolution holds references to loop exit blocks. + if (LPM) { + if (ScalarEvolution *SE = LPM->getAnalysisIfAvailable<ScalarEvolution>()) { + if (Loop *L = LI->getLoopFor(BB)) + SE->forgetLoop(L); + } + } + LI->removeBlock(BB); + + // Inherit predecessor's name if it exists... + if (!OldName.empty() && !OnlyPred->hasName()) + OnlyPred->setName(OldName); + + BB->eraseFromParent(); + + return OnlyPred; +} + +/// Unroll the given loop by Count. The loop must be in LCSSA form. Returns true +/// if unrolling was successful, 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. +/// +/// TripCount is generally defined as the number of times the loop header +/// executes. UnrollLoop relaxes the definition to permit early exits: here +/// TripCount is the iteration on which control exits LatchBlock if no early +/// exits were taken. Note that UnrollLoop assumes that the loop counter test +/// terminates LatchBlock in order to remove unnecesssary instances of the +/// test. In other words, control may exit the loop prior to TripCount +/// iterations via an early branch, but control may not exit the loop from the +/// LatchBlock's terminator prior to TripCount iterations. +/// +/// Similarly, TripMultiple divides the number of times that the LatchBlock may +/// execute without exiting the loop. +/// +/// 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. +/// +/// This utility preserves LoopInfo. If DominatorTree or ScalarEvolution are +/// available it must also preserve those analyses. +bool llvm::UnrollLoop(Loop *L, unsigned Count, unsigned TripCount, + bool AllowRuntime, unsigned TripMultiple, + LoopInfo *LI, LPPassManager *LPM) { + BasicBlock *Preheader = L->getLoopPreheader(); + if (!Preheader) { + DEBUG(dbgs() << " Can't unroll; loop preheader-insertion failed.\n"); + return false; + } + + BasicBlock *LatchBlock = L->getLoopLatch(); + if (!LatchBlock) { + DEBUG(dbgs() << " Can't unroll; loop exit-block-insertion failed.\n"); + return false; + } + + // Loops with indirectbr cannot be cloned. + if (!L->isSafeToClone()) { + DEBUG(dbgs() << " Can't unroll; Loop body cannot be cloned.\n"); + return false; + } + + BasicBlock *Header = L->getHeader(); + BranchInst *BI = dyn_cast<BranchInst>(LatchBlock->getTerminator()); + + if (!BI || BI->isUnconditional()) { + // The loop-rotate pass can be helpful to avoid this in many cases. + DEBUG(dbgs() << + " Can't unroll; loop not terminated by a conditional branch.\n"); + return false; + } + + if (Header->hasAddressTaken()) { + // The loop-rotate pass can be helpful to avoid this in many cases. + DEBUG(dbgs() << + " Won't unroll loop: address of header block is taken.\n"); + return false; + } + + if (TripCount != 0) + DEBUG(dbgs() << " Trip Count = " << TripCount << "\n"); + if (TripMultiple != 1) + DEBUG(dbgs() << " 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; + + // Don't enter the unroll code if there is nothing to do. This way we don't + // need to support "partial unrolling by 1". + if (TripCount == 0 && Count < 2) + return false; + + assert(Count > 0); + assert(TripMultiple > 0); + assert(TripCount == 0 || TripCount % TripMultiple == 0); + + // Are we eliminating the loop control altogether? + bool CompletelyUnroll = Count == TripCount; + + // We assume a run-time trip count if the compiler cannot + // figure out the loop trip count and the unroll-runtime + // flag is specified. + bool RuntimeTripCount = (TripCount == 0 && Count > 0 && AllowRuntime); + + if (RuntimeTripCount && !UnrollRuntimeLoopProlog(L, Count, LI, LPM)) + return false; + + // Notify ScalarEvolution that the loop will be substantially changed, + // if not outright eliminated. + if (LPM) { + ScalarEvolution *SE = LPM->getAnalysisIfAvailable<ScalarEvolution>(); + if (SE) + SE->forgetLoop(L); + } + + // 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(dbgs() << "COMPLETELY UNROLLING loop %" << Header->getName() + << " with trip count " << TripCount << "!\n"); + } else { + DEBUG(dbgs() << "UNROLLING loop %" << Header->getName() + << " by " << Count); + if (TripMultiple == 0 || BreakoutTrip != TripMultiple) { + DEBUG(dbgs() << " with a breakout at trip " << BreakoutTrip); + } else if (TripMultiple != 1) { + DEBUG(dbgs() << " with " << TripMultiple << " trips per branch"); + } else if (RuntimeTripCount) { + DEBUG(dbgs() << " with run-time trip count"); + } + DEBUG(dbgs() << "!\n"); + } + + 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. + ValueToValueMapTy LastValueMap; + std::vector<PHINode*> OrigPHINode; + for (BasicBlock::iterator I = Header->begin(); isa<PHINode>(I); ++I) { + OrigPHINode.push_back(cast<PHINode>(I)); + } + + std::vector<BasicBlock*> Headers; + std::vector<BasicBlock*> Latches; + Headers.push_back(Header); + Latches.push_back(LatchBlock); + + // The current on-the-fly SSA update requires blocks to be processed in + // reverse postorder so that LastValueMap contains the correct value at each + // exit. + LoopBlocksDFS DFS(L); + DFS.perform(LI); + + // Stash the DFS iterators before adding blocks to the loop. + LoopBlocksDFS::RPOIterator BlockBegin = DFS.beginRPO(); + LoopBlocksDFS::RPOIterator BlockEnd = DFS.endRPO(); + + for (unsigned It = 1; It != Count; ++It) { + std::vector<BasicBlock*> NewBlocks; + + for (LoopBlocksDFS::RPOIterator BB = BlockBegin; BB != BlockEnd; ++BB) { + ValueToValueMapTy VMap; + BasicBlock *New = CloneBasicBlock(*BB, VMap, "." + Twine(It)); + 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>(VMap[OrigPHINode[i]]); + Value *InVal = NewPHI->getIncomingValueForBlock(LatchBlock); + if (Instruction *InValI = dyn_cast<Instruction>(InVal)) + if (It > 1 && L->contains(InValI)) + InVal = LastValueMap[InValI]; + VMap[OrigPHINode[i]] = InVal; + New->getInstList().erase(NewPHI); + } + + // Update our running map of newest clones + LastValueMap[*BB] = New; + for (ValueToValueMapTy::iterator VI = VMap.begin(), VE = VMap.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. + for (succ_iterator SI = succ_begin(*BB), SE = succ_end(*BB); + SI != SE; ++SI) { + if (L->contains(*SI)) + continue; + for (BasicBlock::iterator BBI = (*SI)->begin(); + PHINode *phi = dyn_cast<PHINode>(BBI); ++BBI) { + Value *Incoming = phi->getIncomingValueForBlock(*BB); + ValueToValueMapTy::iterator It = LastValueMap.find(Incoming); + if (It != LastValueMap.end()) + Incoming = It->second; + 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); + + 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); + } + + // Loop over the PHI nodes in the original block, setting incoming values. + for (unsigned i = 0, e = OrigPHINode.size(); i != e; ++i) { + PHINode *PN = OrigPHINode[i]; + if (CompletelyUnroll) { + PN->replaceAllUsesWith(PN->getIncomingValueForBlock(Preheader)); + Header->getInstList().erase(PN); + } + else if (Count > 1) { + 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)) + InVal = LastValueMap[InVal]; + } + assert(Latches.back() == LastValueMap[LatchBlock] && "bad last latch"); + PN->addIncoming(InVal, Latches.back()); + } + } + + // 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; + + if (RuntimeTripCount && j != 0) { + NeedConditional = false; + } + + // 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 { + // Remove phi operands at this loop exit + if (Dest != LoopExit) { + BasicBlock *BB = Latches[i]; + for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); + SI != SE; ++SI) { + if (*SI == Headers[i]) + continue; + for (BasicBlock::iterator BBI = (*SI)->begin(); + PHINode *Phi = dyn_cast<PHINode>(BBI); ++BBI) { + Phi->removeIncomingValue(BB, false); + } + } + } + // Replace the conditional branch with an unconditional one. + BranchInst::Create(Dest, Term); + Term->eraseFromParent(); + } + } + + // Merge adjacent basic blocks, if possible. + for (unsigned i = 0, e = Latches.size(); i != e; ++i) { + BranchInst *Term = cast<BranchInst>(Latches[i]->getTerminator()); + if (Term->isUnconditional()) { + BasicBlock *Dest = Term->getSuccessor(0); + if (BasicBlock *Fold = FoldBlockIntoPredecessor(Dest, LI, LPM)) + std::replace(Latches.begin(), Latches.end(), Dest, Fold); + } + } + + if (LPM) { + // FIXME: Reconstruct dom info, because it is not preserved properly. + // Incrementally updating domtree after loop unrolling would be easy. + if (DominatorTree *DT = LPM->getAnalysisIfAvailable<DominatorTree>()) + DT->runOnFunction(*L->getHeader()->getParent()); + + // Simplify any new induction variables in the partially unrolled loop. + ScalarEvolution *SE = LPM->getAnalysisIfAvailable<ScalarEvolution>(); + if (SE && !CompletelyUnroll) { + SmallVector<WeakVH, 16> DeadInsts; + simplifyLoopIVs(L, SE, LPM, DeadInsts); + + // Aggressively clean up dead instructions that simplifyLoopIVs already + // identified. Any remaining should be cleaned up below. + while (!DeadInsts.empty()) + if (Instruction *Inst = + dyn_cast_or_null<Instruction>(&*DeadInsts.pop_back_val())) + RecursivelyDeleteTriviallyDeadInstructions(Inst); + } + } + // 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 (Value *V = SimplifyInstruction(Inst)) + if (LI->replacementPreservesLCSSAForm(Inst, V)) { + Inst->replaceAllUsesWith(V); + (*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); + + return true; +} |