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http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20070416/047888.html git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@36182 91177308-0d34-0410-b5e6-96231b3b80d8
363 lines
13 KiB
C++
363 lines
13 KiB
C++
//===-- LoopUnroll.cpp - Loop unroller pass -------------------------------===//
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//
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// The LLVM Compiler Infrastructure
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//
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// This file was developed by the LLVM research group and is distributed under
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// the University of Illinois Open Source License. See LICENSE.TXT for details.
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//
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//===----------------------------------------------------------------------===//
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//
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// This pass implements a simple loop unroller. It works best when loops have
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// been canonicalized by the -indvars pass, allowing it to determine the trip
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// counts of loops easily.
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//
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// This pass will multi-block loops only if they contain no non-unrolled
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// subloops. The process of unrolling can produce extraneous basic blocks
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// linked with unconditional branches. This will be corrected in the future.
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//
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//===----------------------------------------------------------------------===//
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#define DEBUG_TYPE "loop-unroll"
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#include "llvm/Transforms/Scalar.h"
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#include "llvm/Constants.h"
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#include "llvm/Function.h"
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#include "llvm/Instructions.h"
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#include "llvm/Analysis/ConstantFolding.h"
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#include "llvm/Analysis/LoopInfo.h"
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#include "llvm/Analysis/LoopPass.h"
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#include "llvm/Transforms/Utils/Cloning.h"
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#include "llvm/Transforms/Utils/Local.h"
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#include "llvm/Support/CFG.h"
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#include "llvm/Support/Compiler.h"
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#include "llvm/Support/CommandLine.h"
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#include "llvm/Support/Debug.h"
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#include "llvm/ADT/Statistic.h"
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#include "llvm/ADT/STLExtras.h"
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#include "llvm/ADT/SmallPtrSet.h"
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#include "llvm/IntrinsicInst.h"
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#include <cstdio>
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#include <algorithm>
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using namespace llvm;
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STATISTIC(NumUnrolled, "Number of loops completely unrolled");
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namespace {
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cl::opt<unsigned>
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UnrollThreshold("unroll-threshold", cl::init(100), cl::Hidden,
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cl::desc("The cut-off point for loop unrolling"));
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class VISIBILITY_HIDDEN LoopUnroll : public LoopPass {
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LoopInfo *LI; // The current loop information
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public:
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bool runOnLoop(Loop *L, LPPassManager &LPM);
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BasicBlock* FoldBlockIntoPredecessor(BasicBlock* BB);
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/// This transformation requires natural loop information & requires that
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/// loop preheaders be inserted into the CFG...
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///
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virtual void getAnalysisUsage(AnalysisUsage &AU) const {
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AU.addRequiredID(LoopSimplifyID);
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AU.addRequiredID(LCSSAID);
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AU.addRequired<LoopInfo>();
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AU.addPreservedID(LCSSAID);
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AU.addPreserved<LoopInfo>();
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}
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};
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RegisterPass<LoopUnroll> X("loop-unroll", "Unroll loops");
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}
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LoopPass *llvm::createLoopUnrollPass() { return new LoopUnroll(); }
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/// ApproximateLoopSize - Approximate the size of the loop after it has been
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/// unrolled.
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static unsigned ApproximateLoopSize(const Loop *L) {
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unsigned Size = 0;
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for (unsigned i = 0, e = L->getBlocks().size(); i != e; ++i) {
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BasicBlock *BB = L->getBlocks()[i];
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Instruction *Term = BB->getTerminator();
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for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) {
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if (isa<PHINode>(I) && BB == L->getHeader()) {
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// Ignore PHI nodes in the header.
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} else if (I->hasOneUse() && I->use_back() == Term) {
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// Ignore instructions only used by the loop terminator.
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} else if (isa<DbgInfoIntrinsic>(I)) {
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// Ignore debug instructions
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} else {
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++Size;
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}
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// TODO: Ignore expressions derived from PHI and constants if inval of phi
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// is a constant, or if operation is associative. This will get induction
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// variables.
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}
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}
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return Size;
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}
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// RemapInstruction - Convert the instruction operands from referencing the
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// current values into those specified by ValueMap.
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//
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static inline void RemapInstruction(Instruction *I,
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DenseMap<const Value *, Value*> &ValueMap) {
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for (unsigned op = 0, E = I->getNumOperands(); op != E; ++op) {
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Value *Op = I->getOperand(op);
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DenseMap<const Value *, Value*>::iterator It = ValueMap.find(Op);
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if (It != ValueMap.end()) Op = It->second;
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I->setOperand(op, Op);
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}
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}
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// FoldBlockIntoPredecessor - Folds a basic block into its predecessor if it
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// only has one predecessor, and that predecessor only has one successor.
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// Returns the new combined block.
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BasicBlock* LoopUnroll::FoldBlockIntoPredecessor(BasicBlock* BB) {
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// Merge basic blocks into their predecessor if there is only one distinct
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// pred, and if there is only one distinct successor of the predecessor, and
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// if there are no PHI nodes.
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//
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BasicBlock *OnlyPred = BB->getSinglePredecessor();
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if (!OnlyPred) return 0;
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if (OnlyPred->getTerminator()->getNumSuccessors() != 1)
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return 0;
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DOUT << "Merging: " << *BB << "into: " << *OnlyPred;
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// Resolve any PHI nodes at the start of the block. They are all
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// guaranteed to have exactly one entry if they exist, unless there are
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// multiple duplicate (but guaranteed to be equal) entries for the
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// incoming edges. This occurs when there are multiple edges from
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// OnlyPred to OnlySucc.
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//
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while (PHINode *PN = dyn_cast<PHINode>(&BB->front())) {
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PN->replaceAllUsesWith(PN->getIncomingValue(0));
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BB->getInstList().pop_front(); // Delete the phi node...
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}
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// Delete the unconditional branch from the predecessor...
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OnlyPred->getInstList().pop_back();
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// Move all definitions in the successor to the predecessor...
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OnlyPred->getInstList().splice(OnlyPred->end(), BB->getInstList());
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// Make all PHI nodes that referred to BB now refer to Pred as their
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// source...
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BB->replaceAllUsesWith(OnlyPred);
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std::string OldName = BB->getName();
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// Erase basic block from the function...
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LI->removeBlock(BB);
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BB->eraseFromParent();
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// Inherit predecessors name if it exists...
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if (!OldName.empty() && !OnlyPred->hasName())
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OnlyPred->setName(OldName);
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return OnlyPred;
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}
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bool LoopUnroll::runOnLoop(Loop *L, LPPassManager &LPM) {
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bool Changed = false;
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LI = &getAnalysis<LoopInfo>();
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BasicBlock* Header = L->getHeader();
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BasicBlock* LatchBlock = L->getLoopLatch();
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BranchInst *BI = dyn_cast<BranchInst>(LatchBlock->getTerminator());
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if (BI == 0) return Changed; // Must end in a conditional branch
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ConstantInt *TripCountC = dyn_cast_or_null<ConstantInt>(L->getTripCount());
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if (!TripCountC) return Changed; // Must have constant trip count!
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// Guard against huge trip counts. This also guards against assertions in
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// APInt from the use of getZExtValue, below.
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if (TripCountC->getValue().getActiveBits() > 32)
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return Changed; // More than 2^32 iterations???
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uint64_t TripCountFull = TripCountC->getZExtValue();
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if (TripCountFull == 0)
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return Changed; // Zero iteraitons?
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unsigned LoopSize = ApproximateLoopSize(L);
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DOUT << "Loop Unroll: F[" << Header->getParent()->getName()
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<< "] Loop %" << Header->getName() << " Loop Size = "
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<< LoopSize << " Trip Count = " << TripCountFull << " - ";
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uint64_t Size = (uint64_t)LoopSize*TripCountFull;
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if (Size > UnrollThreshold) {
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DOUT << "TOO LARGE: " << Size << ">" << UnrollThreshold << "\n";
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return Changed;
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}
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DOUT << "UNROLLING!\n";
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std::vector<BasicBlock*> LoopBlocks = L->getBlocks();
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unsigned TripCount = (unsigned)TripCountFull;
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BasicBlock *LoopExit = BI->getSuccessor(L->contains(BI->getSuccessor(0)));
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// For the first iteration of the loop, we should use the precloned values for
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// PHI nodes. Insert associations now.
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DenseMap<const Value*, Value*> LastValueMap;
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std::vector<PHINode*> OrigPHINode;
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for (BasicBlock::iterator I = Header->begin(); isa<PHINode>(I); ++I) {
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PHINode *PN = cast<PHINode>(I);
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OrigPHINode.push_back(PN);
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if (Instruction *I =
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dyn_cast<Instruction>(PN->getIncomingValueForBlock(LatchBlock)))
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if (L->contains(I->getParent()))
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LastValueMap[I] = I;
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}
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// Remove the exit branch from the loop
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LatchBlock->getInstList().erase(BI);
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std::vector<BasicBlock*> Headers;
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std::vector<BasicBlock*> Latches;
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Headers.push_back(Header);
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Latches.push_back(LatchBlock);
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assert(TripCount != 0 && "Trip count of 0 is impossible!");
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for (unsigned It = 1; It != TripCount; ++It) {
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char SuffixBuffer[100];
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sprintf(SuffixBuffer, ".%d", It);
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std::vector<BasicBlock*> NewBlocks;
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for (std::vector<BasicBlock*>::iterator BB = LoopBlocks.begin(),
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E = LoopBlocks.end(); BB != E; ++BB) {
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DenseMap<const Value*, Value*> ValueMap;
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BasicBlock *New = CloneBasicBlock(*BB, ValueMap, SuffixBuffer);
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Header->getParent()->getBasicBlockList().push_back(New);
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// Loop over all of the PHI nodes in the block, changing them to use the
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// incoming values from the previous block.
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if (*BB == Header)
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for (unsigned i = 0, e = OrigPHINode.size(); i != e; ++i) {
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PHINode *NewPHI = cast<PHINode>(ValueMap[OrigPHINode[i]]);
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Value *InVal = NewPHI->getIncomingValueForBlock(LatchBlock);
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if (Instruction *InValI = dyn_cast<Instruction>(InVal))
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if (It > 1 && L->contains(InValI->getParent()))
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InVal = LastValueMap[InValI];
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ValueMap[OrigPHINode[i]] = InVal;
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New->getInstList().erase(NewPHI);
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}
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// Update our running map of newest clones
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LastValueMap[*BB] = New;
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for (DenseMap<const Value*, Value*>::iterator VI = ValueMap.begin(),
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VE = ValueMap.end(); VI != VE; ++VI)
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LastValueMap[VI->first] = VI->second;
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L->addBasicBlockToLoop(New, *LI);
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// Add phi entries for newly created values to all exit blocks except
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// the successor of the latch block. The successor of the exit block will
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// be updated specially after unrolling all the way.
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if (*BB != LatchBlock)
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for (Value::use_iterator UI = (*BB)->use_begin(), UE = (*BB)->use_end();
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UI != UE; ++UI) {
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Instruction* UseInst = cast<Instruction>(*UI);
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if (isa<PHINode>(UseInst) && !L->contains(UseInst->getParent())) {
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PHINode* phi = cast<PHINode>(UseInst);
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Value* Incoming = phi->getIncomingValueForBlock(*BB);
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if (isa<Instruction>(Incoming))
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Incoming = LastValueMap[Incoming];
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phi->addIncoming(Incoming, New);
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}
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}
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// Keep track of new headers and latches as we create them, so that
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// we can insert the proper branches later.
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if (*BB == Header)
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Headers.push_back(New);
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if (*BB == LatchBlock)
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Latches.push_back(New);
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NewBlocks.push_back(New);
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}
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// Remap all instructions in the most recent iteration
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for (unsigned i = 0; i < NewBlocks.size(); ++i)
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for (BasicBlock::iterator I = NewBlocks[i]->begin(),
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E = NewBlocks[i]->end(); I != E; ++I)
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RemapInstruction(I, LastValueMap);
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}
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// Update PHI nodes that reference the final latch block
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if (TripCount > 1) {
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SmallPtrSet<PHINode*, 8> Users;
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for (Value::use_iterator UI = LatchBlock->use_begin(),
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UE = LatchBlock->use_end(); UI != UE; ++UI)
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if (PHINode* phi = dyn_cast<PHINode>(*UI))
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Users.insert(phi);
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for (SmallPtrSet<PHINode*,8>::iterator SI = Users.begin(), SE = Users.end();
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SI != SE; ++SI) {
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Value* InVal = (*SI)->getIncomingValueForBlock(LatchBlock);
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if (isa<Instruction>(InVal))
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InVal = LastValueMap[InVal];
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(*SI)->removeIncomingValue(LatchBlock, false);
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if (InVal)
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(*SI)->addIncoming(InVal, cast<BasicBlock>(LastValueMap[LatchBlock]));
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if ((*SI)->getNumIncomingValues() == 0) {
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// Remove this phi node.
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// If anyone is using this PHI, make them use a dummy value instead...
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(*SI)->replaceAllUsesWith(UndefValue::get((*SI)->getType()));
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(*SI)->eraseFromParent();
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}
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}
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}
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// Now loop over the PHI nodes in the original block, setting them to their
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// incoming values.
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BasicBlock *Preheader = L->getLoopPreheader();
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for (unsigned i = 0, e = OrigPHINode.size(); i != e; ++i) {
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PHINode *PN = OrigPHINode[i];
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PN->replaceAllUsesWith(PN->getIncomingValueForBlock(Preheader));
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Header->getInstList().erase(PN);
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}
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// Insert the branches that link the different iterations together
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for (unsigned i = 0; i < Latches.size()-1; ++i) {
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new BranchInst(Headers[i+1], Latches[i]);
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if(BasicBlock* Fold = FoldBlockIntoPredecessor(Headers[i+1])) {
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std::replace(Latches.begin(), Latches.end(), Headers[i+1], Fold);
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std::replace(Headers.begin(), Headers.end(), Headers[i+1], Fold);
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}
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}
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// Finally, add an unconditional branch to the block to continue into the exit
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// block.
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new BranchInst(LoopExit, Latches[Latches.size()-1]);
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FoldBlockIntoPredecessor(LoopExit);
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// At this point, the code is well formed. We now do a quick sweep over the
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// inserted code, doing constant propagation and dead code elimination as we
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// go.
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const std::vector<BasicBlock*> &NewLoopBlocks = L->getBlocks();
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for (std::vector<BasicBlock*>::const_iterator BB = NewLoopBlocks.begin(),
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BBE = NewLoopBlocks.end(); BB != BBE; ++BB)
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for (BasicBlock::iterator I = (*BB)->begin(), E = (*BB)->end(); I != E; ) {
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Instruction *Inst = I++;
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if (isInstructionTriviallyDead(Inst))
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(*BB)->getInstList().erase(Inst);
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else if (Constant *C = ConstantFoldInstruction(Inst)) {
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Inst->replaceAllUsesWith(C);
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(*BB)->getInstList().erase(Inst);
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}
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}
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// Update the loop information for this loop.
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// Remove the loop from the parent.
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LPM.deleteLoopFromQueue(L);
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++NumUnrolled;
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return true;
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}
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