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Summary: Runtime unrolling of loops needs to emit an expression to compute the loop's runtime trip-count. Avoid runtime unrolling if this computation will be expensive. Depends on D8993. Reviewers: atrick Subscribers: llvm-commits Differential Revision: http://reviews.llvm.org/D8994 git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@234846 91177308-0d34-0410-b5e6-96231b3b80d8
583 lines
22 KiB
C++
583 lines
22 KiB
C++
//===-- UnrollLoop.cpp - Loop unrolling utilities -------------------------===//
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//
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// The LLVM Compiler Infrastructure
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//
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// This file is distributed under the University of Illinois Open Source
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// License. See LICENSE.TXT for details.
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//
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//===----------------------------------------------------------------------===//
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//
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// This file implements some loop unrolling utilities. It does not define any
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// actual pass or policy, but provides a single function to perform loop
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// unrolling.
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//
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// The process of unrolling can produce extraneous basic blocks linked with
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// unconditional branches. This will be corrected in the future.
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/Transforms/Utils/UnrollLoop.h"
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#include "llvm/ADT/SmallPtrSet.h"
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#include "llvm/ADT/Statistic.h"
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#include "llvm/Analysis/AssumptionCache.h"
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#include "llvm/Analysis/InstructionSimplify.h"
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#include "llvm/Analysis/LoopIterator.h"
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#include "llvm/Analysis/LoopPass.h"
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#include "llvm/Analysis/ScalarEvolution.h"
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#include "llvm/IR/BasicBlock.h"
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#include "llvm/IR/DataLayout.h"
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#include "llvm/IR/DiagnosticInfo.h"
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#include "llvm/IR/Dominators.h"
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#include "llvm/IR/LLVMContext.h"
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#include "llvm/Support/Debug.h"
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#include "llvm/Support/raw_ostream.h"
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#include "llvm/Transforms/Utils/BasicBlockUtils.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/Transforms/Utils/LoopUtils.h"
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#include "llvm/Transforms/Utils/SimplifyIndVar.h"
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using namespace llvm;
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#define DEBUG_TYPE "loop-unroll"
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// TODO: Should these be here or in LoopUnroll?
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STATISTIC(NumCompletelyUnrolled, "Number of loops completely unrolled");
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STATISTIC(NumUnrolled, "Number of loops unrolled (completely or otherwise)");
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/// RemapInstruction - Convert the instruction operands from referencing the
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/// current values into those specified by VMap.
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static inline void RemapInstruction(Instruction *I,
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ValueToValueMapTy &VMap) {
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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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ValueToValueMapTy::iterator It = VMap.find(Op);
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if (It != VMap.end())
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I->setOperand(op, It->second);
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}
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if (PHINode *PN = dyn_cast<PHINode>(I)) {
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for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
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ValueToValueMapTy::iterator It = VMap.find(PN->getIncomingBlock(i));
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if (It != VMap.end())
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PN->setIncomingBlock(i, cast<BasicBlock>(It->second));
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}
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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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/// The LoopInfo Analysis that is passed will be kept consistent. If folding is
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/// successful references to the containing loop must be removed from
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/// ScalarEvolution by calling ScalarEvolution::forgetLoop because SE may have
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/// references to the eliminated BB. The argument ForgottenLoops contains a set
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/// of loops that have already been forgotten to prevent redundant, expensive
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/// calls to ScalarEvolution::forgetLoop. Returns the new combined block.
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static BasicBlock *
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FoldBlockIntoPredecessor(BasicBlock *BB, LoopInfo* LI, LPPassManager *LPM,
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SmallPtrSetImpl<Loop *> &ForgottenLoops) {
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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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BasicBlock *OnlyPred = BB->getSinglePredecessor();
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if (!OnlyPred) return nullptr;
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if (OnlyPred->getTerminator()->getNumSuccessors() != 1)
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return nullptr;
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DEBUG(dbgs() << "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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FoldSingleEntryPHINodes(BB);
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// Delete the unconditional branch from the predecessor...
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OnlyPred->getInstList().pop_back();
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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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// Move all definitions in the successor to the predecessor...
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OnlyPred->getInstList().splice(OnlyPred->end(), BB->getInstList());
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// OldName will be valid until erased.
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StringRef OldName = BB->getName();
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// Erase basic block from the function...
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// ScalarEvolution holds references to loop exit blocks.
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if (LPM) {
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if (ScalarEvolution *SE = LPM->getAnalysisIfAvailable<ScalarEvolution>()) {
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if (Loop *L = LI->getLoopFor(BB)) {
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if (ForgottenLoops.insert(L).second)
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SE->forgetLoop(L);
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}
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}
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}
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LI->removeBlock(BB);
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// Inherit predecessor's name if it exists...
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if (!OldName.empty() && !OnlyPred->hasName())
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OnlyPred->setName(OldName);
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BB->eraseFromParent();
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return OnlyPred;
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}
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/// Unroll the given loop by Count. The loop must be in LCSSA form. Returns true
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/// if unrolling was successful, or false if the loop was unmodified. Unrolling
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/// can only fail when the loop's latch block is not terminated by a conditional
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/// branch instruction. However, if the trip count (and multiple) are not known,
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/// loop unrolling will mostly produce more code that is no faster.
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///
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/// TripCount is generally defined as the number of times the loop header
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/// executes. UnrollLoop relaxes the definition to permit early exits: here
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/// TripCount is the iteration on which control exits LatchBlock if no early
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/// exits were taken. Note that UnrollLoop assumes that the loop counter test
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/// terminates LatchBlock in order to remove unnecesssary instances of the
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/// test. In other words, control may exit the loop prior to TripCount
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/// iterations via an early branch, but control may not exit the loop from the
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/// LatchBlock's terminator prior to TripCount iterations.
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///
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/// Similarly, TripMultiple divides the number of times that the LatchBlock may
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/// execute without exiting the loop.
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///
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/// If AllowRuntime is true then UnrollLoop will consider unrolling loops that
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/// have a runtime (i.e. not compile time constant) trip count. Unrolling these
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/// loops require a unroll "prologue" that runs "RuntimeTripCount % Count"
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/// iterations before branching into the unrolled loop. UnrollLoop will not
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/// runtime-unroll the loop if computing RuntimeTripCount will be expensive and
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/// AllowExpensiveTripCount is false.
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///
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/// The LoopInfo Analysis that is passed will be kept consistent.
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///
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/// If a LoopPassManager is passed in, and the loop is fully removed, it will be
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/// removed from the LoopPassManager as well. LPM can also be NULL.
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///
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/// This utility preserves LoopInfo. If DominatorTree or ScalarEvolution are
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/// available from the Pass it must also preserve those analyses.
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bool llvm::UnrollLoop(Loop *L, unsigned Count, unsigned TripCount,
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bool AllowRuntime, bool AllowExpensiveTripCount,
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unsigned TripMultiple, LoopInfo *LI, Pass *PP,
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LPPassManager *LPM, AssumptionCache *AC) {
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BasicBlock *Preheader = L->getLoopPreheader();
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if (!Preheader) {
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DEBUG(dbgs() << " Can't unroll; loop preheader-insertion failed.\n");
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return false;
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}
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BasicBlock *LatchBlock = L->getLoopLatch();
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if (!LatchBlock) {
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DEBUG(dbgs() << " Can't unroll; loop exit-block-insertion failed.\n");
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return false;
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}
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// Loops with indirectbr cannot be cloned.
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if (!L->isSafeToClone()) {
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DEBUG(dbgs() << " Can't unroll; Loop body cannot be cloned.\n");
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return false;
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}
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BasicBlock *Header = L->getHeader();
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BranchInst *BI = dyn_cast<BranchInst>(LatchBlock->getTerminator());
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if (!BI || BI->isUnconditional()) {
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// The loop-rotate pass can be helpful to avoid this in many cases.
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DEBUG(dbgs() <<
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" Can't unroll; loop not terminated by a conditional branch.\n");
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return false;
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}
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if (Header->hasAddressTaken()) {
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// The loop-rotate pass can be helpful to avoid this in many cases.
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DEBUG(dbgs() <<
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" Won't unroll loop: address of header block is taken.\n");
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return false;
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}
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if (TripCount != 0)
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DEBUG(dbgs() << " Trip Count = " << TripCount << "\n");
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if (TripMultiple != 1)
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DEBUG(dbgs() << " Trip Multiple = " << TripMultiple << "\n");
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// Effectively "DCE" unrolled iterations that are beyond the tripcount
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// and will never be executed.
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if (TripCount != 0 && Count > TripCount)
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Count = TripCount;
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// Don't enter the unroll code if there is nothing to do. This way we don't
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// need to support "partial unrolling by 1".
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if (TripCount == 0 && Count < 2)
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return false;
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assert(Count > 0);
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assert(TripMultiple > 0);
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assert(TripCount == 0 || TripCount % TripMultiple == 0);
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// Are we eliminating the loop control altogether?
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bool CompletelyUnroll = Count == TripCount;
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// We assume a run-time trip count if the compiler cannot
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// figure out the loop trip count and the unroll-runtime
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// flag is specified.
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bool RuntimeTripCount = (TripCount == 0 && Count > 0 && AllowRuntime);
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if (RuntimeTripCount &&
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!UnrollRuntimeLoopProlog(L, Count, AllowExpensiveTripCount, LI, LPM))
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return false;
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// Notify ScalarEvolution that the loop will be substantially changed,
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// if not outright eliminated.
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ScalarEvolution *SE =
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PP ? PP->getAnalysisIfAvailable<ScalarEvolution>() : nullptr;
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if (SE)
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SE->forgetLoop(L);
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// If we know the trip count, we know the multiple...
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unsigned BreakoutTrip = 0;
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if (TripCount != 0) {
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BreakoutTrip = TripCount % Count;
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TripMultiple = 0;
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} else {
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// Figure out what multiple to use.
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BreakoutTrip = TripMultiple =
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(unsigned)GreatestCommonDivisor64(Count, TripMultiple);
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}
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// Report the unrolling decision.
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DebugLoc LoopLoc = L->getStartLoc();
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Function *F = Header->getParent();
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LLVMContext &Ctx = F->getContext();
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if (CompletelyUnroll) {
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DEBUG(dbgs() << "COMPLETELY UNROLLING loop %" << Header->getName()
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<< " with trip count " << TripCount << "!\n");
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emitOptimizationRemark(Ctx, DEBUG_TYPE, *F, LoopLoc,
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Twine("completely unrolled loop with ") +
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Twine(TripCount) + " iterations");
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} else {
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auto EmitDiag = [&](const Twine &T) {
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emitOptimizationRemark(Ctx, DEBUG_TYPE, *F, LoopLoc,
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"unrolled loop by a factor of " + Twine(Count) +
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T);
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};
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DEBUG(dbgs() << "UNROLLING loop %" << Header->getName()
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<< " by " << Count);
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if (TripMultiple == 0 || BreakoutTrip != TripMultiple) {
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DEBUG(dbgs() << " with a breakout at trip " << BreakoutTrip);
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EmitDiag(" with a breakout at trip " + Twine(BreakoutTrip));
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} else if (TripMultiple != 1) {
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DEBUG(dbgs() << " with " << TripMultiple << " trips per branch");
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EmitDiag(" with " + Twine(TripMultiple) + " trips per branch");
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} else if (RuntimeTripCount) {
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DEBUG(dbgs() << " with run-time trip count");
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EmitDiag(" with run-time trip count");
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}
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DEBUG(dbgs() << "!\n");
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}
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bool ContinueOnTrue = L->contains(BI->getSuccessor(0));
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BasicBlock *LoopExit = BI->getSuccessor(ContinueOnTrue);
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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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ValueToValueMapTy 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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OrigPHINode.push_back(cast<PHINode>(I));
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}
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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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// The current on-the-fly SSA update requires blocks to be processed in
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// reverse postorder so that LastValueMap contains the correct value at each
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// exit.
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LoopBlocksDFS DFS(L);
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DFS.perform(LI);
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// Stash the DFS iterators before adding blocks to the loop.
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LoopBlocksDFS::RPOIterator BlockBegin = DFS.beginRPO();
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LoopBlocksDFS::RPOIterator BlockEnd = DFS.endRPO();
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for (unsigned It = 1; It != Count; ++It) {
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std::vector<BasicBlock*> NewBlocks;
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SmallDenseMap<const Loop *, Loop *, 4> NewLoops;
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NewLoops[L] = L;
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for (LoopBlocksDFS::RPOIterator BB = BlockBegin; BB != BlockEnd; ++BB) {
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ValueToValueMapTy VMap;
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BasicBlock *New = CloneBasicBlock(*BB, VMap, "." + Twine(It));
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Header->getParent()->getBasicBlockList().push_back(New);
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// Tell LI about New.
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if (*BB == Header) {
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assert(LI->getLoopFor(*BB) == L && "Header should not be in a sub-loop");
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L->addBasicBlockToLoop(New, *LI);
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} else {
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// Figure out which loop New is in.
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const Loop *OldLoop = LI->getLoopFor(*BB);
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assert(OldLoop && "Should (at least) be in the loop being unrolled!");
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Loop *&NewLoop = NewLoops[OldLoop];
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if (!NewLoop) {
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// Found a new sub-loop.
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assert(*BB == OldLoop->getHeader() &&
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"Header should be first in RPO");
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Loop *NewLoopParent = NewLoops.lookup(OldLoop->getParentLoop());
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assert(NewLoopParent &&
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"Expected parent loop before sub-loop in RPO");
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NewLoop = new Loop;
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NewLoopParent->addChildLoop(NewLoop);
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// Forget the old loop, since its inputs may have changed.
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if (SE)
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SE->forgetLoop(OldLoop);
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}
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NewLoop->addBasicBlockToLoop(New, *LI);
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}
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if (*BB == Header)
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// Loop over all of the PHI nodes in the block, changing them to use
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// the incoming values from the previous block.
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for (unsigned i = 0, e = OrigPHINode.size(); i != e; ++i) {
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PHINode *NewPHI = cast<PHINode>(VMap[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))
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InVal = LastValueMap[InValI];
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VMap[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 (ValueToValueMapTy::iterator VI = VMap.begin(), VE = VMap.end();
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VI != VE; ++VI)
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LastValueMap[VI->first] = VI->second;
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// Add phi entries for newly created values to all exit blocks.
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for (succ_iterator SI = succ_begin(*BB), SE = succ_end(*BB);
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SI != SE; ++SI) {
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if (L->contains(*SI))
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continue;
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for (BasicBlock::iterator BBI = (*SI)->begin();
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PHINode *phi = dyn_cast<PHINode>(BBI); ++BBI) {
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Value *Incoming = phi->getIncomingValueForBlock(*BB);
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ValueToValueMapTy::iterator It = LastValueMap.find(Incoming);
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if (It != LastValueMap.end())
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Incoming = It->second;
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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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// Loop over the PHI nodes in the original block, setting incoming values.
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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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if (CompletelyUnroll) {
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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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else if (Count > 1) {
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Value *InVal = PN->removeIncomingValue(LatchBlock, false);
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// If this value was defined in the loop, take the value defined by the
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// last iteration of the loop.
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if (Instruction *InValI = dyn_cast<Instruction>(InVal)) {
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if (L->contains(InValI))
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InVal = LastValueMap[InVal];
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}
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assert(Latches.back() == LastValueMap[LatchBlock] && "bad last latch");
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PN->addIncoming(InVal, Latches.back());
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}
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}
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// Now that all the basic blocks for the unrolled iterations are in place,
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// set up the branches to connect them.
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for (unsigned i = 0, e = Latches.size(); i != e; ++i) {
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// The original branch was replicated in each unrolled iteration.
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BranchInst *Term = cast<BranchInst>(Latches[i]->getTerminator());
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// The branch destination.
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unsigned j = (i + 1) % e;
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BasicBlock *Dest = Headers[j];
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bool NeedConditional = true;
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if (RuntimeTripCount && j != 0) {
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NeedConditional = false;
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}
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// For a complete unroll, make the last iteration end with a branch
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// to the exit block.
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if (CompletelyUnroll && j == 0) {
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Dest = LoopExit;
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NeedConditional = false;
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}
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// If we know the trip count or a multiple of it, we can safely use an
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// unconditional branch for some iterations.
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if (j != BreakoutTrip && (TripMultiple == 0 || j % TripMultiple != 0)) {
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NeedConditional = false;
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}
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if (NeedConditional) {
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// Update the conditional branch's successor for the following
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// iteration.
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Term->setSuccessor(!ContinueOnTrue, Dest);
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} else {
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// Remove phi operands at this loop exit
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if (Dest != LoopExit) {
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BasicBlock *BB = Latches[i];
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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.
|
|
SmallPtrSet<Loop *, 4> ForgottenLoops;
|
|
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,
|
|
ForgottenLoops))
|
|
std::replace(Latches.begin(), Latches.end(), Dest, Fold);
|
|
}
|
|
}
|
|
|
|
// FIXME: We could register any cloned assumptions instead of clearing the
|
|
// whole function's cache.
|
|
AC->clear();
|
|
|
|
DominatorTree *DT = nullptr;
|
|
if (PP) {
|
|
// FIXME: Reconstruct dom info, because it is not preserved properly.
|
|
// Incrementally updating domtree after loop unrolling would be easy.
|
|
if (DominatorTreeWrapperPass *DTWP =
|
|
PP->getAnalysisIfAvailable<DominatorTreeWrapperPass>()) {
|
|
DT = &DTWP->getDomTree();
|
|
DT->recalculate(*L->getHeader()->getParent());
|
|
}
|
|
|
|
// Simplify any new induction variables in the partially unrolled loop.
|
|
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 DataLayout &DL = Header->getModule()->getDataLayout();
|
|
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, DL))
|
|
if (LI->replacementPreservesLCSSAForm(Inst, V)) {
|
|
Inst->replaceAllUsesWith(V);
|
|
(*BB)->getInstList().erase(Inst);
|
|
}
|
|
}
|
|
|
|
NumCompletelyUnrolled += CompletelyUnroll;
|
|
++NumUnrolled;
|
|
|
|
Loop *OuterL = L->getParentLoop();
|
|
// Remove the loop from the LoopPassManager if it's completely removed.
|
|
if (CompletelyUnroll && LPM != nullptr)
|
|
LPM->deleteLoopFromQueue(L);
|
|
|
|
// If we have a pass and a DominatorTree we should re-simplify impacted loops
|
|
// to ensure subsequent analyses can rely on this form. We want to simplify
|
|
// at least one layer outside of the loop that was unrolled so that any
|
|
// changes to the parent loop exposed by the unrolling are considered.
|
|
if (PP && DT) {
|
|
if (!OuterL && !CompletelyUnroll)
|
|
OuterL = L;
|
|
if (OuterL) {
|
|
simplifyLoop(OuterL, DT, LI, PP, /*AliasAnalysis*/ nullptr, SE, AC);
|
|
|
|
// LCSSA must be performed on the outermost affected loop. The unrolled
|
|
// loop's last loop latch is guaranteed to be in the outermost loop after
|
|
// deleteLoopFromQueue updates LoopInfo.
|
|
Loop *LatchLoop = LI->getLoopFor(Latches.back());
|
|
if (!OuterL->contains(LatchLoop))
|
|
while (OuterL->getParentLoop() != LatchLoop)
|
|
OuterL = OuterL->getParentLoop();
|
|
|
|
formLCSSARecursively(*OuterL, *DT, LI, SE);
|
|
}
|
|
}
|
|
|
|
return true;
|
|
}
|
|
|
|
/// Given an llvm.loop loop id metadata node, returns the loop hint metadata
|
|
/// node with the given name (for example, "llvm.loop.unroll.count"). If no
|
|
/// such metadata node exists, then nullptr is returned.
|
|
MDNode *llvm::GetUnrollMetadata(MDNode *LoopID, StringRef Name) {
|
|
// First operand should refer to the loop id itself.
|
|
assert(LoopID->getNumOperands() > 0 && "requires at least one operand");
|
|
assert(LoopID->getOperand(0) == LoopID && "invalid loop id");
|
|
|
|
for (unsigned i = 1, e = LoopID->getNumOperands(); i < e; ++i) {
|
|
MDNode *MD = dyn_cast<MDNode>(LoopID->getOperand(i));
|
|
if (!MD)
|
|
continue;
|
|
|
|
MDString *S = dyn_cast<MDString>(MD->getOperand(0));
|
|
if (!S)
|
|
continue;
|
|
|
|
if (Name.equals(S->getString()))
|
|
return MD;
|
|
}
|
|
return nullptr;
|
|
}
|