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- Some clients which used DOUT have moved to DEBUG. We are deprecating the "magic" DOUT behavior which avoided calling printing functions when the statement was disabled. In addition to being unnecessary magic, it had the downside of leaving code in -Asserts builds, and of hiding potentially unnecessary computations. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@77019 91177308-0d34-0410-b5e6-96231b3b80d8
372 lines
14 KiB
C++
372 lines
14 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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// It works best when loops have been canonicalized by the -indvars pass,
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// allowing it to determine the trip counts of loops easily.
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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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#define DEBUG_TYPE "loop-unroll"
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#include "llvm/Transforms/Utils/UnrollLoop.h"
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#include "llvm/BasicBlock.h"
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#include "llvm/ADT/Statistic.h"
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#include "llvm/Analysis/ConstantFolding.h"
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#include "llvm/Analysis/LoopPass.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 <cstdio>
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using namespace llvm;
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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 ValueMap.
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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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/// The LoopInfo Analysis that is passed will be kept consistent.
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/// Returns the new combined block.
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static BasicBlock *FoldBlockIntoPredecessor(BasicBlock *BB, LoopInfo* LI) {
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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 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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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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// 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 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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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 succesful, 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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/// 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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bool llvm::UnrollLoop(Loop *L, unsigned Count, LoopInfo* LI, LPPassManager* LPM) {
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assert(L->isLCSSAForm());
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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 || BI->isUnconditional()) {
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// The loop-rotate pass can be helpful to avoid this in many cases.
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DOUT << " 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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// Find trip count
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unsigned TripCount = L->getSmallConstantTripCount();
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// Find trip multiple if count is not available
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unsigned TripMultiple = 1;
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if (TripCount == 0)
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TripMultiple = L->getSmallConstantTripMultiple();
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if (TripCount != 0)
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DOUT << " Trip Count = " << TripCount << "\n";
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if (TripMultiple != 1)
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DOUT << " 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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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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// 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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if (CompletelyUnroll) {
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DEBUG(errs() << "COMPLETELY UNROLLING loop %" << Header->getName()
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<< " with trip count " << TripCount << "!\n");
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} else {
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DEBUG(errs() << "UNROLLING loop %" << Header->getName()
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<< " by " << Count);
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if (TripMultiple == 0 || BreakoutTrip != TripMultiple) {
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DOUT << " with a breakout at trip " << BreakoutTrip;
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} else if (TripMultiple != 1) {
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DOUT << " with " << TripMultiple << " trips per branch";
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}
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DOUT << "!\n";
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}
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std::vector<BasicBlock*> LoopBlocks = L->getBlocks();
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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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typedef DenseMap<const Value*, Value*> ValueMapTy;
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ValueMapTy 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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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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for (unsigned It = 1; It != Count; ++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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ValueMapTy 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 (ValueMapTy::iterator VI = ValueMap.begin(), VE = ValueMap.end();
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VI != VE; ++VI)
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LastValueMap[VI->first] = VI->second;
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L->addBasicBlockToLoop(New, LI->getBase());
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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;) {
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Instruction *UseInst = cast<Instruction>(*UI);
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++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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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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// Also, clear out the new latch's back edge so that it doesn't look
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// like a new loop, so that it's amenable to being merged with adjacent
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// blocks later on.
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TerminatorInst *Term = New->getTerminator();
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assert(L->contains(Term->getSuccessor(!ContinueOnTrue)));
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assert(Term->getSuccessor(ContinueOnTrue) == LoopExit);
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Term->setSuccessor(!ContinueOnTrue, NULL);
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}
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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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// The latch block exits the loop. If there are any PHI nodes in the
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// successor blocks, update them to use the appropriate values computed as the
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// last iteration of the loop.
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if (Count != 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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BasicBlock *LastIterationBB = cast<BasicBlock>(LastValueMap[LatchBlock]);
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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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PHINode *PN = *SI;
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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->getParent()))
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InVal = LastValueMap[InVal];
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}
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PN->addIncoming(InVal, LastIterationBB);
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}
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}
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// Now, if we're doing complete unrolling, loop over the PHI nodes in the
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// original block, setting them to their incoming values.
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if (CompletelyUnroll) {
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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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}
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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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// 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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Term->setUnconditionalDest(Dest);
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// Merge adjacent basic blocks, if possible.
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if (BasicBlock *Fold = FoldBlockIntoPredecessor(Dest, LI)) {
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std::replace(Latches.begin(), Latches.end(), Dest, Fold);
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std::replace(Headers.begin(), Headers.end(), Dest, Fold);
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}
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}
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}
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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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Header->getContext())) {
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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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NumCompletelyUnrolled += CompletelyUnroll;
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++NumUnrolled;
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// Remove the loop from the LoopPassManager if it's completely removed.
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if (CompletelyUnroll && LPM != NULL)
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LPM->deleteLoopFromQueue(L);
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// If we didn't completely unroll the loop, it should still be in LCSSA form.
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if (!CompletelyUnroll)
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assert(L->isLCSSAForm());
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return true;
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}
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