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4520dd2b7b
Patch by Samuel Tardieu. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@57291 91177308-0d34-0410-b5e6-96231b3b80d8
373 lines
14 KiB
C++
373 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/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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//
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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 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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DOUT << "COMPLETELY UNROLLING loop %" << Header->getName()
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<< " with trip count " << TripCount << "!\n";
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} else {
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DOUT << "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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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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