301 lines
11 KiB
C++
301 lines
11 KiB
C++
//===-- LoopUnroll.cpp - Loop unroller pass -------------------------------===//
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//
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// The LLVM Compiler Infrastructure
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//
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// This file was developed by the LLVM research group and is distributed under
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// the University of Illinois Open Source License. See LICENSE.TXT for details.
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//
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//===----------------------------------------------------------------------===//
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//
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// This pass implements a simple loop unroller. It works best when loops have
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// been canonicalized by the -indvars pass, allowing it to determine the trip
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// counts of loops easily.
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//
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// This pass is currently extremely limited. It only currently only unrolls
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// single basic block loops that execute a constant number of times.
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//
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//===----------------------------------------------------------------------===//
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#define DEBUG_TYPE "loop-unroll"
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#include "llvm/Transforms/Scalar.h"
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#include "llvm/Constants.h"
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#include "llvm/Function.h"
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#include "llvm/Instructions.h"
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#include "llvm/Analysis/LoopInfo.h"
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#include "llvm/Transforms/Utils/Cloning.h"
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#include "llvm/Transforms/Utils/Local.h"
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#include "llvm/Support/CommandLine.h"
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#include "llvm/Support/Debug.h"
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#include "llvm/ADT/Statistic.h"
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#include "llvm/ADT/STLExtras.h"
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#include "llvm/IntrinsicInst.h"
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#include <cstdio>
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#include <set>
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#include <algorithm>
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using namespace llvm;
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namespace {
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Statistic<> NumUnrolled("loop-unroll", "Number of loops completely unrolled");
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cl::opt<unsigned>
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UnrollThreshold("unroll-threshold", cl::init(100), cl::Hidden,
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cl::desc("The cut-off point for loop unrolling"));
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class LoopUnroll : public FunctionPass {
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LoopInfo *LI; // The current loop information
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public:
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virtual bool runOnFunction(Function &F);
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bool visitLoop(Loop *L);
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/// This transformation requires natural loop information & requires that
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/// loop preheaders be inserted into the CFG...
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///
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virtual void getAnalysisUsage(AnalysisUsage &AU) const {
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AU.addRequiredID(LoopSimplifyID);
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AU.addRequired<LoopInfo>();
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AU.addPreserved<LoopInfo>();
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}
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};
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RegisterOpt<LoopUnroll> X("loop-unroll", "Unroll loops");
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}
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FunctionPass *llvm::createLoopUnrollPass() { return new LoopUnroll(); }
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bool LoopUnroll::runOnFunction(Function &F) {
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bool Changed = false;
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LI = &getAnalysis<LoopInfo>();
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// Transform all the top-level loops. Copy the loop list so that the child
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// can update the loop tree if it needs to delete the loop.
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std::vector<Loop*> SubLoops(LI->begin(), LI->end());
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for (unsigned i = 0, e = SubLoops.size(); i != e; ++i)
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Changed |= visitLoop(SubLoops[i]);
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return Changed;
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}
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/// ApproximateLoopSize - Approximate the size of the loop after it has been
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/// unrolled.
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static unsigned ApproximateLoopSize(const Loop *L) {
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unsigned Size = 0;
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for (unsigned i = 0, e = L->getBlocks().size(); i != e; ++i) {
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BasicBlock *BB = L->getBlocks()[i];
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Instruction *Term = BB->getTerminator();
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for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) {
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if (isa<PHINode>(I) && BB == L->getHeader()) {
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// Ignore PHI nodes in the header.
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} else if (I->hasOneUse() && I->use_back() == Term) {
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// Ignore instructions only used by the loop terminator.
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} else if (DbgInfoIntrinsic *DbgI = dyn_cast<DbgInfoIntrinsic>(I)) {
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// Ignore debug instructions
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} else {
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++Size;
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}
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// TODO: Ignore expressions derived from PHI and constants if inval of phi
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// is a constant, or if operation is associative. This will get induction
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// variables.
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}
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}
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return Size;
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}
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// RemapInstruction - Convert the instruction operands from referencing the
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// current values into those specified by ValueMap.
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//
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static inline void RemapInstruction(Instruction *I,
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std::map<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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std::map<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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bool LoopUnroll::visitLoop(Loop *L) {
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bool Changed = false;
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// Recurse through all subloops before we process this loop. Copy the loop
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// list so that the child can update the loop tree if it needs to delete the
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// loop.
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std::vector<Loop*> SubLoops(L->begin(), L->end());
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for (unsigned i = 0, e = SubLoops.size(); i != e; ++i)
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Changed |= visitLoop(SubLoops[i]);
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// We only handle single basic block loops right now.
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if (L->getBlocks().size() != 1)
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return Changed;
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BasicBlock *BB = L->getHeader();
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BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator());
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if (BI == 0) return Changed; // Must end in a conditional branch
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ConstantInt *TripCountC = dyn_cast_or_null<ConstantInt>(L->getTripCount());
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if (!TripCountC) return Changed; // Must have constant trip count!
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unsigned TripCount = TripCountC->getRawValue();
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if (TripCount != TripCountC->getRawValue() || TripCount == 0)
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return Changed; // More than 2^32 iterations???
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unsigned LoopSize = ApproximateLoopSize(L);
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DEBUG(std::cerr << "Loop Unroll: F[" << BB->getParent()->getName()
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<< "] Loop %" << BB->getName() << " Loop Size = " << LoopSize
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<< " Trip Count = " << TripCount << " - ");
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uint64_t Size = (uint64_t)LoopSize*(uint64_t)TripCount;
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if (Size > UnrollThreshold) {
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DEBUG(std::cerr << "TOO LARGE: " << Size << ">" << UnrollThreshold << "\n");
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return Changed;
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}
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DEBUG(std::cerr << "UNROLLING!\n");
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BasicBlock *LoopExit = BI->getSuccessor(L->contains(BI->getSuccessor(0)));
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// Create a new basic block to temporarily hold all of the cloned code.
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BasicBlock *NewBlock = new BasicBlock();
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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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std::map<const Value*, Value*> LastValueMap;
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std::vector<PHINode*> OrigPHINode;
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for (BasicBlock::iterator I = BB->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 =dyn_cast<Instruction>(PN->getIncomingValueForBlock(BB)))
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if (I->getParent() == BB)
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LastValueMap[I] = I;
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}
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// Remove the exit branch from the loop
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BB->getInstList().erase(BI);
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assert(TripCount != 0 && "Trip count of 0 is impossible!");
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for (unsigned It = 1; It != TripCount; ++It) {
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char SuffixBuffer[100];
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sprintf(SuffixBuffer, ".%d", It);
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std::map<const Value*, Value*> ValueMap;
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BasicBlock *New = CloneBasicBlock(BB, ValueMap, SuffixBuffer);
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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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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(BB);
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if (Instruction *InValI = dyn_cast<Instruction>(InVal))
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if (InValI->getParent() == BB)
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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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for (BasicBlock::iterator I = New->begin(), E = New->end(); I != E; ++I)
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RemapInstruction(I, ValueMap);
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// Now that all of the instructions are remapped, splice them into the end
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// of the NewBlock.
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NewBlock->getInstList().splice(NewBlock->end(), New->getInstList());
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delete New;
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// LastValue map now contains values from this iteration.
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std::swap(LastValueMap, ValueMap);
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}
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// If there was more than one iteration, replace any uses of values computed
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// in the loop with values computed during the last iteration of the loop.
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if (TripCount != 1) {
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std::set<User*> Users;
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for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
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Users.insert(I->use_begin(), I->use_end());
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// We don't want to reprocess entries with PHI nodes in them. For this
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// reason, we look at each operand of each user exactly once, performing the
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// stubstitution exactly once.
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for (std::set<User*>::iterator UI = Users.begin(), E = Users.end(); UI != E;
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++UI) {
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Instruction *I = cast<Instruction>(*UI);
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if (I->getParent() != BB && I->getParent() != NewBlock)
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RemapInstruction(I, LastValueMap);
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}
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}
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// Now that we cloned the block as many times as we needed, stitch the new
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// code into the original block and delete the temporary block.
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BB->getInstList().splice(BB->end(), NewBlock->getInstList());
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delete NewBlock;
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// Now loop over the PHI nodes in the original block, setting them to their
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// incoming values.
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BasicBlock *Preheader = L->getLoopPreheader();
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for (unsigned i = 0, e = OrigPHINode.size(); i != e; ++i) {
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PHINode *PN = OrigPHINode[i];
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PN->replaceAllUsesWith(PN->getIncomingValueForBlock(Preheader));
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BB->getInstList().erase(PN);
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}
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// Finally, add an unconditional branch to the block to continue into the exit
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// block.
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new BranchInst(LoopExit, BB);
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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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for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ) {
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Instruction *Inst = I++;
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if (isInstructionTriviallyDead(Inst))
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BB->getInstList().erase(Inst);
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else if (Constant *C = ConstantFoldInstruction(Inst)) {
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Inst->replaceAllUsesWith(C);
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BB->getInstList().erase(Inst);
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}
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}
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// Update the loop information for this loop.
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Loop *Parent = L->getParentLoop();
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// Move all of the basic blocks in the loop into the parent loop.
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LI->changeLoopFor(BB, Parent);
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// Remove the loop from the parent.
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if (Parent)
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delete Parent->removeChildLoop(std::find(Parent->begin(), Parent->end(),L));
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else
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delete LI->removeLoop(std::find(LI->begin(), LI->end(), L));
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// FIXME: Should update dominator analyses
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// Now that everything is up-to-date that will be, we fold the loop block into
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// the preheader and exit block, updating our analyses as we go.
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LoopExit->getInstList().splice(LoopExit->begin(), BB->getInstList(),
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BB->getInstList().begin(),
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prior(BB->getInstList().end()));
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LoopExit->getInstList().splice(LoopExit->begin(), Preheader->getInstList(),
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Preheader->getInstList().begin(),
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prior(Preheader->getInstList().end()));
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// Make all other blocks in the program branch to LoopExit now instead of
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// Preheader.
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Preheader->replaceAllUsesWith(LoopExit);
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// Remove BB and LoopExit from our analyses.
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LI->removeBlock(Preheader);
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LI->removeBlock(BB);
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// If the preheader was the entry block of this function, move the exit block
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// to be the new entry of the loop.
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Function *F = LoopExit->getParent();
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if (Preheader == &F->front())
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F->getBasicBlockList().splice(F->begin(), F->getBasicBlockList(), LoopExit);
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// Actually delete the blocks now.
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F->getBasicBlockList().erase(Preheader);
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F->getBasicBlockList().erase(BB);
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++NumUnrolled;
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return true;
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}
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