[llvm-commits] CVS: llvm/lib/Transforms/Scalar/LoopUnroll.cpp
Chris Lattner
lattner at cs.uiuc.edu
Sun Apr 18 00:21:21 PDT 2004
Changes in directory llvm/lib/Transforms/Scalar:
LoopUnroll.cpp added (r1.1)
---
Log message:
Initial checkin of a simple loop unroller. This pass is extremely basic and
limited. Even in it's extremely simple state (it can only *fully* unroll single
basic block loops that execute a constant number of times), it already helps improve
performance a LOT on some benchmarks, particularly with the native code generators.
---
Diffs of the changes: (+247 -0)
Index: llvm/lib/Transforms/Scalar/LoopUnroll.cpp
diff -c /dev/null llvm/lib/Transforms/Scalar/LoopUnroll.cpp:1.1
*** /dev/null Sun Apr 18 00:20:28 2004
--- llvm/lib/Transforms/Scalar/LoopUnroll.cpp Sun Apr 18 00:20:17 2004
***************
*** 0 ****
--- 1,247 ----
+ //===-- LoopUnroll.cpp - Loop unroller pass -------------------------------===//
+ //
+ // The LLVM Compiler Infrastructure
+ //
+ // This file was developed by the LLVM research group and is distributed under
+ // the University of Illinois Open Source License. See LICENSE.TXT for details.
+ //
+ //===----------------------------------------------------------------------===//
+ //
+ // This pass implements a simple loop unroller. It works best when loops have
+ // been canonicalized by the -indvars pass, allowing it to determine the trip
+ // counts of loops easily.
+ //
+ // This pass is currently extremely limited. It only currently only unrolls
+ // single basic block loops that execute a constant number of times.
+ //
+ //===----------------------------------------------------------------------===//
+
+ #define DEBUG_TYPE "loop-unroll"
+ #include "llvm/Transforms/Scalar.h"
+ #include "llvm/Constants.h"
+ #include "llvm/Function.h"
+ #include "llvm/Instructions.h"
+ #include "llvm/Analysis/LoopInfo.h"
+ #include "llvm/Transforms/Utils/Cloning.h"
+ #include "llvm/Transforms/Utils/Local.h"
+ #include "Support/CommandLine.h"
+ #include "Support/Debug.h"
+ #include "Support/Statistic.h"
+ #include <cstdio>
+ using namespace llvm;
+
+ namespace {
+ Statistic<> NumUnrolled("loop-unroll", "Number of loops completely unrolled");
+
+ cl::opt<unsigned>
+ UnrollThreshold("unroll-threshold", cl::init(100), cl::Hidden,
+ cl::desc("The cut-off point for loop unrolling"));
+
+ class LoopUnroll : public FunctionPass {
+ LoopInfo *LI; // The current loop information
+ public:
+ virtual bool runOnFunction(Function &F);
+ bool visitLoop(Loop *L);
+
+ /// This transformation requires natural loop information & requires that
+ /// loop preheaders be inserted into the CFG...
+ ///
+ virtual void getAnalysisUsage(AnalysisUsage &AU) const {
+ AU.setPreservesCFG();
+ AU.addRequiredID(LoopSimplifyID);
+ AU.addRequired<LoopInfo>();
+ }
+ };
+ RegisterOpt<LoopUnroll> X("loop-unroll", "Unroll loops");
+ }
+
+ FunctionPass *llvm::createLoopUnrollPass() { return new LoopUnroll(); }
+
+ bool LoopUnroll::runOnFunction(Function &F) {
+ bool Changed = false;
+ LI = &getAnalysis<LoopInfo>();
+
+ for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I)
+ Changed |= visitLoop(*I);
+
+ return Changed;
+ }
+
+ /// ApproximateLoopSize - Approximate the size of the loop after it has been
+ /// unrolled.
+ static unsigned ApproximateLoopSize(const Loop *L) {
+ unsigned Size = 0;
+ for (unsigned i = 0, e = L->getBlocks().size(); i != e; ++i) {
+ BasicBlock *BB = L->getBlocks()[i];
+ Instruction *Term = BB->getTerminator();
+ for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) {
+ if (isa<PHINode>(I) && BB == L->getHeader()) {
+ // Ignore PHI nodes in the header.
+ } else if (I->hasOneUse() && I->use_back() == Term) {
+ // Ignore instructions only used by the loop terminator.
+ } else {
+ ++Size;
+ }
+
+ // TODO: Ignore expressions derived from PHI and constants if inval of phi
+ // is a constant, or if operation is associative. This will get induction
+ // variables.
+ }
+ }
+
+ return Size;
+ }
+
+ // RemapInstruction - Convert the instruction operands from referencing the
+ // current values into those specified by ValueMap.
+ //
+ static inline void RemapInstruction(Instruction *I,
+ std::map<const Value *, Value*> &ValueMap) {
+ for (unsigned op = 0, E = I->getNumOperands(); op != E; ++op) {
+ Value *Op = I->getOperand(op);
+ std::map<const Value *, Value*>::iterator It = ValueMap.find(Op);
+ if (It != ValueMap.end()) Op = It->second;
+ I->setOperand(op, Op);
+ }
+ }
+
+
+ bool LoopUnroll::visitLoop(Loop *L) {
+ bool Changed = false;
+
+ // Recurse through all subloops before we process this loop. Copy the loop
+ // list so that the child can update the loop tree if it needs to delete the
+ // loop.
+ std::vector<Loop*> SubLoops(L->begin(), L->end());
+ for (unsigned i = 0, e = SubLoops.size(); i != e; ++i)
+ Changed |= visitLoop(SubLoops[i]);
+
+ // We only handle single basic block loops right now.
+ if (L->getBlocks().size() != 1)
+ return Changed;
+
+ BasicBlock *BB = L->getHeader();
+ BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator());
+ if (BI == 0) return Changed; // Must end in a conditional branch
+
+ ConstantInt *TripCountC = dyn_cast_or_null<ConstantInt>(L->getTripCount());
+ if (!TripCountC) return Changed; // Must have constant trip count!
+
+ unsigned TripCount = TripCountC->getRawValue();
+ if (TripCount != TripCountC->getRawValue())
+ return Changed; // More than 2^32 iterations???
+
+ unsigned LoopSize = ApproximateLoopSize(L);
+ DEBUG(std::cerr << "Loop Unroll: F[" << BB->getParent()->getName()
+ << "] Loop %" << BB->getName() << " Loop Size = " << LoopSize
+ << " Trip Count = " << TripCount << " - ");
+ if (LoopSize*TripCount > UnrollThreshold) {
+ DEBUG(std::cerr << "TOO LARGE: " << LoopSize*TripCount << ">"
+ << UnrollThreshold << "\n");
+ return Changed;
+ }
+ DEBUG(std::cerr << "UNROLLING!\n");
+
+ assert(L->getExitBlocks().size() == 1 && "Must have exactly one exit block!");
+ BasicBlock *LoopExit = L->getExitBlocks()[0];
+
+ // Create a new basic block to temporarily hold all of the cloned code.
+ BasicBlock *NewBlock = new BasicBlock();
+
+ // For the first iteration of the loop, we should use the precloned values for
+ // PHI nodes. Insert associations now.
+ std::map<const Value*, Value*> LastValueMap;
+ std::vector<PHINode*> OrigPHINode;
+ for (BasicBlock::iterator I = BB->begin();
+ PHINode *PN = dyn_cast<PHINode>(I); ++I) {
+ OrigPHINode.push_back(PN);
+ if (Instruction *I =dyn_cast<Instruction>(PN->getIncomingValueForBlock(BB)))
+ if (I->getParent() == BB)
+ LastValueMap[I] = I;
+ }
+
+ // Remove the exit branch from the loop
+ BB->getInstList().erase(BI);
+
+ assert(TripCount != 0 && "Trip count of 0 is impossible!");
+ for (unsigned It = 1; It != TripCount; ++It) {
+ char SuffixBuffer[100];
+ sprintf(SuffixBuffer, ".%d", It);
+ std::map<const Value*, Value*> ValueMap;
+ BasicBlock *New = CloneBasicBlock(BB, ValueMap, SuffixBuffer);
+
+ // Loop over all of the PHI nodes in the block, changing them to use the
+ // incoming values from the previous block.
+ for (unsigned i = 0, e = OrigPHINode.size(); i != e; ++i) {
+ PHINode *NewPHI = cast<PHINode>(ValueMap[OrigPHINode[i]]);
+ Value *InVal = NewPHI->getIncomingValueForBlock(BB);
+ if (Instruction *InValI = dyn_cast<Instruction>(InVal))
+ if (InValI->getParent() == BB)
+ InVal = LastValueMap[InValI];
+ ValueMap[OrigPHINode[i]] = InVal;
+ New->getInstList().erase(NewPHI);
+ }
+
+ for (BasicBlock::iterator I = New->begin(), E = New->end(); I != E; ++I)
+ RemapInstruction(I, ValueMap);
+
+ // Now that all of the instructions are remapped, splice them into the end
+ // of the NewBlock.
+ NewBlock->getInstList().splice(NewBlock->end(), New->getInstList());
+ delete New;
+
+ // LastValue map now contains values from this iteration.
+ std::swap(LastValueMap, ValueMap);
+ }
+
+ // If there was more than one iteration, replace any uses of values computed
+ // in the loop with values computed during last iteration of the loop.
+ if (TripCount != 1)
+ for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) {
+ std::vector<User*> Users(I->use_begin(), I->use_end());
+ for (unsigned i = 0, e = Users.size(); i != e; ++i) {
+ Instruction *UI = cast<Instruction>(Users[i]);
+ if (UI->getParent() != BB && UI->getParent() != NewBlock)
+ UI->replaceUsesOfWith(I, LastValueMap[I]);
+ }
+ }
+
+ // Now that we cloned the block as many times as we needed, stitch the new
+ // code into the original block and delete the temporary block.
+ BB->getInstList().splice(BB->end(), NewBlock->getInstList());
+ delete NewBlock;
+
+ // Now loop over the PHI nodes in the original block, setting them to their
+ // incoming values.
+ BasicBlock *Preheader = L->getLoopPreheader();
+ for (unsigned i = 0, e = OrigPHINode.size(); i != e; ++i) {
+ PHINode *PN = OrigPHINode[i];
+ PN->replaceAllUsesWith(PN->getIncomingValueForBlock(Preheader));
+ BB->getInstList().erase(PN);
+ }
+
+ // Finally, add an unconditional branch to the block to continue into the exit
+ // block.
+ new BranchInst(LoopExit, BB);
+
+ // 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.
+ for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ) {
+ Instruction *Inst = I++;
+
+ if (isInstructionTriviallyDead(Inst))
+ BB->getInstList().erase(Inst);
+ else if (Constant *C = ConstantFoldInstruction(Inst)) {
+ Inst->replaceAllUsesWith(C);
+ BB->getInstList().erase(Inst);
+ }
+ }
+
+ // FIXME: Should update analyses
+
+ // FIXME: Should fold into preheader and exit block
+
+ ++NumUnrolled;
+ return true;
+ }
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