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//===-- LoopUnroll.cpp - Loop unroller pass -------------------------------===//
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// The LLVM Compiler Infrastructure
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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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// 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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#define DEBUG_TYPE "loop-unroll"
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#include "llvm/IntrinsicInst.h"
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#include "llvm/Transforms/Scalar.h"
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#include "llvm/Analysis/LoopPass.h"
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#include "llvm/Analysis/InlineCost.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/Support/raw_ostream.h"
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#include "llvm/Transforms/Utils/UnrollLoop.h"
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static 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 automatic loop unrolling"));
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static cl::opt<unsigned>
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UnrollCount("unroll-count", cl::init(0), cl::Hidden,
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cl::desc("Use this unroll count for all loops, for testing purposes"));
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UnrollAllowPartial("unroll-allow-partial", cl::init(false), cl::Hidden,
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cl::desc("Allows loops to be partially unrolled until "
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"-unroll-threshold loop size is reached."));
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class LoopUnroll : public LoopPass {
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static char ID; // Pass ID, replacement for typeid
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LoopUnroll() : LoopPass(&ID) {}
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/// A magic value for use with the Threshold parameter to indicate
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/// that the loop unroll should be performed regardless of how much
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/// code expansion would result.
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static const unsigned NoThreshold = UINT_MAX;
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bool runOnLoop(Loop *L, LPPassManager &LPM);
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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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virtual void getAnalysisUsage(AnalysisUsage &AU) const {
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AU.addRequiredID(LoopSimplifyID);
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AU.addRequiredID(LCSSAID);
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AU.addRequired<LoopInfo>();
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AU.addPreservedID(LCSSAID);
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AU.addPreserved<LoopInfo>();
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// FIXME: Loop unroll requires LCSSA. And LCSSA requires dom info.
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// If loop unroll does not preserve dom info then LCSSA pass on next
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// loop will receive invalid dom info.
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// For now, recreate dom info, if loop is unrolled.
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AU.addPreserved<DominatorTree>();
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AU.addPreserved<DominanceFrontier>();
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char LoopUnroll::ID = 0;
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static RegisterPass<LoopUnroll> X("loop-unroll", "Unroll loops");
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Pass *llvm::createLoopUnrollPass() { return new LoopUnroll(); }
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/// ApproximateLoopSize - Approximate the size of the loop.
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static unsigned ApproximateLoopSize(const Loop *L, unsigned &NumCalls) {
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for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
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Metrics.analyzeBasicBlock(*I);
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NumCalls = Metrics.NumCalls;
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return Metrics.NumInsts;
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bool LoopUnroll::runOnLoop(Loop *L, LPPassManager &LPM) {
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assert(L->isLCSSAForm());
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LoopInfo *LI = &getAnalysis<LoopInfo>();
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BasicBlock *Header = L->getHeader();
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DEBUG(dbgs() << "Loop Unroll: F[" << Header->getParent()->getName()
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<< "] Loop %" << Header->getName() << "\n");
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unsigned TripCount = L->getSmallConstantTripCount();
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unsigned Count = UnrollCount;
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// Automatically select an unroll count.
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// Conservative heuristic: if we know the trip count, see if we can
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// completely unroll (subject to the threshold, checked below); otherwise
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// try to find greatest modulo of the trip count which is still under
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// Enforce the threshold.
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if (UnrollThreshold != NoThreshold) {
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unsigned LoopSize = ApproximateLoopSize(L, NumCalls);
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DEBUG(dbgs() << " Loop Size = " << LoopSize << "\n");
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DEBUG(dbgs() << " Not unrolling loop with function calls.\n");
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uint64_t Size = (uint64_t)LoopSize*Count;
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if (TripCount != 1 && Size > UnrollThreshold) {
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DEBUG(dbgs() << " Too large to fully unroll with count: " << Count
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<< " because size: " << Size << ">" << UnrollThreshold << "\n");
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if (!UnrollAllowPartial) {
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DEBUG(dbgs() << " will not try to unroll partially because "
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<< "-unroll-allow-partial not given\n");
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// Reduce unroll count to be modulo of TripCount for partial unrolling
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Count = UnrollThreshold / LoopSize;
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while (Count != 0 && TripCount%Count != 0) {
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DEBUG(dbgs() << " could not unroll partially\n");
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DEBUG(dbgs() << " partially unrolling with count: " << Count << "\n");
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Function *F = L->getHeader()->getParent();
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if (!UnrollLoop(L, Count, LI, &LPM))
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// FIXME: Reconstruct dom info, because it is not preserved properly.
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DominatorTree *DT = getAnalysisIfAvailable<DominatorTree>();
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DT->runOnFunction(*F);
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DominanceFrontier *DF = getAnalysisIfAvailable<DominanceFrontier>();
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DF->runOnFunction(*F);