1 //===-- LoopUnroll.cpp - Loop unroller pass -------------------------------===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This pass implements a simple loop unroller. It works best when loops have
11 // been canonicalized by the -indvars pass, allowing it to determine the trip
12 // counts of loops easily.
13 //===----------------------------------------------------------------------===//
15 #define DEBUG_TYPE "loop-unroll"
16 #include "llvm/IntrinsicInst.h"
17 #include "llvm/Transforms/Scalar.h"
18 #include "llvm/Analysis/LoopInfo.h"
19 #include "llvm/Analysis/LoopPass.h"
20 #include "llvm/Support/Compiler.h"
21 #include "llvm/Support/CommandLine.h"
22 #include "llvm/Support/Debug.h"
23 #include "llvm/Transforms/Utils/UnrollLoop.h"
28 static cl::opt<unsigned>
29 UnrollThreshold("unroll-threshold", cl::init(100), cl::Hidden,
30 cl::desc("The cut-off point for automatic loop unrolling"));
32 static cl::opt<unsigned>
33 UnrollCount("unroll-count", cl::init(0), cl::Hidden,
34 cl::desc("Use this unroll count for all loops, for testing purposes"));
37 class VISIBILITY_HIDDEN LoopUnroll : public LoopPass {
39 static char ID; // Pass ID, replacement for typeid
40 LoopUnroll() : LoopPass((intptr_t)&ID) {}
42 /// A magic value for use with the Threshold parameter to indicate
43 /// that the loop unroll should be performed regardless of how much
44 /// code expansion would result.
45 static const unsigned NoThreshold = UINT_MAX;
47 bool runOnLoop(Loop *L, LPPassManager &LPM);
49 /// This transformation requires natural loop information & requires that
50 /// loop preheaders be inserted into the CFG...
52 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
53 AU.addRequiredID(LoopSimplifyID);
54 AU.addRequiredID(LCSSAID);
55 AU.addRequired<LoopInfo>();
56 AU.addPreservedID(LCSSAID);
57 AU.addPreserved<LoopInfo>();
58 // FIXME: Loop unroll requires LCSSA. And LCSSA requires dom info.
59 // If loop unroll does not preserve dom info then LCSSA pass on next
60 // loop will receive invalid dom info.
61 // For now, recreate dom info, if loop is unrolled.
62 AU.addPreserved<DominatorTree>();
63 AU.addPreserved<DominanceFrontier>();
68 char LoopUnroll::ID = 0;
69 static RegisterPass<LoopUnroll> X("loop-unroll", "Unroll loops");
71 LoopPass *llvm::createLoopUnrollPass() { return new LoopUnroll(); }
73 /// ApproximateLoopSize - Approximate the size of the loop.
74 static unsigned ApproximateLoopSize(const Loop *L) {
76 for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
79 Instruction *Term = BB->getTerminator();
80 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) {
81 if (isa<PHINode>(I) && BB == L->getHeader()) {
82 // Ignore PHI nodes in the header.
83 } else if (I->hasOneUse() && I->use_back() == Term) {
84 // Ignore instructions only used by the loop terminator.
85 } else if (isa<DbgInfoIntrinsic>(I)) {
86 // Ignore debug instructions
87 } else if (isa<CallInst>(I)) {
88 // Estimate size overhead introduced by call instructions which
89 // is higher than other instructions. Here 3 and 10 are magic
90 // numbers that help one isolated test case from PR2067 without
91 // negatively impacting measured benchmarks.
92 if (isa<IntrinsicInst>(I))
100 // TODO: Ignore expressions derived from PHI and constants if inval of phi
101 // is a constant, or if operation is associative. This will get induction
109 bool LoopUnroll::runOnLoop(Loop *L, LPPassManager &LPM) {
110 assert(L->isLCSSAForm());
111 LoopInfo *LI = &getAnalysis<LoopInfo>();
113 BasicBlock *Header = L->getHeader();
114 DOUT << "Loop Unroll: F[" << Header->getParent()->getName()
115 << "] Loop %" << Header->getName() << "\n";
118 unsigned TripCount = L->getSmallConstantTripCount();
119 unsigned Count = UnrollCount;
121 // Automatically select an unroll count.
123 // Conservative heuristic: if we know the trip count, see if we can
124 // completely unroll (subject to the threshold, checked below); otherwise
126 if (TripCount != 0) {
133 // Enforce the threshold.
134 if (UnrollThreshold != NoThreshold) {
135 unsigned LoopSize = ApproximateLoopSize(L);
136 DOUT << " Loop Size = " << LoopSize << "\n";
137 uint64_t Size = (uint64_t)LoopSize*Count;
138 if (TripCount != 1 && Size > UnrollThreshold) {
139 DOUT << " TOO LARGE TO UNROLL: "
140 << Size << ">" << UnrollThreshold << "\n";
146 Function *F = L->getHeader()->getParent();
147 if (!UnrollLoop(L, Count, LI, &LPM))
150 // FIXME: Reconstruct dom info, because it is not preserved properly.
151 DominatorTree *DT = getAnalysisToUpdate<DominatorTree>();
153 DT->runOnFunction(*F);
154 DominanceFrontier *DF = getAnalysisToUpdate<DominanceFrontier>();
156 DF->runOnFunction(*F);