LoopUnrollPass.cpp revision 90c579de5a383cee278acc3f7e7b9d0a656e6a35 
1//===-- LoopUnroll.cpp - Loop unroller pass -------------------------------===//
2//
3//                     The LLVM Compiler Infrastructure
4//
5// This file is distributed under the University of Illinois Open Source
6// License. See LICENSE.TXT for details.
7//
8//===----------------------------------------------------------------------===//
9//
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//===----------------------------------------------------------------------===//
14
15#define DEBUG_TYPE "loop-unroll"
16#include "llvm/IntrinsicInst.h"
17#include "llvm/Transforms/Scalar.h"
18#include "llvm/Analysis/LoopPass.h"
19#include "llvm/Analysis/InlineCost.h"
20#include "llvm/Analysis/ScalarEvolution.h"
21#include "llvm/Support/CommandLine.h"
22#include "llvm/Support/Debug.h"
23#include "llvm/Support/raw_ostream.h"
24#include "llvm/Transforms/Utils/UnrollLoop.h"
25#include <climits>
26
27using namespace llvm;
28
29static cl::opt<unsigned>
30UnrollThreshold("unroll-threshold", cl::init(200), cl::Hidden,
31  cl::desc("The cut-off point for automatic loop unrolling"));
32
33static cl::opt<unsigned>
34UnrollCount("unroll-count", cl::init(0), cl::Hidden,
35  cl::desc("Use this unroll count for all loops, for testing purposes"));
36
37static cl::opt<bool>
38UnrollAllowPartial("unroll-allow-partial", cl::init(false), cl::Hidden,
39  cl::desc("Allows loops to be partially unrolled until "
40           "-unroll-threshold loop size is reached."));
41
42namespace {
43  class LoopUnroll : public LoopPass {
44  public:
45    static char ID; // Pass ID, replacement for typeid
46    LoopUnroll() : LoopPass(ID) {}
47
48    /// A magic value for use with the Threshold parameter to indicate
49    /// that the loop unroll should be performed regardless of how much
50    /// code expansion would result.
51    static const unsigned NoThreshold = UINT_MAX;
52
53    bool runOnLoop(Loop *L, LPPassManager &LPM);
54
55    /// This transformation requires natural loop information & requires that
56    /// loop preheaders be inserted into the CFG...
57    ///
58    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
59      AU.addRequired<LoopInfo>();
60      AU.addPreserved<LoopInfo>();
61      AU.addRequiredID(LoopSimplifyID);
62      AU.addPreservedID(LoopSimplifyID);
63      AU.addRequiredID(LCSSAID);
64      AU.addPreservedID(LCSSAID);
65      // FIXME: Loop unroll requires LCSSA. And LCSSA requires dom info.
66      // If loop unroll does not preserve dom info then LCSSA pass on next
67      // loop will receive invalid dom info.
68      // For now, recreate dom info, if loop is unrolled.
69      AU.addPreserved<DominatorTree>();
70      AU.addPreserved<DominanceFrontier>();
71      AU.addPreserved<ScalarEvolution>();
72    }
73  };
74}
75
76char LoopUnroll::ID = 0;
77INITIALIZE_PASS(LoopUnroll, "loop-unroll", "Unroll loops", false, false);
78
79Pass *llvm::createLoopUnrollPass() { return new LoopUnroll(); }
80
81/// ApproximateLoopSize - Approximate the size of the loop.
82static unsigned ApproximateLoopSize(const Loop *L, unsigned &NumCalls) {
83  CodeMetrics Metrics;
84  for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
85       I != E; ++I)
86    Metrics.analyzeBasicBlock(*I);
87  NumCalls = Metrics.NumCalls;
88  return Metrics.NumInsts;
89}
90
91bool LoopUnroll::runOnLoop(Loop *L, LPPassManager &LPM) {
92  LoopInfo *LI = &getAnalysis<LoopInfo>();
93
94  BasicBlock *Header = L->getHeader();
95  DEBUG(dbgs() << "Loop Unroll: F[" << Header->getParent()->getName()
96        << "] Loop %" << Header->getName() << "\n");
97  (void)Header;
98
99  // Find trip count
100  unsigned TripCount = L->getSmallConstantTripCount();
101  unsigned Count = UnrollCount;
102
103  // Automatically select an unroll count.
104  if (Count == 0) {
105    // Conservative heuristic: if we know the trip count, see if we can
106    // completely unroll (subject to the threshold, checked below); otherwise
107    // try to find greatest modulo of the trip count which is still under
108    // threshold value.
109    if (TripCount == 0)
110      return false;
111    Count = TripCount;
112  }
113
114  // Enforce the threshold.
115  if (UnrollThreshold != NoThreshold) {
116    unsigned NumCalls;
117    unsigned LoopSize = ApproximateLoopSize(L, NumCalls);
118    DEBUG(dbgs() << "  Loop Size = " << LoopSize << "\n");
119    if (NumCalls != 0) {
120      DEBUG(dbgs() << "  Not unrolling loop with function calls.\n");
121      return false;
122    }
123    uint64_t Size = (uint64_t)LoopSize*Count;
124    if (TripCount != 1 && Size > UnrollThreshold) {
125      DEBUG(dbgs() << "  Too large to fully unroll with count: " << Count
126            << " because size: " << Size << ">" << UnrollThreshold << "\n");
127      if (!UnrollAllowPartial) {
128        DEBUG(dbgs() << "  will not try to unroll partially because "
129              << "-unroll-allow-partial not given\n");
130        return false;
131      }
132      // Reduce unroll count to be modulo of TripCount for partial unrolling
133      Count = UnrollThreshold / LoopSize;
134      while (Count != 0 && TripCount%Count != 0) {
135        Count--;
136      }
137      if (Count < 2) {
138        DEBUG(dbgs() << "  could not unroll partially\n");
139        return false;
140      }
141      DEBUG(dbgs() << "  partially unrolling with count: " << Count << "\n");
142    }
143  }
144
145  // Unroll the loop.
146  Function *F = L->getHeader()->getParent();
147  if (!UnrollLoop(L, Count, LI, &LPM))
148    return false;
149
150  // FIXME: Reconstruct dom info, because it is not preserved properly.
151  DominatorTree *DT = getAnalysisIfAvailable<DominatorTree>();
152  if (DT) {
153    DT->runOnFunction(*F);
154    DominanceFrontier *DF = getAnalysisIfAvailable<DominanceFrontier>();
155    if (DF)
156      DF->runOnFunction(*F);
157  }
158  return true;
159}
160