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/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"
29 static cl::opt<unsigned>
30 UnrollThreshold("unroll-threshold", cl::init(150), cl::Hidden,
31 cl::desc("The cut-off point for automatic loop unrolling"));
33 static cl::opt<unsigned>
34 UnrollCount("unroll-count", cl::init(0), cl::Hidden,
35 cl::desc("Use this unroll count for all loops, for testing purposes"));
38 UnrollAllowPartial("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."));
43 class LoopUnroll : public LoopPass {
45 static char ID; // Pass ID, replacement for typeid
46 LoopUnroll() : LoopPass(ID) {}
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;
53 // Threshold to use when optsize is specified (and there is no
54 // explicit -unroll-threshold).
55 static const unsigned OptSizeUnrollThreshold = 50;
57 unsigned CurrentThreshold;
59 bool runOnLoop(Loop *L, LPPassManager &LPM);
61 /// This transformation requires natural loop information & requires that
62 /// loop preheaders be inserted into the CFG...
64 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
65 AU.addRequired<LoopInfo>();
66 AU.addPreserved<LoopInfo>();
67 AU.addRequiredID(LoopSimplifyID);
68 AU.addPreservedID(LoopSimplifyID);
69 AU.addRequiredID(LCSSAID);
70 AU.addPreservedID(LCSSAID);
71 AU.addPreserved<ScalarEvolution>();
72 // FIXME: Loop unroll requires LCSSA. And LCSSA requires dom info.
73 // If loop unroll does not preserve dom info then LCSSA pass on next
74 // loop will receive invalid dom info.
75 // For now, recreate dom info, if loop is unrolled.
76 AU.addPreserved<DominatorTree>();
81 char LoopUnroll::ID = 0;
82 INITIALIZE_PASS(LoopUnroll, "loop-unroll", "Unroll loops", false, false);
84 Pass *llvm::createLoopUnrollPass() { return new LoopUnroll(); }
86 /// ApproximateLoopSize - Approximate the size of the loop.
87 static unsigned ApproximateLoopSize(const Loop *L, unsigned &NumCalls) {
89 for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
91 Metrics.analyzeBasicBlock(*I);
92 NumCalls = Metrics.NumInlineCandidates;
94 unsigned LoopSize = Metrics.NumInsts;
96 // If we can identify the induction variable, we know that it will become
97 // constant when we unroll the loop, so factor that into our loop size
99 // FIXME: We have to divide by InlineConstants::InstrCost because the
100 // measure returned by CountCodeReductionForConstant is not an instruction
101 // count, but rather a weight as defined by InlineConstants. It would
102 // probably be a good idea to standardize on a single weighting scheme by
103 // pushing more of the logic for weighting into CodeMetrics.
104 if (PHINode *IndVar = L->getCanonicalInductionVariable()) {
105 unsigned SizeDecrease = Metrics.CountCodeReductionForConstant(IndVar);
106 // NOTE: Because SizeDecrease is a fuzzy estimate, we don't want to allow
107 // it to totally negate the cost of unrolling a loop.
108 SizeDecrease = SizeDecrease > LoopSize / 2 ? LoopSize / 2 : SizeDecrease;
111 // Don't allow an estimate of size zero. This would allows unrolling of loops
112 // with huge iteration counts, which is a compile time problem even if it's
113 // not a problem for code quality.
114 if (LoopSize == 0) LoopSize = 1;
119 bool LoopUnroll::runOnLoop(Loop *L, LPPassManager &LPM) {
121 LoopInfo *LI = &getAnalysis<LoopInfo>();
123 BasicBlock *Header = L->getHeader();
124 DEBUG(dbgs() << "Loop Unroll: F[" << Header->getParent()->getName()
125 << "] Loop %" << Header->getName() << "\n");
128 // Determine the current unrolling threshold. While this is normally set
129 // from UnrollThreshold, it is overridden to a smaller value if the current
130 // function is marked as optimize-for-size, and the unroll threshold was
131 // not user specified.
132 CurrentThreshold = UnrollThreshold;
133 if (Header->getParent()->hasFnAttr(Attribute::OptimizeForSize) &&
134 UnrollThreshold.getNumOccurrences() == 0)
135 CurrentThreshold = OptSizeUnrollThreshold;
138 unsigned TripCount = L->getSmallConstantTripCount();
139 unsigned Count = UnrollCount;
141 // Automatically select an unroll count.
143 // Conservative heuristic: if we know the trip count, see if we can
144 // completely unroll (subject to the threshold, checked below); otherwise
145 // try to find greatest modulo of the trip count which is still under
152 // Enforce the threshold.
153 if (CurrentThreshold != NoThreshold) {
154 unsigned NumInlineCandidates;
155 unsigned LoopSize = ApproximateLoopSize(L, NumInlineCandidates);
156 DEBUG(dbgs() << " Loop Size = " << LoopSize << "\n");
157 if (NumInlineCandidates != 0) {
158 DEBUG(dbgs() << " Not unrolling loop with inlinable calls.\n");
161 uint64_t Size = (uint64_t)LoopSize*Count;
162 if (TripCount != 1 && Size > CurrentThreshold) {
163 DEBUG(dbgs() << " Too large to fully unroll with count: " << Count
164 << " because size: " << Size << ">" << CurrentThreshold << "\n");
165 if (!UnrollAllowPartial) {
166 DEBUG(dbgs() << " will not try to unroll partially because "
167 << "-unroll-allow-partial not given\n");
170 // Reduce unroll count to be modulo of TripCount for partial unrolling
171 Count = CurrentThreshold / LoopSize;
172 while (Count != 0 && TripCount%Count != 0) {
176 DEBUG(dbgs() << " could not unroll partially\n");
179 DEBUG(dbgs() << " partially unrolling with count: " << Count << "\n");
184 Function *F = L->getHeader()->getParent();
185 if (!UnrollLoop(L, Count, LI, &LPM))
188 // FIXME: Reconstruct dom info, because it is not preserved properly.
189 if (DominatorTree *DT = getAnalysisIfAvailable<DominatorTree>())
190 DT->runOnFunction(*F);