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/Transforms/Scalar.h"
17 #include "llvm/Analysis/CodeMetrics.h"
18 #include "llvm/Analysis/LoopPass.h"
19 #include "llvm/Analysis/ScalarEvolution.h"
20 #include "llvm/Analysis/TargetTransformInfo.h"
21 #include "llvm/IR/DataLayout.h"
22 #include "llvm/IR/IntrinsicInst.h"
23 #include "llvm/Support/CommandLine.h"
24 #include "llvm/Support/Debug.h"
25 #include "llvm/Support/raw_ostream.h"
26 #include "llvm/Transforms/Utils/UnrollLoop.h"
31 static cl::opt<unsigned>
32 UnrollThreshold("unroll-threshold", cl::init(150), cl::Hidden,
33 cl::desc("The cut-off point for automatic loop unrolling"));
35 static cl::opt<unsigned>
36 UnrollCount("unroll-count", cl::init(0), cl::Hidden,
37 cl::desc("Use this unroll count for all loops, for testing purposes"));
40 UnrollAllowPartial("unroll-allow-partial", cl::init(false), cl::Hidden,
41 cl::desc("Allows loops to be partially unrolled until "
42 "-unroll-threshold loop size is reached."));
45 UnrollRuntime("unroll-runtime", cl::ZeroOrMore, cl::init(false), cl::Hidden,
46 cl::desc("Unroll loops with run-time trip counts"));
49 class LoopUnroll : public LoopPass {
51 static char ID; // Pass ID, replacement for typeid
52 LoopUnroll(int T = -1, int C = -1, int P = -1) : LoopPass(ID) {
53 CurrentThreshold = (T == -1) ? UnrollThreshold : unsigned(T);
54 CurrentCount = (C == -1) ? UnrollCount : unsigned(C);
55 CurrentAllowPartial = (P == -1) ? UnrollAllowPartial : (bool)P;
57 UserThreshold = (T != -1) || (UnrollThreshold.getNumOccurrences() > 0);
59 initializeLoopUnrollPass(*PassRegistry::getPassRegistry());
62 /// A magic value for use with the Threshold parameter to indicate
63 /// that the loop unroll should be performed regardless of how much
64 /// code expansion would result.
65 static const unsigned NoThreshold = UINT_MAX;
67 // Threshold to use when optsize is specified (and there is no
68 // explicit -unroll-threshold).
69 static const unsigned OptSizeUnrollThreshold = 50;
71 // Default unroll count for loops with run-time trip count if
72 // -unroll-count is not set
73 static const unsigned UnrollRuntimeCount = 8;
75 unsigned CurrentCount;
76 unsigned CurrentThreshold;
77 bool CurrentAllowPartial;
78 bool UserThreshold; // CurrentThreshold is user-specified.
80 bool runOnLoop(Loop *L, LPPassManager &LPM);
82 /// This transformation requires natural loop information & requires that
83 /// loop preheaders be inserted into the CFG...
85 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
86 AU.addRequired<LoopInfo>();
87 AU.addPreserved<LoopInfo>();
88 AU.addRequiredID(LoopSimplifyID);
89 AU.addPreservedID(LoopSimplifyID);
90 AU.addRequiredID(LCSSAID);
91 AU.addPreservedID(LCSSAID);
92 AU.addRequired<ScalarEvolution>();
93 AU.addPreserved<ScalarEvolution>();
94 AU.addRequired<TargetTransformInfo>();
95 // FIXME: Loop unroll requires LCSSA. And LCSSA requires dom info.
96 // If loop unroll does not preserve dom info then LCSSA pass on next
97 // loop will receive invalid dom info.
98 // For now, recreate dom info, if loop is unrolled.
99 AU.addPreserved<DominatorTree>();
104 char LoopUnroll::ID = 0;
105 INITIALIZE_PASS_BEGIN(LoopUnroll, "loop-unroll", "Unroll loops", false, false)
106 INITIALIZE_AG_DEPENDENCY(TargetTransformInfo)
107 INITIALIZE_PASS_DEPENDENCY(LoopInfo)
108 INITIALIZE_PASS_DEPENDENCY(LoopSimplify)
109 INITIALIZE_PASS_DEPENDENCY(LCSSA)
110 INITIALIZE_PASS_DEPENDENCY(ScalarEvolution)
111 INITIALIZE_PASS_END(LoopUnroll, "loop-unroll", "Unroll loops", false, false)
113 Pass *llvm::createLoopUnrollPass(int Threshold, int Count, int AllowPartial) {
114 return new LoopUnroll(Threshold, Count, AllowPartial);
117 /// ApproximateLoopSize - Approximate the size of the loop.
118 static unsigned ApproximateLoopSize(const Loop *L, unsigned &NumCalls,
119 bool &NotDuplicatable,
120 const TargetTransformInfo &TTI) {
122 for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
124 Metrics.analyzeBasicBlock(*I, TTI);
125 NumCalls = Metrics.NumInlineCandidates;
126 NotDuplicatable = Metrics.notDuplicatable;
128 unsigned LoopSize = Metrics.NumInsts;
130 // Don't allow an estimate of size zero. This would allows unrolling of loops
131 // with huge iteration counts, which is a compile time problem even if it's
132 // not a problem for code quality.
133 if (LoopSize == 0) LoopSize = 1;
138 bool LoopUnroll::runOnLoop(Loop *L, LPPassManager &LPM) {
139 LoopInfo *LI = &getAnalysis<LoopInfo>();
140 ScalarEvolution *SE = &getAnalysis<ScalarEvolution>();
141 const TargetTransformInfo &TTI = getAnalysis<TargetTransformInfo>();
143 BasicBlock *Header = L->getHeader();
144 DEBUG(dbgs() << "Loop Unroll: F[" << Header->getParent()->getName()
145 << "] Loop %" << Header->getName() << "\n");
148 // Determine the current unrolling threshold. While this is normally set
149 // from UnrollThreshold, it is overridden to a smaller value if the current
150 // function is marked as optimize-for-size, and the unroll threshold was
151 // not user specified.
152 unsigned Threshold = CurrentThreshold;
153 if (!UserThreshold &&
154 Header->getParent()->getAttributes().
155 hasAttribute(AttributeSet::FunctionIndex,
156 Attribute::OptimizeForSize))
157 Threshold = OptSizeUnrollThreshold;
159 // Find trip count and trip multiple if count is not available
160 unsigned TripCount = 0;
161 unsigned TripMultiple = 1;
162 // Find "latch trip count". UnrollLoop assumes that control cannot exit
163 // via the loop latch on any iteration prior to TripCount. The loop may exit
164 // early via an earlier branch.
165 BasicBlock *LatchBlock = L->getLoopLatch();
167 TripCount = SE->getSmallConstantTripCount(L, LatchBlock);
168 TripMultiple = SE->getSmallConstantTripMultiple(L, LatchBlock);
170 // Use a default unroll-count if the user doesn't specify a value
171 // and the trip count is a run-time value. The default is different
172 // for run-time or compile-time trip count loops.
173 unsigned Count = CurrentCount;
174 if (UnrollRuntime && CurrentCount == 0 && TripCount == 0)
175 Count = UnrollRuntimeCount;
178 // Conservative heuristic: if we know the trip count, see if we can
179 // completely unroll (subject to the threshold, checked below); otherwise
180 // try to find greatest modulo of the trip count which is still under
187 // Enforce the threshold.
188 if (Threshold != NoThreshold) {
189 unsigned NumInlineCandidates;
190 bool notDuplicatable;
191 unsigned LoopSize = ApproximateLoopSize(L, NumInlineCandidates,
192 notDuplicatable, TTI);
193 DEBUG(dbgs() << " Loop Size = " << LoopSize << "\n");
194 if (notDuplicatable) {
195 DEBUG(dbgs() << " Not unrolling loop which contains non duplicatable"
196 << " instructions.\n");
199 if (NumInlineCandidates != 0) {
200 DEBUG(dbgs() << " Not unrolling loop with inlinable calls.\n");
203 uint64_t Size = (uint64_t)LoopSize*Count;
204 if (TripCount != 1 && Size > Threshold) {
205 DEBUG(dbgs() << " Too large to fully unroll with count: " << Count
206 << " because size: " << Size << ">" << Threshold << "\n");
207 if (!CurrentAllowPartial && !(UnrollRuntime && TripCount == 0)) {
208 DEBUG(dbgs() << " will not try to unroll partially because "
209 << "-unroll-allow-partial not given\n");
213 // Reduce unroll count to be modulo of TripCount for partial unrolling
214 Count = Threshold / LoopSize;
215 while (Count != 0 && TripCount%Count != 0)
218 else if (UnrollRuntime) {
219 // Reduce unroll count to be a lower power-of-two value
220 while (Count != 0 && Size > Threshold) {
222 Size = LoopSize*Count;
226 DEBUG(dbgs() << " could not unroll partially\n");
229 DEBUG(dbgs() << " partially unrolling with count: " << Count << "\n");
234 if (!UnrollLoop(L, Count, TripCount, UnrollRuntime, TripMultiple, LI, &LPM))