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 #include "llvm/Transforms/Scalar.h"
16 #include "llvm/Analysis/CodeMetrics.h"
17 #include "llvm/Analysis/LoopPass.h"
18 #include "llvm/Analysis/ScalarEvolution.h"
19 #include "llvm/Analysis/TargetTransformInfo.h"
20 #include "llvm/IR/DataLayout.h"
21 #include "llvm/IR/Dominators.h"
22 #include "llvm/IR/IntrinsicInst.h"
23 #include "llvm/IR/Metadata.h"
24 #include "llvm/Support/CommandLine.h"
25 #include "llvm/Support/Debug.h"
26 #include "llvm/Support/raw_ostream.h"
27 #include "llvm/Transforms/Utils/UnrollLoop.h"
32 #define DEBUG_TYPE "loop-unroll"
34 static cl::opt<unsigned>
35 UnrollThreshold("unroll-threshold", cl::init(150), cl::Hidden,
36 cl::desc("The cut-off point for automatic loop unrolling"));
38 static cl::opt<unsigned>
39 UnrollCount("unroll-count", cl::init(0), cl::Hidden,
40 cl::desc("Use this unroll count for all loops including those with "
41 "unroll_count pragma values, for testing purposes"));
44 UnrollAllowPartial("unroll-allow-partial", cl::init(false), cl::Hidden,
45 cl::desc("Allows loops to be partially unrolled until "
46 "-unroll-threshold loop size is reached."));
49 UnrollRuntime("unroll-runtime", cl::ZeroOrMore, cl::init(false), cl::Hidden,
50 cl::desc("Unroll loops with run-time trip counts"));
52 // Maximum allowed unroll count for a loop being fully unrolled
53 // because of a pragma unroll(enable) statement (ie, metadata
54 // "llvm.loopunroll.enable" is true). This prevents unexpected
55 // behavior like crashing when using this pragma on high trip count
57 static const unsigned PragmaFullUnrollCountLimit = 1024;
60 class LoopUnroll : public LoopPass {
62 static char ID; // Pass ID, replacement for typeid
63 LoopUnroll(int T = -1, int C = -1, int P = -1, int R = -1) : LoopPass(ID) {
64 CurrentThreshold = (T == -1) ? UnrollThreshold : unsigned(T);
65 CurrentCount = (C == -1) ? UnrollCount : unsigned(C);
66 CurrentAllowPartial = (P == -1) ? UnrollAllowPartial : (bool)P;
67 CurrentRuntime = (R == -1) ? UnrollRuntime : (bool)R;
69 UserThreshold = (T != -1) || (UnrollThreshold.getNumOccurrences() > 0);
70 UserAllowPartial = (P != -1) ||
71 (UnrollAllowPartial.getNumOccurrences() > 0);
72 UserRuntime = (R != -1) || (UnrollRuntime.getNumOccurrences() > 0);
73 UserCount = (C != -1) || (UnrollCount.getNumOccurrences() > 0);
75 initializeLoopUnrollPass(*PassRegistry::getPassRegistry());
78 /// A magic value for use with the Threshold parameter to indicate
79 /// that the loop unroll should be performed regardless of how much
80 /// code expansion would result.
81 static const unsigned NoThreshold = UINT_MAX;
83 // Threshold to use when optsize is specified (and there is no
84 // explicit -unroll-threshold).
85 static const unsigned OptSizeUnrollThreshold = 50;
87 // Default unroll count for loops with run-time trip count if
88 // -unroll-count is not set
89 static const unsigned UnrollRuntimeCount = 8;
91 unsigned CurrentCount;
92 unsigned CurrentThreshold;
93 bool CurrentAllowPartial;
95 bool UserCount; // CurrentCount is user-specified.
96 bool UserThreshold; // CurrentThreshold is user-specified.
97 bool UserAllowPartial; // CurrentAllowPartial is user-specified.
98 bool UserRuntime; // CurrentRuntime is user-specified.
100 bool runOnLoop(Loop *L, LPPassManager &LPM) override;
102 /// This transformation requires natural loop information & requires that
103 /// loop preheaders be inserted into the CFG...
105 void getAnalysisUsage(AnalysisUsage &AU) const override {
106 AU.addRequired<LoopInfo>();
107 AU.addPreserved<LoopInfo>();
108 AU.addRequiredID(LoopSimplifyID);
109 AU.addPreservedID(LoopSimplifyID);
110 AU.addRequiredID(LCSSAID);
111 AU.addPreservedID(LCSSAID);
112 AU.addRequired<ScalarEvolution>();
113 AU.addPreserved<ScalarEvolution>();
114 AU.addRequired<TargetTransformInfo>();
115 // FIXME: Loop unroll requires LCSSA. And LCSSA requires dom info.
116 // If loop unroll does not preserve dom info then LCSSA pass on next
117 // loop will receive invalid dom info.
118 // For now, recreate dom info, if loop is unrolled.
119 AU.addPreserved<DominatorTreeWrapperPass>();
124 char LoopUnroll::ID = 0;
125 INITIALIZE_PASS_BEGIN(LoopUnroll, "loop-unroll", "Unroll loops", false, false)
126 INITIALIZE_AG_DEPENDENCY(TargetTransformInfo)
127 INITIALIZE_PASS_DEPENDENCY(LoopInfo)
128 INITIALIZE_PASS_DEPENDENCY(LoopSimplify)
129 INITIALIZE_PASS_DEPENDENCY(LCSSA)
130 INITIALIZE_PASS_DEPENDENCY(ScalarEvolution)
131 INITIALIZE_PASS_END(LoopUnroll, "loop-unroll", "Unroll loops", false, false)
133 Pass *llvm::createLoopUnrollPass(int Threshold, int Count, int AllowPartial,
135 return new LoopUnroll(Threshold, Count, AllowPartial, Runtime);
138 Pass *llvm::createSimpleLoopUnrollPass() {
139 return llvm::createLoopUnrollPass(-1, -1, 0, 0);
142 /// ApproximateLoopSize - Approximate the size of the loop.
143 static unsigned ApproximateLoopSize(const Loop *L, unsigned &NumCalls,
144 bool &NotDuplicatable,
145 const TargetTransformInfo &TTI) {
147 for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
149 Metrics.analyzeBasicBlock(*I, TTI);
150 NumCalls = Metrics.NumInlineCandidates;
151 NotDuplicatable = Metrics.notDuplicatable;
153 unsigned LoopSize = Metrics.NumInsts;
155 // Don't allow an estimate of size zero. This would allows unrolling of loops
156 // with huge iteration counts, which is a compile time problem even if it's
157 // not a problem for code quality.
158 if (LoopSize == 0) LoopSize = 1;
163 // Returns the value associated with the given metadata node name (for
164 // example, "llvm.loopunroll.count"). If no such named metadata node
165 // exists, then nullptr is returned.
166 static const ConstantInt *GetUnrollMetadataValue(const Loop *L,
168 MDNode *LoopID = L->getLoopID();
169 if (!LoopID) return nullptr;
171 // First operand should refer to the loop id itself.
172 assert(LoopID->getNumOperands() > 0 && "requires at least one operand");
173 assert(LoopID->getOperand(0) == LoopID && "invalid loop id");
175 for (unsigned i = 1, e = LoopID->getNumOperands(); i < e; ++i) {
176 const MDNode *MD = dyn_cast<MDNode>(LoopID->getOperand(i));
179 const MDString *S = dyn_cast<MDString>(MD->getOperand(0));
182 if (Name.equals(S->getString())) {
183 assert(MD->getNumOperands() == 2 &&
184 "Unroll hint metadata should have two operands.");
185 return cast<ConstantInt>(MD->getOperand(1));
191 // Returns true if the loop has an unroll(enable) pragma.
192 static bool HasUnrollEnablePragma(const Loop *L) {
193 const ConstantInt *EnableValue =
194 GetUnrollMetadataValue(L, "llvm.loopunroll.enable");
195 return (EnableValue && EnableValue->getZExtValue());
199 // Returns true if the loop has an unroll(disable) pragma.
200 static bool HasUnrollDisablePragma(const Loop *L) {
201 const ConstantInt *EnableValue =
202 GetUnrollMetadataValue(L, "llvm.loopunroll.enable");
203 return (EnableValue && !EnableValue->getZExtValue());
207 // Check for unroll_count(N) pragma. If found, return true and set
208 // Count to the integer parameter of the pragma.
209 static bool HasUnrollCountPragma(const Loop *L, int &Count) {
210 const ConstantInt *CountValue =
211 GetUnrollMetadataValue(L, "llvm.loopunroll.count");
213 Count = CountValue->getZExtValue();
214 assert(Count >= 1 && "Unroll count must be positive.");
220 bool LoopUnroll::runOnLoop(Loop *L, LPPassManager &LPM) {
221 if (skipOptnoneFunction(L))
224 LoopInfo *LI = &getAnalysis<LoopInfo>();
225 ScalarEvolution *SE = &getAnalysis<ScalarEvolution>();
226 const TargetTransformInfo &TTI = getAnalysis<TargetTransformInfo>();
228 BasicBlock *Header = L->getHeader();
229 DEBUG(dbgs() << "Loop Unroll: F[" << Header->getParent()->getName()
230 << "] Loop %" << Header->getName() << "\n");
233 TargetTransformInfo::UnrollingPreferences UP;
234 UP.Threshold = CurrentThreshold;
235 UP.OptSizeThreshold = OptSizeUnrollThreshold;
236 UP.PartialThreshold = CurrentThreshold;
237 UP.PartialOptSizeThreshold = OptSizeUnrollThreshold;
238 UP.Count = CurrentCount;
239 UP.MaxCount = UINT_MAX;
240 UP.Partial = CurrentAllowPartial;
241 UP.Runtime = CurrentRuntime;
242 TTI.getUnrollingPreferences(L, UP);
244 // Determine the current unrolling threshold. While this is normally set
245 // from UnrollThreshold, it is overridden to a smaller value if the current
246 // function is marked as optimize-for-size, and the unroll threshold was
247 // not user specified.
248 unsigned Threshold = UserThreshold ? CurrentThreshold : UP.Threshold;
249 unsigned PartialThreshold =
250 UserThreshold ? CurrentThreshold : UP.PartialThreshold;
251 if (!UserThreshold &&
252 Header->getParent()->getAttributes().
253 hasAttribute(AttributeSet::FunctionIndex,
254 Attribute::OptimizeForSize)) {
255 Threshold = UP.OptSizeThreshold;
256 PartialThreshold = UP.PartialOptSizeThreshold;
259 // Find trip count and trip multiple if count is not available
260 unsigned TripCount = 0;
261 unsigned TripMultiple = 1;
262 // Find "latch trip count". UnrollLoop assumes that control cannot exit
263 // via the loop latch on any iteration prior to TripCount. The loop may exit
264 // early via an earlier branch.
265 BasicBlock *LatchBlock = L->getLoopLatch();
267 TripCount = SE->getSmallConstantTripCount(L, LatchBlock);
268 TripMultiple = SE->getSmallConstantTripMultiple(L, LatchBlock);
271 // User-specified count (either as a command-line option or
272 // constructor parameter) has highest precedence.
273 unsigned Count = UserCount ? CurrentCount : 0;
275 // If there is no user-specified count, unroll pragmas have the next
276 // highest precendence.
278 if (HasUnrollDisablePragma(L)) {
279 // Loop has unroll(disable) pragma.
284 if (HasUnrollCountPragma(L, PragmaCount)) {
285 if (PragmaCount == 1) {
290 Threshold = NoThreshold;
291 } else if (HasUnrollEnablePragma(L)) {
292 // Loop has unroll(enable) pragma without a unroll_count pragma,
293 // so unroll loop fully if possible.
294 if (TripCount == 0) {
295 DEBUG(dbgs() << " Loop has unroll(enable) pragma but loop cannot be "
296 "fully unrolled because trip count is unknown.\n");
297 // Continue with standard heuristic unrolling.
298 } else if (TripCount > PragmaFullUnrollCountLimit) {
299 DEBUG(dbgs() << " Loop has unroll(enable) pragma but loop cannot be "
300 "fully unrolled because loop count is greater than "
301 << PragmaFullUnrollCountLimit);
302 // Continue with standard heuristic unrolling.
305 Threshold = NoThreshold;
313 bool Runtime = UserRuntime ? CurrentRuntime : UP.Runtime;
314 if (Runtime && Count == 0 && TripCount == 0)
315 Count = UnrollRuntimeCount;
318 // Conservative heuristic: if we know the trip count, see if we can
319 // completely unroll (subject to the threshold, checked below); otherwise
320 // try to find greatest modulo of the trip count which is still under
327 // Enforce the threshold.
328 if (Threshold != NoThreshold && PartialThreshold != NoThreshold) {
329 unsigned NumInlineCandidates;
330 bool notDuplicatable;
331 unsigned LoopSize = ApproximateLoopSize(L, NumInlineCandidates,
332 notDuplicatable, TTI);
333 DEBUG(dbgs() << " Loop Size = " << LoopSize << "\n");
334 if (notDuplicatable) {
335 DEBUG(dbgs() << " Not unrolling loop which contains non-duplicatable"
336 << " instructions.\n");
339 if (NumInlineCandidates != 0) {
340 DEBUG(dbgs() << " Not unrolling loop with inlinable calls.\n");
343 uint64_t Size = (uint64_t)LoopSize*Count;
344 if (TripCount != 1 &&
345 (Size > Threshold || (Count != TripCount && Size > PartialThreshold))) {
346 if (Size > Threshold)
347 DEBUG(dbgs() << " Too large to fully unroll with count: " << Count
348 << " because size: " << Size << ">" << Threshold << "\n");
350 bool AllowPartial = UserAllowPartial ? CurrentAllowPartial : UP.Partial;
351 if (!AllowPartial && !(Runtime && TripCount == 0)) {
352 DEBUG(dbgs() << " will not try to unroll partially because "
353 << "-unroll-allow-partial not given\n");
357 // Reduce unroll count to be modulo of TripCount for partial unrolling
358 Count = PartialThreshold / LoopSize;
359 while (Count != 0 && TripCount%Count != 0)
363 // Reduce unroll count to be a lower power-of-two value
364 while (Count != 0 && Size > PartialThreshold) {
366 Size = LoopSize*Count;
369 if (Count > UP.MaxCount)
372 DEBUG(dbgs() << " could not unroll partially\n");
375 DEBUG(dbgs() << " partially unrolling with count: " << Count << "\n");
380 if (!UnrollLoop(L, Count, TripCount, Runtime, TripMultiple, LI, this, &LPM))