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 #include "llvm/Transforms/Scalar.h" 16 #include "llvm/Analysis/AssumptionCache.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/DiagnosticInfo.h" 23 #include "llvm/IR/Dominators.h" 24 #include "llvm/IR/IntrinsicInst.h" 25 #include "llvm/IR/Metadata.h" 26 #include "llvm/Support/CommandLine.h" 27 #include "llvm/Support/Debug.h" 28 #include "llvm/Support/raw_ostream.h" 29 #include "llvm/Transforms/Utils/UnrollLoop.h" 30 #include <climits> 31 32 using namespace llvm; 33 34 #define DEBUG_TYPE "loop-unroll" 35 36 static cl::opt<unsigned> 37 UnrollThreshold("unroll-threshold", cl::init(150), cl::Hidden, 38 cl::desc("The cut-off point for automatic loop unrolling")); 39 40 static cl::opt<unsigned> 41 UnrollCount("unroll-count", cl::init(0), cl::Hidden, 42 cl::desc("Use this unroll count for all loops including those with " 43 "unroll_count pragma values, for testing purposes")); 44 45 static cl::opt<bool> 46 UnrollAllowPartial("unroll-allow-partial", cl::init(false), cl::Hidden, 47 cl::desc("Allows loops to be partially unrolled until " 48 "-unroll-threshold loop size is reached.")); 49 50 static cl::opt<bool> 51 UnrollRuntime("unroll-runtime", cl::ZeroOrMore, cl::init(false), cl::Hidden, 52 cl::desc("Unroll loops with run-time trip counts")); 53 54 static cl::opt<unsigned> 55 PragmaUnrollThreshold("pragma-unroll-threshold", cl::init(16 * 1024), cl::Hidden, 56 cl::desc("Unrolled size limit for loops with an unroll(full) or " 57 "unroll_count pragma.")); 58 59 namespace { 60 class LoopUnroll : public LoopPass { 61 public: 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; 68 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); 74 75 initializeLoopUnrollPass(*PassRegistry::getPassRegistry()); 76 } 77 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; 82 83 // Threshold to use when optsize is specified (and there is no 84 // explicit -unroll-threshold). 85 static const unsigned OptSizeUnrollThreshold = 50; 86 87 // Default unroll count for loops with run-time trip count if 88 // -unroll-count is not set 89 static const unsigned UnrollRuntimeCount = 8; 90 91 unsigned CurrentCount; 92 unsigned CurrentThreshold; 93 bool CurrentAllowPartial; 94 bool CurrentRuntime; 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. 99 100 bool runOnLoop(Loop *L, LPPassManager &LPM) override; 101 102 /// This transformation requires natural loop information & requires that 103 /// loop preheaders be inserted into the CFG... 104 /// 105 void getAnalysisUsage(AnalysisUsage &AU) const override { 106 AU.addRequired<AssumptionCacheTracker>(); 107 AU.addRequired<LoopInfoWrapperPass>(); 108 AU.addPreserved<LoopInfoWrapperPass>(); 109 AU.addRequiredID(LoopSimplifyID); 110 AU.addPreservedID(LoopSimplifyID); 111 AU.addRequiredID(LCSSAID); 112 AU.addPreservedID(LCSSAID); 113 AU.addRequired<ScalarEvolution>(); 114 AU.addPreserved<ScalarEvolution>(); 115 AU.addRequired<TargetTransformInfoWrapperPass>(); 116 // FIXME: Loop unroll requires LCSSA. And LCSSA requires dom info. 117 // If loop unroll does not preserve dom info then LCSSA pass on next 118 // loop will receive invalid dom info. 119 // For now, recreate dom info, if loop is unrolled. 120 AU.addPreserved<DominatorTreeWrapperPass>(); 121 } 122 123 // Fill in the UnrollingPreferences parameter with values from the 124 // TargetTransformationInfo. 125 void getUnrollingPreferences(Loop *L, const TargetTransformInfo &TTI, 126 TargetTransformInfo::UnrollingPreferences &UP) { 127 UP.Threshold = CurrentThreshold; 128 UP.OptSizeThreshold = OptSizeUnrollThreshold; 129 UP.PartialThreshold = CurrentThreshold; 130 UP.PartialOptSizeThreshold = OptSizeUnrollThreshold; 131 UP.Count = CurrentCount; 132 UP.MaxCount = UINT_MAX; 133 UP.Partial = CurrentAllowPartial; 134 UP.Runtime = CurrentRuntime; 135 TTI.getUnrollingPreferences(L, UP); 136 } 137 138 // Select and return an unroll count based on parameters from 139 // user, unroll preferences, unroll pragmas, or a heuristic. 140 // SetExplicitly is set to true if the unroll count is is set by 141 // the user or a pragma rather than selected heuristically. 142 unsigned 143 selectUnrollCount(const Loop *L, unsigned TripCount, bool PragmaFullUnroll, 144 unsigned PragmaCount, 145 const TargetTransformInfo::UnrollingPreferences &UP, 146 bool &SetExplicitly); 147 148 // Select threshold values used to limit unrolling based on a 149 // total unrolled size. Parameters Threshold and PartialThreshold 150 // are set to the maximum unrolled size for fully and partially 151 // unrolled loops respectively. 152 void selectThresholds(const Loop *L, bool HasPragma, 153 const TargetTransformInfo::UnrollingPreferences &UP, 154 unsigned &Threshold, unsigned &PartialThreshold) { 155 // Determine the current unrolling threshold. While this is 156 // normally set from UnrollThreshold, it is overridden to a 157 // smaller value if the current function is marked as 158 // optimize-for-size, and the unroll threshold was not user 159 // specified. 160 Threshold = UserThreshold ? CurrentThreshold : UP.Threshold; 161 PartialThreshold = UserThreshold ? CurrentThreshold : UP.PartialThreshold; 162 if (!UserThreshold && 163 L->getHeader()->getParent()->getAttributes(). 164 hasAttribute(AttributeSet::FunctionIndex, 165 Attribute::OptimizeForSize)) { 166 Threshold = UP.OptSizeThreshold; 167 PartialThreshold = UP.PartialOptSizeThreshold; 168 } 169 if (HasPragma) { 170 // If the loop has an unrolling pragma, we want to be more 171 // aggressive with unrolling limits. Set thresholds to at 172 // least the PragmaTheshold value which is larger than the 173 // default limits. 174 if (Threshold != NoThreshold) 175 Threshold = std::max<unsigned>(Threshold, PragmaUnrollThreshold); 176 if (PartialThreshold != NoThreshold) 177 PartialThreshold = 178 std::max<unsigned>(PartialThreshold, PragmaUnrollThreshold); 179 } 180 } 181 }; 182 } 183 184 char LoopUnroll::ID = 0; 185 INITIALIZE_PASS_BEGIN(LoopUnroll, "loop-unroll", "Unroll loops", false, false) 186 INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass) 187 INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker) 188 INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass) 189 INITIALIZE_PASS_DEPENDENCY(LoopSimplify) 190 INITIALIZE_PASS_DEPENDENCY(LCSSA) 191 INITIALIZE_PASS_DEPENDENCY(ScalarEvolution) 192 INITIALIZE_PASS_END(LoopUnroll, "loop-unroll", "Unroll loops", false, false) 193 194 Pass *llvm::createLoopUnrollPass(int Threshold, int Count, int AllowPartial, 195 int Runtime) { 196 return new LoopUnroll(Threshold, Count, AllowPartial, Runtime); 197 } 198 199 Pass *llvm::createSimpleLoopUnrollPass() { 200 return llvm::createLoopUnrollPass(-1, -1, 0, 0); 201 } 202 203 /// ApproximateLoopSize - Approximate the size of the loop. 204 static unsigned ApproximateLoopSize(const Loop *L, unsigned &NumCalls, 205 bool &NotDuplicatable, 206 const TargetTransformInfo &TTI, 207 AssumptionCache *AC) { 208 SmallPtrSet<const Value *, 32> EphValues; 209 CodeMetrics::collectEphemeralValues(L, AC, EphValues); 210 211 CodeMetrics Metrics; 212 for (Loop::block_iterator I = L->block_begin(), E = L->block_end(); 213 I != E; ++I) 214 Metrics.analyzeBasicBlock(*I, TTI, EphValues); 215 NumCalls = Metrics.NumInlineCandidates; 216 NotDuplicatable = Metrics.notDuplicatable; 217 218 unsigned LoopSize = Metrics.NumInsts; 219 220 // Don't allow an estimate of size zero. This would allows unrolling of loops 221 // with huge iteration counts, which is a compile time problem even if it's 222 // not a problem for code quality. Also, the code using this size may assume 223 // that each loop has at least three instructions (likely a conditional 224 // branch, a comparison feeding that branch, and some kind of loop increment 225 // feeding that comparison instruction). 226 LoopSize = std::max(LoopSize, 3u); 227 228 return LoopSize; 229 } 230 231 // Returns the loop hint metadata node with the given name (for example, 232 // "llvm.loop.unroll.count"). If no such metadata node exists, then nullptr is 233 // returned. 234 static MDNode *GetUnrollMetadataForLoop(const Loop *L, StringRef Name) { 235 if (MDNode *LoopID = L->getLoopID()) 236 return GetUnrollMetadata(LoopID, Name); 237 return nullptr; 238 } 239 240 // Returns true if the loop has an unroll(full) pragma. 241 static bool HasUnrollFullPragma(const Loop *L) { 242 return GetUnrollMetadataForLoop(L, "llvm.loop.unroll.full"); 243 } 244 245 // Returns true if the loop has an unroll(disable) pragma. 246 static bool HasUnrollDisablePragma(const Loop *L) { 247 return GetUnrollMetadataForLoop(L, "llvm.loop.unroll.disable"); 248 } 249 250 // If loop has an unroll_count pragma return the (necessarily 251 // positive) value from the pragma. Otherwise return 0. 252 static unsigned UnrollCountPragmaValue(const Loop *L) { 253 MDNode *MD = GetUnrollMetadataForLoop(L, "llvm.loop.unroll.count"); 254 if (MD) { 255 assert(MD->getNumOperands() == 2 && 256 "Unroll count hint metadata should have two operands."); 257 unsigned Count = 258 mdconst::extract<ConstantInt>(MD->getOperand(1))->getZExtValue(); 259 assert(Count >= 1 && "Unroll count must be positive."); 260 return Count; 261 } 262 return 0; 263 } 264 265 // Remove existing unroll metadata and add unroll disable metadata to 266 // indicate the loop has already been unrolled. This prevents a loop 267 // from being unrolled more than is directed by a pragma if the loop 268 // unrolling pass is run more than once (which it generally is). 269 static void SetLoopAlreadyUnrolled(Loop *L) { 270 MDNode *LoopID = L->getLoopID(); 271 if (!LoopID) return; 272 273 // First remove any existing loop unrolling metadata. 274 SmallVector<Metadata *, 4> MDs; 275 // Reserve first location for self reference to the LoopID metadata node. 276 MDs.push_back(nullptr); 277 for (unsigned i = 1, ie = LoopID->getNumOperands(); i < ie; ++i) { 278 bool IsUnrollMetadata = false; 279 MDNode *MD = dyn_cast<MDNode>(LoopID->getOperand(i)); 280 if (MD) { 281 const MDString *S = dyn_cast<MDString>(MD->getOperand(0)); 282 IsUnrollMetadata = S && S->getString().startswith("llvm.loop.unroll."); 283 } 284 if (!IsUnrollMetadata) 285 MDs.push_back(LoopID->getOperand(i)); 286 } 287 288 // Add unroll(disable) metadata to disable future unrolling. 289 LLVMContext &Context = L->getHeader()->getContext(); 290 SmallVector<Metadata *, 1> DisableOperands; 291 DisableOperands.push_back(MDString::get(Context, "llvm.loop.unroll.disable")); 292 MDNode *DisableNode = MDNode::get(Context, DisableOperands); 293 MDs.push_back(DisableNode); 294 295 MDNode *NewLoopID = MDNode::get(Context, MDs); 296 // Set operand 0 to refer to the loop id itself. 297 NewLoopID->replaceOperandWith(0, NewLoopID); 298 L->setLoopID(NewLoopID); 299 } 300 301 unsigned LoopUnroll::selectUnrollCount( 302 const Loop *L, unsigned TripCount, bool PragmaFullUnroll, 303 unsigned PragmaCount, const TargetTransformInfo::UnrollingPreferences &UP, 304 bool &SetExplicitly) { 305 SetExplicitly = true; 306 307 // User-specified count (either as a command-line option or 308 // constructor parameter) has highest precedence. 309 unsigned Count = UserCount ? CurrentCount : 0; 310 311 // If there is no user-specified count, unroll pragmas have the next 312 // highest precendence. 313 if (Count == 0) { 314 if (PragmaCount) { 315 Count = PragmaCount; 316 } else if (PragmaFullUnroll) { 317 Count = TripCount; 318 } 319 } 320 321 if (Count == 0) 322 Count = UP.Count; 323 324 if (Count == 0) { 325 SetExplicitly = false; 326 if (TripCount == 0) 327 // Runtime trip count. 328 Count = UnrollRuntimeCount; 329 else 330 // Conservative heuristic: if we know the trip count, see if we can 331 // completely unroll (subject to the threshold, checked below); otherwise 332 // try to find greatest modulo of the trip count which is still under 333 // threshold value. 334 Count = TripCount; 335 } 336 if (TripCount && Count > TripCount) 337 return TripCount; 338 return Count; 339 } 340 341 bool LoopUnroll::runOnLoop(Loop *L, LPPassManager &LPM) { 342 if (skipOptnoneFunction(L)) 343 return false; 344 345 Function &F = *L->getHeader()->getParent(); 346 347 LoopInfo *LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo(); 348 ScalarEvolution *SE = &getAnalysis<ScalarEvolution>(); 349 const TargetTransformInfo &TTI = 350 getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F); 351 auto &AC = getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F); 352 353 BasicBlock *Header = L->getHeader(); 354 DEBUG(dbgs() << "Loop Unroll: F[" << Header->getParent()->getName() 355 << "] Loop %" << Header->getName() << "\n"); 356 357 if (HasUnrollDisablePragma(L)) { 358 return false; 359 } 360 bool PragmaFullUnroll = HasUnrollFullPragma(L); 361 unsigned PragmaCount = UnrollCountPragmaValue(L); 362 bool HasPragma = PragmaFullUnroll || PragmaCount > 0; 363 364 TargetTransformInfo::UnrollingPreferences UP; 365 getUnrollingPreferences(L, TTI, UP); 366 367 // Find trip count and trip multiple if count is not available 368 unsigned TripCount = 0; 369 unsigned TripMultiple = 1; 370 // If there are multiple exiting blocks but one of them is the latch, use the 371 // latch for the trip count estimation. Otherwise insist on a single exiting 372 // block for the trip count estimation. 373 BasicBlock *ExitingBlock = L->getLoopLatch(); 374 if (!ExitingBlock || !L->isLoopExiting(ExitingBlock)) 375 ExitingBlock = L->getExitingBlock(); 376 if (ExitingBlock) { 377 TripCount = SE->getSmallConstantTripCount(L, ExitingBlock); 378 TripMultiple = SE->getSmallConstantTripMultiple(L, ExitingBlock); 379 } 380 381 // Select an initial unroll count. This may be reduced later based 382 // on size thresholds. 383 bool CountSetExplicitly; 384 unsigned Count = selectUnrollCount(L, TripCount, PragmaFullUnroll, 385 PragmaCount, UP, CountSetExplicitly); 386 387 unsigned NumInlineCandidates; 388 bool notDuplicatable; 389 unsigned LoopSize = 390 ApproximateLoopSize(L, NumInlineCandidates, notDuplicatable, TTI, &AC); 391 DEBUG(dbgs() << " Loop Size = " << LoopSize << "\n"); 392 393 // When computing the unrolled size, note that the conditional branch on the 394 // backedge and the comparison feeding it are not replicated like the rest of 395 // the loop body (which is why 2 is subtracted). 396 uint64_t UnrolledSize = (uint64_t)(LoopSize-2) * Count + 2; 397 if (notDuplicatable) { 398 DEBUG(dbgs() << " Not unrolling loop which contains non-duplicatable" 399 << " instructions.\n"); 400 return false; 401 } 402 if (NumInlineCandidates != 0) { 403 DEBUG(dbgs() << " Not unrolling loop with inlinable calls.\n"); 404 return false; 405 } 406 407 unsigned Threshold, PartialThreshold; 408 selectThresholds(L, HasPragma, UP, Threshold, PartialThreshold); 409 410 // Given Count, TripCount and thresholds determine the type of 411 // unrolling which is to be performed. 412 enum { Full = 0, Partial = 1, Runtime = 2 }; 413 int Unrolling; 414 if (TripCount && Count == TripCount) { 415 if (Threshold != NoThreshold && UnrolledSize > Threshold) { 416 DEBUG(dbgs() << " Too large to fully unroll with count: " << Count 417 << " because size: " << UnrolledSize << ">" << Threshold 418 << "\n"); 419 Unrolling = Partial; 420 } else { 421 Unrolling = Full; 422 } 423 } else if (TripCount && Count < TripCount) { 424 Unrolling = Partial; 425 } else { 426 Unrolling = Runtime; 427 } 428 429 // Reduce count based on the type of unrolling and the threshold values. 430 unsigned OriginalCount = Count; 431 bool AllowRuntime = UserRuntime ? CurrentRuntime : UP.Runtime; 432 if (Unrolling == Partial) { 433 bool AllowPartial = UserAllowPartial ? CurrentAllowPartial : UP.Partial; 434 if (!AllowPartial && !CountSetExplicitly) { 435 DEBUG(dbgs() << " will not try to unroll partially because " 436 << "-unroll-allow-partial not given\n"); 437 return false; 438 } 439 if (PartialThreshold != NoThreshold && UnrolledSize > PartialThreshold) { 440 // Reduce unroll count to be modulo of TripCount for partial unrolling. 441 Count = (std::max(PartialThreshold, 3u)-2) / (LoopSize-2); 442 while (Count != 0 && TripCount % Count != 0) 443 Count--; 444 } 445 } else if (Unrolling == Runtime) { 446 if (!AllowRuntime && !CountSetExplicitly) { 447 DEBUG(dbgs() << " will not try to unroll loop with runtime trip count " 448 << "-unroll-runtime not given\n"); 449 return false; 450 } 451 // Reduce unroll count to be the largest power-of-two factor of 452 // the original count which satisfies the threshold limit. 453 while (Count != 0 && UnrolledSize > PartialThreshold) { 454 Count >>= 1; 455 UnrolledSize = (LoopSize-2) * Count + 2; 456 } 457 if (Count > UP.MaxCount) 458 Count = UP.MaxCount; 459 DEBUG(dbgs() << " partially unrolling with count: " << Count << "\n"); 460 } 461 462 if (HasPragma) { 463 if (PragmaCount != 0) 464 // If loop has an unroll count pragma mark loop as unrolled to prevent 465 // unrolling beyond that requested by the pragma. 466 SetLoopAlreadyUnrolled(L); 467 468 // Emit optimization remarks if we are unable to unroll the loop 469 // as directed by a pragma. 470 DebugLoc LoopLoc = L->getStartLoc(); 471 Function *F = Header->getParent(); 472 LLVMContext &Ctx = F->getContext(); 473 if (PragmaFullUnroll && PragmaCount == 0) { 474 if (TripCount && Count != TripCount) { 475 emitOptimizationRemarkMissed( 476 Ctx, DEBUG_TYPE, *F, LoopLoc, 477 "Unable to fully unroll loop as directed by unroll(full) pragma " 478 "because unrolled size is too large."); 479 } else if (!TripCount) { 480 emitOptimizationRemarkMissed( 481 Ctx, DEBUG_TYPE, *F, LoopLoc, 482 "Unable to fully unroll loop as directed by unroll(full) pragma " 483 "because loop has a runtime trip count."); 484 } 485 } else if (PragmaCount > 0 && Count != OriginalCount) { 486 emitOptimizationRemarkMissed( 487 Ctx, DEBUG_TYPE, *F, LoopLoc, 488 "Unable to unroll loop the number of times directed by " 489 "unroll_count pragma because unrolled size is too large."); 490 } 491 } 492 493 if (Unrolling != Full && Count < 2) { 494 // Partial unrolling by 1 is a nop. For full unrolling, a factor 495 // of 1 makes sense because loop control can be eliminated. 496 return false; 497 } 498 499 // Unroll the loop. 500 if (!UnrollLoop(L, Count, TripCount, AllowRuntime, TripMultiple, LI, this, 501 &LPM, &AC)) 502 return false; 503 504 return true; 505 } 506