1 //===-- ConstraintElimination.cpp - Eliminate conds using constraints. ----===// 2 // 3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. 4 // See https://llvm.org/LICENSE.txt for license information. 5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception 6 // 7 //===----------------------------------------------------------------------===// 8 // 9 // Eliminate conditions based on constraints collected from dominating 10 // conditions. 11 // 12 //===----------------------------------------------------------------------===// 13 14 #include "llvm/Transforms/Scalar/ConstraintElimination.h" 15 #include "llvm/ADT/STLExtras.h" 16 #include "llvm/ADT/ScopeExit.h" 17 #include "llvm/ADT/SmallVector.h" 18 #include "llvm/ADT/Statistic.h" 19 #include "llvm/Analysis/ConstraintSystem.h" 20 #include "llvm/Analysis/GlobalsModRef.h" 21 #include "llvm/IR/DataLayout.h" 22 #include "llvm/IR/Dominators.h" 23 #include "llvm/IR/Function.h" 24 #include "llvm/IR/Instructions.h" 25 #include "llvm/IR/PatternMatch.h" 26 #include "llvm/InitializePasses.h" 27 #include "llvm/Pass.h" 28 #include "llvm/Support/Debug.h" 29 #include "llvm/Support/DebugCounter.h" 30 #include "llvm/Transforms/Scalar.h" 31 32 #include <string> 33 34 using namespace llvm; 35 using namespace PatternMatch; 36 37 #define DEBUG_TYPE "constraint-elimination" 38 39 STATISTIC(NumCondsRemoved, "Number of instructions removed"); 40 DEBUG_COUNTER(EliminatedCounter, "conds-eliminated", 41 "Controls which conditions are eliminated"); 42 43 static int64_t MaxConstraintValue = std::numeric_limits<int64_t>::max(); 44 45 // Decomposes \p V into a vector of pairs of the form { c, X } where c * X. The 46 // sum of the pairs equals \p V. The first pair is the constant-factor and X 47 // must be nullptr. If the expression cannot be decomposed, returns an empty 48 // vector. 49 static SmallVector<std::pair<int64_t, Value *>, 4> decompose(Value *V) { 50 if (auto *CI = dyn_cast<ConstantInt>(V)) { 51 if (CI->isNegative() || CI->uge(MaxConstraintValue)) 52 return {}; 53 return {{CI->getSExtValue(), nullptr}}; 54 } 55 auto *GEP = dyn_cast<GetElementPtrInst>(V); 56 if (GEP && GEP->getNumOperands() == 2) { 57 if (isa<ConstantInt>(GEP->getOperand(GEP->getNumOperands() - 1))) { 58 return {{cast<ConstantInt>(GEP->getOperand(GEP->getNumOperands() - 1)) 59 ->getSExtValue(), 60 nullptr}, 61 {1, GEP->getPointerOperand()}}; 62 } 63 Value *Op0; 64 ConstantInt *CI; 65 if (match(GEP->getOperand(GEP->getNumOperands() - 1), 66 m_NUWShl(m_Value(Op0), m_ConstantInt(CI)))) 67 return {{0, nullptr}, 68 {1, GEP->getPointerOperand()}, 69 {std::pow(int64_t(2), CI->getSExtValue()), Op0}}; 70 if (match(GEP->getOperand(GEP->getNumOperands() - 1), 71 m_ZExt(m_NUWShl(m_Value(Op0), m_ConstantInt(CI))))) 72 return {{0, nullptr}, 73 {1, GEP->getPointerOperand()}, 74 {std::pow(int64_t(2), CI->getSExtValue()), Op0}}; 75 76 return {{0, nullptr}, 77 {1, GEP->getPointerOperand()}, 78 {1, GEP->getOperand(GEP->getNumOperands() - 1)}}; 79 } 80 81 Value *Op0; 82 Value *Op1; 83 ConstantInt *CI; 84 if (match(V, m_NUWAdd(m_Value(Op0), m_ConstantInt(CI)))) 85 return {{CI->getSExtValue(), nullptr}, {1, Op0}}; 86 if (match(V, m_NUWAdd(m_Value(Op0), m_Value(Op1)))) 87 return {{0, nullptr}, {1, Op0}, {1, Op1}}; 88 89 if (match(V, m_NUWSub(m_Value(Op0), m_ConstantInt(CI)))) 90 return {{-1 * CI->getSExtValue(), nullptr}, {1, Op0}}; 91 if (match(V, m_NUWSub(m_Value(Op0), m_Value(Op1)))) 92 return {{0, nullptr}, {1, Op0}, {1, Op1}}; 93 94 return {{0, nullptr}, {1, V}}; 95 } 96 97 struct ConstraintTy { 98 SmallVector<int64_t, 8> Coefficients; 99 100 ConstraintTy(SmallVector<int64_t, 8> Coefficients) 101 : Coefficients(Coefficients) {} 102 103 unsigned size() const { return Coefficients.size(); } 104 }; 105 106 /// Turn a condition \p CmpI into a constraint vector, using indices from \p 107 /// Value2Index. If \p ShouldAdd is true, new indices are added for values not 108 /// yet in \p Value2Index. 109 static SmallVector<ConstraintTy, 4> 110 getConstraint(CmpInst::Predicate Pred, Value *Op0, Value *Op1, 111 DenseMap<Value *, unsigned> &Value2Index, bool ShouldAdd) { 112 int64_t Offset1 = 0; 113 int64_t Offset2 = 0; 114 115 auto TryToGetIndex = [ShouldAdd, 116 &Value2Index](Value *V) -> Optional<unsigned> { 117 if (ShouldAdd) { 118 Value2Index.insert({V, Value2Index.size() + 1}); 119 return Value2Index[V]; 120 } 121 auto I = Value2Index.find(V); 122 if (I == Value2Index.end()) 123 return None; 124 return I->second; 125 }; 126 127 if (Pred == CmpInst::ICMP_UGT || Pred == CmpInst::ICMP_UGE) 128 return getConstraint(CmpInst::getSwappedPredicate(Pred), Op1, Op0, 129 Value2Index, ShouldAdd); 130 131 if (Pred == CmpInst::ICMP_EQ) { 132 auto A = getConstraint(CmpInst::ICMP_UGE, Op0, Op1, Value2Index, ShouldAdd); 133 auto B = getConstraint(CmpInst::ICMP_ULE, Op0, Op1, Value2Index, ShouldAdd); 134 append_range(A, B); 135 return A; 136 } 137 138 // Only ULE and ULT predicates are supported at the moment. 139 if (Pred != CmpInst::ICMP_ULE && Pred != CmpInst::ICMP_ULT) 140 return {}; 141 142 auto ADec = decompose(Op0); 143 auto BDec = decompose(Op1); 144 // Skip if decomposing either of the values failed. 145 if (ADec.empty() || BDec.empty()) 146 return {}; 147 148 // Skip trivial constraints without any variables. 149 if (ADec.size() == 1 && BDec.size() == 1) 150 return {}; 151 152 Offset1 = ADec[0].first; 153 Offset2 = BDec[0].first; 154 Offset1 *= -1; 155 156 // Create iterator ranges that skip the constant-factor. 157 auto VariablesA = make_range(std::next(ADec.begin()), ADec.end()); 158 auto VariablesB = make_range(std::next(BDec.begin()), BDec.end()); 159 160 // Check if each referenced value in the constraint is already in the system 161 // or can be added (if ShouldAdd is true). 162 for (const auto &KV : 163 concat<std::pair<int64_t, Value *>>(VariablesA, VariablesB)) 164 if (!TryToGetIndex(KV.second)) 165 return {}; 166 167 // Build result constraint, by first adding all coefficients from A and then 168 // subtracting all coefficients from B. 169 SmallVector<int64_t, 8> R(Value2Index.size() + 1, 0); 170 for (const auto &KV : VariablesA) 171 R[Value2Index[KV.second]] += KV.first; 172 173 for (const auto &KV : VariablesB) 174 R[Value2Index[KV.second]] -= KV.first; 175 176 R[0] = Offset1 + Offset2 + (Pred == CmpInst::ICMP_ULT ? -1 : 0); 177 return {R}; 178 } 179 180 static SmallVector<ConstraintTy, 4> 181 getConstraint(CmpInst *Cmp, DenseMap<Value *, unsigned> &Value2Index, 182 bool ShouldAdd) { 183 return getConstraint(Cmp->getPredicate(), Cmp->getOperand(0), 184 Cmp->getOperand(1), Value2Index, ShouldAdd); 185 } 186 187 namespace { 188 /// Represents either a condition that holds on entry to a block or a basic 189 /// block, with their respective Dominator DFS in and out numbers. 190 struct ConstraintOrBlock { 191 unsigned NumIn; 192 unsigned NumOut; 193 bool IsBlock; 194 bool Not; 195 union { 196 BasicBlock *BB; 197 CmpInst *Condition; 198 }; 199 200 ConstraintOrBlock(DomTreeNode *DTN) 201 : NumIn(DTN->getDFSNumIn()), NumOut(DTN->getDFSNumOut()), IsBlock(true), 202 BB(DTN->getBlock()) {} 203 ConstraintOrBlock(DomTreeNode *DTN, CmpInst *Condition, bool Not) 204 : NumIn(DTN->getDFSNumIn()), NumOut(DTN->getDFSNumOut()), IsBlock(false), 205 Not(Not), Condition(Condition) {} 206 }; 207 208 struct StackEntry { 209 unsigned NumIn; 210 unsigned NumOut; 211 CmpInst *Condition; 212 bool IsNot; 213 214 StackEntry(unsigned NumIn, unsigned NumOut, CmpInst *Condition, bool IsNot) 215 : NumIn(NumIn), NumOut(NumOut), Condition(Condition), IsNot(IsNot) {} 216 }; 217 } // namespace 218 219 static void dumpWithNames(ConstraintTy &C, 220 DenseMap<Value *, unsigned> &Value2Index) { 221 SmallVector<std::string> Names(Value2Index.size(), ""); 222 for (auto &KV : Value2Index) { 223 Names[KV.second - 1] = std::string("%") + KV.first->getName().str(); 224 } 225 ConstraintSystem CS; 226 CS.addVariableRowFill(C.Coefficients); 227 CS.dump(Names); 228 } 229 230 static bool eliminateConstraints(Function &F, DominatorTree &DT) { 231 bool Changed = false; 232 DT.updateDFSNumbers(); 233 ConstraintSystem CS; 234 235 SmallVector<ConstraintOrBlock, 64> WorkList; 236 237 // First, collect conditions implied by branches and blocks with their 238 // Dominator DFS in and out numbers. 239 for (BasicBlock &BB : F) { 240 if (!DT.getNode(&BB)) 241 continue; 242 WorkList.emplace_back(DT.getNode(&BB)); 243 244 auto *Br = dyn_cast<BranchInst>(BB.getTerminator()); 245 if (!Br || !Br->isConditional()) 246 continue; 247 248 // If the condition is an OR of 2 compares and the false successor only has 249 // the current block as predecessor, queue both negated conditions for the 250 // false successor. 251 Value *Op0, *Op1; 252 if (match(Br->getCondition(), m_LogicalOr(m_Value(Op0), m_Value(Op1))) && 253 match(Op0, m_Cmp()) && match(Op1, m_Cmp())) { 254 BasicBlock *FalseSuccessor = Br->getSuccessor(1); 255 if (FalseSuccessor->getSinglePredecessor()) { 256 WorkList.emplace_back(DT.getNode(FalseSuccessor), cast<CmpInst>(Op0), 257 true); 258 WorkList.emplace_back(DT.getNode(FalseSuccessor), cast<CmpInst>(Op1), 259 true); 260 } 261 continue; 262 } 263 264 // If the condition is an AND of 2 compares and the true successor only has 265 // the current block as predecessor, queue both conditions for the true 266 // successor. 267 if (match(Br->getCondition(), m_LogicalAnd(m_Value(Op0), m_Value(Op1))) && 268 match(Op0, m_Cmp()) && match(Op1, m_Cmp())) { 269 BasicBlock *TrueSuccessor = Br->getSuccessor(0); 270 if (TrueSuccessor->getSinglePredecessor()) { 271 WorkList.emplace_back(DT.getNode(TrueSuccessor), cast<CmpInst>(Op0), 272 false); 273 WorkList.emplace_back(DT.getNode(TrueSuccessor), cast<CmpInst>(Op1), 274 false); 275 } 276 continue; 277 } 278 279 auto *CmpI = dyn_cast<CmpInst>(Br->getCondition()); 280 if (!CmpI) 281 continue; 282 if (Br->getSuccessor(0)->getSinglePredecessor()) 283 WorkList.emplace_back(DT.getNode(Br->getSuccessor(0)), CmpI, false); 284 if (Br->getSuccessor(1)->getSinglePredecessor()) 285 WorkList.emplace_back(DT.getNode(Br->getSuccessor(1)), CmpI, true); 286 } 287 288 // Next, sort worklist by dominance, so that dominating blocks and conditions 289 // come before blocks and conditions dominated by them. If a block and a 290 // condition have the same numbers, the condition comes before the block, as 291 // it holds on entry to the block. 292 sort(WorkList, [](const ConstraintOrBlock &A, const ConstraintOrBlock &B) { 293 return std::tie(A.NumIn, A.IsBlock) < std::tie(B.NumIn, B.IsBlock); 294 }); 295 296 // Finally, process ordered worklist and eliminate implied conditions. 297 SmallVector<StackEntry, 16> DFSInStack; 298 DenseMap<Value *, unsigned> Value2Index; 299 for (ConstraintOrBlock &CB : WorkList) { 300 // First, pop entries from the stack that are out-of-scope for CB. Remove 301 // the corresponding entry from the constraint system. 302 while (!DFSInStack.empty()) { 303 auto &E = DFSInStack.back(); 304 LLVM_DEBUG(dbgs() << "Top of stack : " << E.NumIn << " " << E.NumOut 305 << "\n"); 306 LLVM_DEBUG(dbgs() << "CB: " << CB.NumIn << " " << CB.NumOut << "\n"); 307 assert(E.NumIn <= CB.NumIn); 308 if (CB.NumOut <= E.NumOut) 309 break; 310 LLVM_DEBUG(dbgs() << "Removing " << *E.Condition << " " << E.IsNot 311 << "\n"); 312 DFSInStack.pop_back(); 313 CS.popLastConstraint(); 314 } 315 316 LLVM_DEBUG({ 317 dbgs() << "Processing "; 318 if (CB.IsBlock) 319 dbgs() << *CB.BB; 320 else 321 dbgs() << *CB.Condition; 322 dbgs() << "\n"; 323 }); 324 325 // For a block, check if any CmpInsts become known based on the current set 326 // of constraints. 327 if (CB.IsBlock) { 328 for (Instruction &I : *CB.BB) { 329 auto *Cmp = dyn_cast<CmpInst>(&I); 330 if (!Cmp) 331 continue; 332 auto R = getConstraint(Cmp, Value2Index, false); 333 if (R.size() != 1 || R[0].size() == 1) 334 continue; 335 if (CS.isConditionImplied(R[0].Coefficients)) { 336 if (!DebugCounter::shouldExecute(EliminatedCounter)) 337 continue; 338 339 LLVM_DEBUG(dbgs() << "Condition " << *Cmp 340 << " implied by dominating constraints\n"); 341 LLVM_DEBUG({ 342 for (auto &E : reverse(DFSInStack)) 343 dbgs() << " C " << *E.Condition << " " << E.IsNot << "\n"; 344 }); 345 Cmp->replaceAllUsesWith( 346 ConstantInt::getTrue(F.getParent()->getContext())); 347 NumCondsRemoved++; 348 Changed = true; 349 } 350 if (CS.isConditionImplied( 351 ConstraintSystem::negate(R[0].Coefficients))) { 352 if (!DebugCounter::shouldExecute(EliminatedCounter)) 353 continue; 354 355 LLVM_DEBUG(dbgs() << "Condition !" << *Cmp 356 << " implied by dominating constraints\n"); 357 LLVM_DEBUG({ 358 for (auto &E : reverse(DFSInStack)) 359 dbgs() << " C " << *E.Condition << " " << E.IsNot << "\n"; 360 }); 361 Cmp->replaceAllUsesWith( 362 ConstantInt::getFalse(F.getParent()->getContext())); 363 NumCondsRemoved++; 364 Changed = true; 365 } 366 } 367 continue; 368 } 369 370 // Set up a function to restore the predicate at the end of the scope if it 371 // has been negated. Negate the predicate in-place, if required. 372 auto *CI = dyn_cast<CmpInst>(CB.Condition); 373 auto PredicateRestorer = make_scope_exit([CI, &CB]() { 374 if (CB.Not && CI) 375 CI->setPredicate(CI->getInversePredicate()); 376 }); 377 if (CB.Not) { 378 if (CI) { 379 CI->setPredicate(CI->getInversePredicate()); 380 } else { 381 LLVM_DEBUG(dbgs() << "Can only negate compares so far.\n"); 382 continue; 383 } 384 } 385 386 // Otherwise, add the condition to the system and stack, if we can transform 387 // it into a constraint. 388 auto R = getConstraint(CB.Condition, Value2Index, true); 389 if (R.empty()) 390 continue; 391 392 LLVM_DEBUG(dbgs() << "Adding " << *CB.Condition << " " << CB.Not << "\n"); 393 bool Added = false; 394 for (auto &C : R) { 395 auto Coeffs = C.Coefficients; 396 LLVM_DEBUG({ 397 dbgs() << " constraint: "; 398 dumpWithNames(C, Value2Index); 399 }); 400 Added |= CS.addVariableRowFill(Coeffs); 401 // If R has been added to the system, queue it for removal once it goes 402 // out-of-scope. 403 if (Added) 404 DFSInStack.emplace_back(CB.NumIn, CB.NumOut, CB.Condition, CB.Not); 405 } 406 } 407 408 assert(CS.size() == DFSInStack.size() && 409 "updates to CS and DFSInStack are out of sync"); 410 return Changed; 411 } 412 413 PreservedAnalyses ConstraintEliminationPass::run(Function &F, 414 FunctionAnalysisManager &AM) { 415 auto &DT = AM.getResult<DominatorTreeAnalysis>(F); 416 if (!eliminateConstraints(F, DT)) 417 return PreservedAnalyses::all(); 418 419 PreservedAnalyses PA; 420 PA.preserve<DominatorTreeAnalysis>(); 421 PA.preserve<GlobalsAA>(); 422 PA.preserveSet<CFGAnalyses>(); 423 return PA; 424 } 425 426 namespace { 427 428 class ConstraintElimination : public FunctionPass { 429 public: 430 static char ID; 431 432 ConstraintElimination() : FunctionPass(ID) { 433 initializeConstraintEliminationPass(*PassRegistry::getPassRegistry()); 434 } 435 436 bool runOnFunction(Function &F) override { 437 auto &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree(); 438 return eliminateConstraints(F, DT); 439 } 440 441 void getAnalysisUsage(AnalysisUsage &AU) const override { 442 AU.setPreservesCFG(); 443 AU.addRequired<DominatorTreeWrapperPass>(); 444 AU.addPreserved<GlobalsAAWrapperPass>(); 445 AU.addPreserved<DominatorTreeWrapperPass>(); 446 } 447 }; 448 449 } // end anonymous namespace 450 451 char ConstraintElimination::ID = 0; 452 453 INITIALIZE_PASS_BEGIN(ConstraintElimination, "constraint-elimination", 454 "Constraint Elimination", false, false) 455 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass) 456 INITIALIZE_PASS_DEPENDENCY(LazyValueInfoWrapperPass) 457 INITIALIZE_PASS_END(ConstraintElimination, "constraint-elimination", 458 "Constraint Elimination", false, false) 459 460 FunctionPass *llvm::createConstraintEliminationPass() { 461 return new ConstraintElimination(); 462 } 463