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