Local.cpp revision ad80981a106c9d0ec83351e63ee3ac75ed646bf4
1//===-- Local.cpp - Functions to perform local transformations ------------===// 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 family of functions perform various local transformations to the 11// program. 12// 13//===----------------------------------------------------------------------===// 14 15#include "llvm/Transforms/Utils/Local.h" 16#include "llvm/Constants.h" 17#include "llvm/GlobalAlias.h" 18#include "llvm/GlobalVariable.h" 19#include "llvm/DerivedTypes.h" 20#include "llvm/Instructions.h" 21#include "llvm/Intrinsics.h" 22#include "llvm/IntrinsicInst.h" 23#include "llvm/LLVMContext.h" 24#include "llvm/ADT/SmallPtrSet.h" 25#include "llvm/Analysis/ConstantFolding.h" 26#include "llvm/Analysis/DebugInfo.h" 27#include "llvm/Analysis/ProfileInfo.h" 28#include "llvm/Target/TargetData.h" 29#include "llvm/Support/GetElementPtrTypeIterator.h" 30#include "llvm/Support/MathExtras.h" 31using namespace llvm; 32 33//===----------------------------------------------------------------------===// 34// Local analysis. 35// 36 37/// isSafeToLoadUnconditionally - Return true if we know that executing a load 38/// from this value cannot trap. If it is not obviously safe to load from the 39/// specified pointer, we do a quick local scan of the basic block containing 40/// ScanFrom, to determine if the address is already accessed. 41bool llvm::isSafeToLoadUnconditionally(Value *V, Instruction *ScanFrom) { 42 // If it is an alloca it is always safe to load from. 43 if (isa<AllocaInst>(V)) return true; 44 45 // If it is a global variable it is mostly safe to load from. 46 if (const GlobalValue *GV = dyn_cast<GlobalVariable>(V)) 47 // Don't try to evaluate aliases. External weak GV can be null. 48 return !isa<GlobalAlias>(GV) && !GV->hasExternalWeakLinkage(); 49 50 // Otherwise, be a little bit agressive by scanning the local block where we 51 // want to check to see if the pointer is already being loaded or stored 52 // from/to. If so, the previous load or store would have already trapped, 53 // so there is no harm doing an extra load (also, CSE will later eliminate 54 // the load entirely). 55 BasicBlock::iterator BBI = ScanFrom, E = ScanFrom->getParent()->begin(); 56 57 while (BBI != E) { 58 --BBI; 59 60 // If we see a free or a call which may write to memory (i.e. which might do 61 // a free) the pointer could be marked invalid. 62 if (isa<FreeInst>(BBI) || 63 (isa<CallInst>(BBI) && BBI->mayWriteToMemory() && 64 !isa<DbgInfoIntrinsic>(BBI))) 65 return false; 66 67 if (LoadInst *LI = dyn_cast<LoadInst>(BBI)) { 68 if (LI->getOperand(0) == V) return true; 69 } else if (StoreInst *SI = dyn_cast<StoreInst>(BBI)) { 70 if (SI->getOperand(1) == V) return true; 71 } 72 } 73 return false; 74} 75 76 77//===----------------------------------------------------------------------===// 78// Local constant propagation. 79// 80 81// ConstantFoldTerminator - If a terminator instruction is predicated on a 82// constant value, convert it into an unconditional branch to the constant 83// destination. 84// 85bool llvm::ConstantFoldTerminator(BasicBlock *BB) { 86 TerminatorInst *T = BB->getTerminator(); 87 88 // Branch - See if we are conditional jumping on constant 89 if (BranchInst *BI = dyn_cast<BranchInst>(T)) { 90 if (BI->isUnconditional()) return false; // Can't optimize uncond branch 91 BasicBlock *Dest1 = BI->getSuccessor(0); 92 BasicBlock *Dest2 = BI->getSuccessor(1); 93 94 if (ConstantInt *Cond = dyn_cast<ConstantInt>(BI->getCondition())) { 95 // Are we branching on constant? 96 // YES. Change to unconditional branch... 97 BasicBlock *Destination = Cond->getZExtValue() ? Dest1 : Dest2; 98 BasicBlock *OldDest = Cond->getZExtValue() ? Dest2 : Dest1; 99 100 //cerr << "Function: " << T->getParent()->getParent() 101 // << "\nRemoving branch from " << T->getParent() 102 // << "\n\nTo: " << OldDest << endl; 103 104 // Let the basic block know that we are letting go of it. Based on this, 105 // it will adjust it's PHI nodes. 106 assert(BI->getParent() && "Terminator not inserted in block!"); 107 OldDest->removePredecessor(BI->getParent()); 108 109 // Set the unconditional destination, and change the insn to be an 110 // unconditional branch. 111 BI->setUnconditionalDest(Destination); 112 return true; 113 } else if (Dest2 == Dest1) { // Conditional branch to same location? 114 // This branch matches something like this: 115 // br bool %cond, label %Dest, label %Dest 116 // and changes it into: br label %Dest 117 118 // Let the basic block know that we are letting go of one copy of it. 119 assert(BI->getParent() && "Terminator not inserted in block!"); 120 Dest1->removePredecessor(BI->getParent()); 121 122 // Change a conditional branch to unconditional. 123 BI->setUnconditionalDest(Dest1); 124 return true; 125 } 126 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(T)) { 127 // If we are switching on a constant, we can convert the switch into a 128 // single branch instruction! 129 ConstantInt *CI = dyn_cast<ConstantInt>(SI->getCondition()); 130 BasicBlock *TheOnlyDest = SI->getSuccessor(0); // The default dest 131 BasicBlock *DefaultDest = TheOnlyDest; 132 assert(TheOnlyDest == SI->getDefaultDest() && 133 "Default destination is not successor #0?"); 134 135 // Figure out which case it goes to... 136 for (unsigned i = 1, e = SI->getNumSuccessors(); i != e; ++i) { 137 // Found case matching a constant operand? 138 if (SI->getSuccessorValue(i) == CI) { 139 TheOnlyDest = SI->getSuccessor(i); 140 break; 141 } 142 143 // Check to see if this branch is going to the same place as the default 144 // dest. If so, eliminate it as an explicit compare. 145 if (SI->getSuccessor(i) == DefaultDest) { 146 // Remove this entry... 147 DefaultDest->removePredecessor(SI->getParent()); 148 SI->removeCase(i); 149 --i; --e; // Don't skip an entry... 150 continue; 151 } 152 153 // Otherwise, check to see if the switch only branches to one destination. 154 // We do this by reseting "TheOnlyDest" to null when we find two non-equal 155 // destinations. 156 if (SI->getSuccessor(i) != TheOnlyDest) TheOnlyDest = 0; 157 } 158 159 if (CI && !TheOnlyDest) { 160 // Branching on a constant, but not any of the cases, go to the default 161 // successor. 162 TheOnlyDest = SI->getDefaultDest(); 163 } 164 165 // If we found a single destination that we can fold the switch into, do so 166 // now. 167 if (TheOnlyDest) { 168 // Insert the new branch.. 169 BranchInst::Create(TheOnlyDest, SI); 170 BasicBlock *BB = SI->getParent(); 171 172 // Remove entries from PHI nodes which we no longer branch to... 173 for (unsigned i = 0, e = SI->getNumSuccessors(); i != e; ++i) { 174 // Found case matching a constant operand? 175 BasicBlock *Succ = SI->getSuccessor(i); 176 if (Succ == TheOnlyDest) 177 TheOnlyDest = 0; // Don't modify the first branch to TheOnlyDest 178 else 179 Succ->removePredecessor(BB); 180 } 181 182 // Delete the old switch... 183 BB->getInstList().erase(SI); 184 return true; 185 } else if (SI->getNumSuccessors() == 2) { 186 // Otherwise, we can fold this switch into a conditional branch 187 // instruction if it has only one non-default destination. 188 Value *Cond = new ICmpInst(SI, ICmpInst::ICMP_EQ, SI->getCondition(), 189 SI->getSuccessorValue(1), "cond"); 190 // Insert the new branch... 191 BranchInst::Create(SI->getSuccessor(1), SI->getSuccessor(0), Cond, SI); 192 193 // Delete the old switch... 194 SI->eraseFromParent(); 195 return true; 196 } 197 } 198 return false; 199} 200 201 202//===----------------------------------------------------------------------===// 203// Local dead code elimination... 204// 205 206/// isInstructionTriviallyDead - Return true if the result produced by the 207/// instruction is not used, and the instruction has no side effects. 208/// 209bool llvm::isInstructionTriviallyDead(Instruction *I) { 210 if (!I->use_empty() || isa<TerminatorInst>(I)) return false; 211 212 // We don't want debug info removed by anything this general. 213 if (isa<DbgInfoIntrinsic>(I)) return false; 214 215 if (!I->mayHaveSideEffects()) return true; 216 217 // Special case intrinsics that "may have side effects" but can be deleted 218 // when dead. 219 if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) 220 // Safe to delete llvm.stacksave if dead. 221 if (II->getIntrinsicID() == Intrinsic::stacksave) 222 return true; 223 return false; 224} 225 226/// RecursivelyDeleteTriviallyDeadInstructions - If the specified value is a 227/// trivially dead instruction, delete it. If that makes any of its operands 228/// trivially dead, delete them too, recursively. 229void llvm::RecursivelyDeleteTriviallyDeadInstructions(Value *V) { 230 Instruction *I = dyn_cast<Instruction>(V); 231 if (!I || !I->use_empty() || !isInstructionTriviallyDead(I)) 232 return; 233 234 SmallVector<Instruction*, 16> DeadInsts; 235 DeadInsts.push_back(I); 236 237 while (!DeadInsts.empty()) { 238 I = DeadInsts.pop_back_val(); 239 240 // Null out all of the instruction's operands to see if any operand becomes 241 // dead as we go. 242 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) { 243 Value *OpV = I->getOperand(i); 244 I->setOperand(i, 0); 245 246 if (!OpV->use_empty()) continue; 247 248 // If the operand is an instruction that became dead as we nulled out the 249 // operand, and if it is 'trivially' dead, delete it in a future loop 250 // iteration. 251 if (Instruction *OpI = dyn_cast<Instruction>(OpV)) 252 if (isInstructionTriviallyDead(OpI)) 253 DeadInsts.push_back(OpI); 254 } 255 256 I->eraseFromParent(); 257 } 258} 259 260/// RecursivelyDeleteDeadPHINode - If the specified value is an effectively 261/// dead PHI node, due to being a def-use chain of single-use nodes that 262/// either forms a cycle or is terminated by a trivially dead instruction, 263/// delete it. If that makes any of its operands trivially dead, delete them 264/// too, recursively. 265void 266llvm::RecursivelyDeleteDeadPHINode(PHINode *PN) { 267 // We can remove a PHI if it is on a cycle in the def-use graph 268 // where each node in the cycle has degree one, i.e. only one use, 269 // and is an instruction with no side effects. 270 if (!PN->hasOneUse()) 271 return; 272 273 SmallPtrSet<PHINode *, 4> PHIs; 274 PHIs.insert(PN); 275 for (Instruction *J = cast<Instruction>(*PN->use_begin()); 276 J->hasOneUse() && !J->mayHaveSideEffects(); 277 J = cast<Instruction>(*J->use_begin())) 278 // If we find a PHI more than once, we're on a cycle that 279 // won't prove fruitful. 280 if (PHINode *JP = dyn_cast<PHINode>(J)) 281 if (!PHIs.insert(cast<PHINode>(JP))) { 282 // Break the cycle and delete the PHI and its operands. 283 JP->replaceAllUsesWith(UndefValue::get(JP->getType())); 284 RecursivelyDeleteTriviallyDeadInstructions(JP); 285 break; 286 } 287} 288 289//===----------------------------------------------------------------------===// 290// Control Flow Graph Restructuring... 291// 292 293/// MergeBasicBlockIntoOnlyPred - DestBB is a block with one predecessor and its 294/// predecessor is known to have one successor (DestBB!). Eliminate the edge 295/// between them, moving the instructions in the predecessor into DestBB and 296/// deleting the predecessor block. 297/// 298void llvm::MergeBasicBlockIntoOnlyPred(BasicBlock *DestBB, Pass *P) { 299 // If BB has single-entry PHI nodes, fold them. 300 while (PHINode *PN = dyn_cast<PHINode>(DestBB->begin())) { 301 Value *NewVal = PN->getIncomingValue(0); 302 // Replace self referencing PHI with undef, it must be dead. 303 if (NewVal == PN) NewVal = UndefValue::get(PN->getType()); 304 PN->replaceAllUsesWith(NewVal); 305 PN->eraseFromParent(); 306 } 307 308 BasicBlock *PredBB = DestBB->getSinglePredecessor(); 309 assert(PredBB && "Block doesn't have a single predecessor!"); 310 311 // Splice all the instructions from PredBB to DestBB. 312 PredBB->getTerminator()->eraseFromParent(); 313 DestBB->getInstList().splice(DestBB->begin(), PredBB->getInstList()); 314 315 // Anything that branched to PredBB now branches to DestBB. 316 PredBB->replaceAllUsesWith(DestBB); 317 318 if (P) { 319 ProfileInfo *PI = P->getAnalysisIfAvailable<ProfileInfo>(); 320 if (PI) { 321 PI->replaceAllUses(PredBB, DestBB); 322 PI->removeEdge(ProfileInfo::getEdge(PredBB, DestBB)); 323 } 324 } 325 // Nuke BB. 326 PredBB->eraseFromParent(); 327} 328 329/// OnlyUsedByDbgIntrinsics - Return true if the instruction I is only used 330/// by DbgIntrinsics. If DbgInUses is specified then the vector is filled 331/// with the DbgInfoIntrinsic that use the instruction I. 332bool llvm::OnlyUsedByDbgInfoIntrinsics(Instruction *I, 333 SmallVectorImpl<DbgInfoIntrinsic *> *DbgInUses) { 334 if (DbgInUses) 335 DbgInUses->clear(); 336 337 for (Value::use_iterator UI = I->use_begin(), UE = I->use_end(); UI != UE; 338 ++UI) { 339 if (DbgInfoIntrinsic *DI = dyn_cast<DbgInfoIntrinsic>(*UI)) { 340 if (DbgInUses) 341 DbgInUses->push_back(DI); 342 } else { 343 if (DbgInUses) 344 DbgInUses->clear(); 345 return false; 346 } 347 } 348 return true; 349} 350 351