Local.cpp revision 9e9a0d5fc26878e51a58a8b57900fcbf952c2691
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/Target/TargetData.h" 28#include "llvm/Support/GetElementPtrTypeIterator.h" 29#include "llvm/Support/MathExtras.h" 30using namespace llvm; 31 32//===----------------------------------------------------------------------===// 33// Local analysis. 34// 35 36/// isSafeToLoadUnconditionally - Return true if we know that executing a load 37/// from this value cannot trap. If it is not obviously safe to load from the 38/// specified pointer, we do a quick local scan of the basic block containing 39/// ScanFrom, to determine if the address is already accessed. 40bool llvm::isSafeToLoadUnconditionally(Value *V, Instruction *ScanFrom) { 41 // If it is an alloca it is always safe to load from. 42 if (isa<AllocaInst>(V)) return true; 43 44 // If it is a global variable it is mostly safe to load from. 45 if (const GlobalValue *GV = dyn_cast<GlobalVariable>(V)) 46 // Don't try to evaluate aliases. External weak GV can be null. 47 return !isa<GlobalAlias>(GV) && !GV->hasExternalWeakLinkage(); 48 49 // Otherwise, be a little bit agressive by scanning the local block where we 50 // want to check to see if the pointer is already being loaded or stored 51 // from/to. If so, the previous load or store would have already trapped, 52 // so there is no harm doing an extra load (also, CSE will later eliminate 53 // the load entirely). 54 BasicBlock::iterator BBI = ScanFrom, E = ScanFrom->getParent()->begin(); 55 56 while (BBI != E) { 57 --BBI; 58 59 // If we see a free or a call which may write to memory (i.e. which might do 60 // a free) the pointer could be marked invalid. 61 if (isa<FreeInst>(BBI) || 62 (isa<CallInst>(BBI) && BBI->mayWriteToMemory() && 63 !isa<DbgInfoIntrinsic>(BBI))) 64 return false; 65 66 if (LoadInst *LI = dyn_cast<LoadInst>(BBI)) { 67 if (LI->getOperand(0) == V) return true; 68 } else if (StoreInst *SI = dyn_cast<StoreInst>(BBI)) { 69 if (SI->getOperand(1) == V) return true; 70 } 71 } 72 return false; 73} 74 75 76//===----------------------------------------------------------------------===// 77// Local constant propagation. 78// 79 80// ConstantFoldTerminator - If a terminator instruction is predicated on a 81// constant value, convert it into an unconditional branch to the constant 82// destination. 83// 84bool llvm::ConstantFoldTerminator(BasicBlock *BB) { 85 TerminatorInst *T = BB->getTerminator(); 86 87 // Branch - See if we are conditional jumping on constant 88 if (BranchInst *BI = dyn_cast<BranchInst>(T)) { 89 if (BI->isUnconditional()) return false; // Can't optimize uncond branch 90 BasicBlock *Dest1 = BI->getSuccessor(0); 91 BasicBlock *Dest2 = BI->getSuccessor(1); 92 93 if (ConstantInt *Cond = dyn_cast<ConstantInt>(BI->getCondition())) { 94 // Are we branching on constant? 95 // YES. Change to unconditional branch... 96 BasicBlock *Destination = Cond->getZExtValue() ? Dest1 : Dest2; 97 BasicBlock *OldDest = Cond->getZExtValue() ? Dest2 : Dest1; 98 99 //cerr << "Function: " << T->getParent()->getParent() 100 // << "\nRemoving branch from " << T->getParent() 101 // << "\n\nTo: " << OldDest << endl; 102 103 // Let the basic block know that we are letting go of it. Based on this, 104 // it will adjust it's PHI nodes. 105 assert(BI->getParent() && "Terminator not inserted in block!"); 106 OldDest->removePredecessor(BI->getParent()); 107 108 // Set the unconditional destination, and change the insn to be an 109 // unconditional branch. 110 BI->setUnconditionalDest(Destination); 111 return true; 112 } else if (Dest2 == Dest1) { // Conditional branch to same location? 113 // This branch matches something like this: 114 // br bool %cond, label %Dest, label %Dest 115 // and changes it into: br label %Dest 116 117 // Let the basic block know that we are letting go of one copy of it. 118 assert(BI->getParent() && "Terminator not inserted in block!"); 119 Dest1->removePredecessor(BI->getParent()); 120 121 // Change a conditional branch to unconditional. 122 BI->setUnconditionalDest(Dest1); 123 return true; 124 } 125 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(T)) { 126 // If we are switching on a constant, we can convert the switch into a 127 // single branch instruction! 128 ConstantInt *CI = dyn_cast<ConstantInt>(SI->getCondition()); 129 BasicBlock *TheOnlyDest = SI->getSuccessor(0); // The default dest 130 BasicBlock *DefaultDest = TheOnlyDest; 131 assert(TheOnlyDest == SI->getDefaultDest() && 132 "Default destination is not successor #0?"); 133 134 // Figure out which case it goes to... 135 for (unsigned i = 1, e = SI->getNumSuccessors(); i != e; ++i) { 136 // Found case matching a constant operand? 137 if (SI->getSuccessorValue(i) == CI) { 138 TheOnlyDest = SI->getSuccessor(i); 139 break; 140 } 141 142 // Check to see if this branch is going to the same place as the default 143 // dest. If so, eliminate it as an explicit compare. 144 if (SI->getSuccessor(i) == DefaultDest) { 145 // Remove this entry... 146 DefaultDest->removePredecessor(SI->getParent()); 147 SI->removeCase(i); 148 --i; --e; // Don't skip an entry... 149 continue; 150 } 151 152 // Otherwise, check to see if the switch only branches to one destination. 153 // We do this by reseting "TheOnlyDest" to null when we find two non-equal 154 // destinations. 155 if (SI->getSuccessor(i) != TheOnlyDest) TheOnlyDest = 0; 156 } 157 158 if (CI && !TheOnlyDest) { 159 // Branching on a constant, but not any of the cases, go to the default 160 // successor. 161 TheOnlyDest = SI->getDefaultDest(); 162 } 163 164 // If we found a single destination that we can fold the switch into, do so 165 // now. 166 if (TheOnlyDest) { 167 // Insert the new branch.. 168 BranchInst::Create(TheOnlyDest, SI); 169 BasicBlock *BB = SI->getParent(); 170 171 // Remove entries from PHI nodes which we no longer branch to... 172 for (unsigned i = 0, e = SI->getNumSuccessors(); i != e; ++i) { 173 // Found case matching a constant operand? 174 BasicBlock *Succ = SI->getSuccessor(i); 175 if (Succ == TheOnlyDest) 176 TheOnlyDest = 0; // Don't modify the first branch to TheOnlyDest 177 else 178 Succ->removePredecessor(BB); 179 } 180 181 // Delete the old switch... 182 BB->getInstList().erase(SI); 183 return true; 184 } else if (SI->getNumSuccessors() == 2) { 185 // Otherwise, we can fold this switch into a conditional branch 186 // instruction if it has only one non-default destination. 187 Value *Cond = new ICmpInst(SI, ICmpInst::ICMP_EQ, SI->getCondition(), 188 SI->getSuccessorValue(1), "cond"); 189 // Insert the new branch... 190 BranchInst::Create(SI->getSuccessor(1), SI->getSuccessor(0), Cond, SI); 191 192 // Delete the old switch... 193 SI->eraseFromParent(); 194 return true; 195 } 196 } 197 return false; 198} 199 200 201//===----------------------------------------------------------------------===// 202// Local dead code elimination... 203// 204 205/// isInstructionTriviallyDead - Return true if the result produced by the 206/// instruction is not used, and the instruction has no side effects. 207/// 208bool llvm::isInstructionTriviallyDead(Instruction *I) { 209 if (!I->use_empty() || isa<TerminatorInst>(I)) return false; 210 211 // We don't want debug info removed by anything this general. 212 if (isa<DbgInfoIntrinsic>(I)) return false; 213 214 if (!I->mayHaveSideEffects()) return true; 215 216 // Special case intrinsics that "may have side effects" but can be deleted 217 // when dead. 218 if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) 219 // Safe to delete llvm.stacksave if dead. 220 if (II->getIntrinsicID() == Intrinsic::stacksave) 221 return true; 222 return false; 223} 224 225/// RecursivelyDeleteTriviallyDeadInstructions - If the specified value is a 226/// trivially dead instruction, delete it. If that makes any of its operands 227/// trivially dead, delete them too, recursively. 228void llvm::RecursivelyDeleteTriviallyDeadInstructions(Value *V) { 229 Instruction *I = dyn_cast<Instruction>(V); 230 if (!I || !I->use_empty() || !isInstructionTriviallyDead(I)) 231 return; 232 233 SmallVector<Instruction*, 16> DeadInsts; 234 DeadInsts.push_back(I); 235 236 while (!DeadInsts.empty()) { 237 I = DeadInsts.pop_back_val(); 238 239 // Null out all of the instruction's operands to see if any operand becomes 240 // dead as we go. 241 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) { 242 Value *OpV = I->getOperand(i); 243 I->setOperand(i, 0); 244 245 if (!OpV->use_empty()) continue; 246 247 // If the operand is an instruction that became dead as we nulled out the 248 // operand, and if it is 'trivially' dead, delete it in a future loop 249 // iteration. 250 if (Instruction *OpI = dyn_cast<Instruction>(OpV)) 251 if (isInstructionTriviallyDead(OpI)) 252 DeadInsts.push_back(OpI); 253 } 254 255 I->eraseFromParent(); 256 } 257} 258 259/// RecursivelyDeleteDeadPHINode - If the specified value is an effectively 260/// dead PHI node, due to being a def-use chain of single-use nodes that 261/// either forms a cycle or is terminated by a trivially dead instruction, 262/// delete it. If that makes any of its operands trivially dead, delete them 263/// too, recursively. 264void 265llvm::RecursivelyDeleteDeadPHINode(PHINode *PN) { 266 // We can remove a PHI if it is on a cycle in the def-use graph 267 // where each node in the cycle has degree one, i.e. only one use, 268 // and is an instruction with no side effects. 269 if (!PN->hasOneUse()) 270 return; 271 272 SmallPtrSet<PHINode *, 4> PHIs; 273 PHIs.insert(PN); 274 for (Instruction *J = cast<Instruction>(*PN->use_begin()); 275 J->hasOneUse() && !J->mayHaveSideEffects(); 276 J = cast<Instruction>(*J->use_begin())) 277 // If we find a PHI more than once, we're on a cycle that 278 // won't prove fruitful. 279 if (PHINode *JP = dyn_cast<PHINode>(J)) 280 if (!PHIs.insert(cast<PHINode>(JP))) { 281 // Break the cycle and delete the PHI and its operands. 282 JP->replaceAllUsesWith(UndefValue::get(JP->getType())); 283 RecursivelyDeleteTriviallyDeadInstructions(JP); 284 break; 285 } 286} 287 288//===----------------------------------------------------------------------===// 289// Control Flow Graph Restructuring... 290// 291 292/// MergeBasicBlockIntoOnlyPred - DestBB is a block with one predecessor and its 293/// predecessor is known to have one successor (DestBB!). Eliminate the edge 294/// between them, moving the instructions in the predecessor into DestBB and 295/// deleting the predecessor block. 296/// 297void llvm::MergeBasicBlockIntoOnlyPred(BasicBlock *DestBB) { 298 // If BB has single-entry PHI nodes, fold them. 299 while (PHINode *PN = dyn_cast<PHINode>(DestBB->begin())) { 300 Value *NewVal = PN->getIncomingValue(0); 301 // Replace self referencing PHI with undef, it must be dead. 302 if (NewVal == PN) NewVal = UndefValue::get(PN->getType()); 303 PN->replaceAllUsesWith(NewVal); 304 PN->eraseFromParent(); 305 } 306 307 BasicBlock *PredBB = DestBB->getSinglePredecessor(); 308 assert(PredBB && "Block doesn't have a single predecessor!"); 309 310 // Splice all the instructions from PredBB to DestBB. 311 PredBB->getTerminator()->eraseFromParent(); 312 DestBB->getInstList().splice(DestBB->begin(), PredBB->getInstList()); 313 314 // Anything that branched to PredBB now branches to DestBB. 315 PredBB->replaceAllUsesWith(DestBB); 316 317 // Nuke BB. 318 PredBB->eraseFromParent(); 319} 320 321/// OnlyUsedByDbgIntrinsics - Return true if the instruction I is only used 322/// by DbgIntrinsics. If DbgInUses is specified then the vector is filled 323/// with the DbgInfoIntrinsic that use the instruction I. 324bool llvm::OnlyUsedByDbgInfoIntrinsics(Instruction *I, 325 SmallVectorImpl<DbgInfoIntrinsic *> *DbgInUses) { 326 if (DbgInUses) 327 DbgInUses->clear(); 328 329 for (Value::use_iterator UI = I->use_begin(), UE = I->use_end(); UI != UE; 330 ++UI) { 331 if (DbgInfoIntrinsic *DI = dyn_cast<DbgInfoIntrinsic>(*UI)) { 332 if (DbgInUses) 333 DbgInUses->push_back(DI); 334 } else { 335 if (DbgInUses) 336 DbgInUses->clear(); 337 return false; 338 } 339 } 340 return true; 341} 342 343