Local.cpp revision 7af1c78b98d2df7d0ab9154461ca3d835706716e
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/GlobalVariable.h" 18#include "llvm/DerivedTypes.h" 19#include "llvm/Instructions.h" 20#include "llvm/Intrinsics.h" 21#include "llvm/IntrinsicInst.h" 22#include "llvm/ADT/SmallPtrSet.h" 23#include "llvm/Analysis/ConstantFolding.h" 24#include "llvm/Analysis/DebugInfo.h" 25#include "llvm/Target/TargetData.h" 26#include "llvm/Support/GetElementPtrTypeIterator.h" 27#include "llvm/Support/MathExtras.h" 28using namespace llvm; 29 30//===----------------------------------------------------------------------===// 31// Local constant propagation. 32// 33 34// ConstantFoldTerminator - If a terminator instruction is predicated on a 35// constant value, convert it into an unconditional branch to the constant 36// destination. 37// 38bool llvm::ConstantFoldTerminator(BasicBlock *BB) { 39 TerminatorInst *T = BB->getTerminator(); 40 41 // Branch - See if we are conditional jumping on constant 42 if (BranchInst *BI = dyn_cast<BranchInst>(T)) { 43 if (BI->isUnconditional()) return false; // Can't optimize uncond branch 44 BasicBlock *Dest1 = BI->getSuccessor(0); 45 BasicBlock *Dest2 = BI->getSuccessor(1); 46 47 if (ConstantInt *Cond = dyn_cast<ConstantInt>(BI->getCondition())) { 48 // Are we branching on constant? 49 // YES. Change to unconditional branch... 50 BasicBlock *Destination = Cond->getZExtValue() ? Dest1 : Dest2; 51 BasicBlock *OldDest = Cond->getZExtValue() ? Dest2 : Dest1; 52 53 //cerr << "Function: " << T->getParent()->getParent() 54 // << "\nRemoving branch from " << T->getParent() 55 // << "\n\nTo: " << OldDest << endl; 56 57 // Let the basic block know that we are letting go of it. Based on this, 58 // it will adjust it's PHI nodes. 59 assert(BI->getParent() && "Terminator not inserted in block!"); 60 OldDest->removePredecessor(BI->getParent()); 61 62 // Set the unconditional destination, and change the insn to be an 63 // unconditional branch. 64 BI->setUnconditionalDest(Destination); 65 return true; 66 } else if (Dest2 == Dest1) { // Conditional branch to same location? 67 // This branch matches something like this: 68 // br bool %cond, label %Dest, label %Dest 69 // and changes it into: br label %Dest 70 71 // Let the basic block know that we are letting go of one copy of it. 72 assert(BI->getParent() && "Terminator not inserted in block!"); 73 Dest1->removePredecessor(BI->getParent()); 74 75 // Change a conditional branch to unconditional. 76 BI->setUnconditionalDest(Dest1); 77 return true; 78 } 79 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(T)) { 80 // If we are switching on a constant, we can convert the switch into a 81 // single branch instruction! 82 ConstantInt *CI = dyn_cast<ConstantInt>(SI->getCondition()); 83 BasicBlock *TheOnlyDest = SI->getSuccessor(0); // The default dest 84 BasicBlock *DefaultDest = TheOnlyDest; 85 assert(TheOnlyDest == SI->getDefaultDest() && 86 "Default destination is not successor #0?"); 87 88 // Figure out which case it goes to... 89 for (unsigned i = 1, e = SI->getNumSuccessors(); i != e; ++i) { 90 // Found case matching a constant operand? 91 if (SI->getSuccessorValue(i) == CI) { 92 TheOnlyDest = SI->getSuccessor(i); 93 break; 94 } 95 96 // Check to see if this branch is going to the same place as the default 97 // dest. If so, eliminate it as an explicit compare. 98 if (SI->getSuccessor(i) == DefaultDest) { 99 // Remove this entry... 100 DefaultDest->removePredecessor(SI->getParent()); 101 SI->removeCase(i); 102 --i; --e; // Don't skip an entry... 103 continue; 104 } 105 106 // Otherwise, check to see if the switch only branches to one destination. 107 // We do this by reseting "TheOnlyDest" to null when we find two non-equal 108 // destinations. 109 if (SI->getSuccessor(i) != TheOnlyDest) TheOnlyDest = 0; 110 } 111 112 if (CI && !TheOnlyDest) { 113 // Branching on a constant, but not any of the cases, go to the default 114 // successor. 115 TheOnlyDest = SI->getDefaultDest(); 116 } 117 118 // If we found a single destination that we can fold the switch into, do so 119 // now. 120 if (TheOnlyDest) { 121 // Insert the new branch.. 122 BranchInst::Create(TheOnlyDest, SI); 123 BasicBlock *BB = SI->getParent(); 124 125 // Remove entries from PHI nodes which we no longer branch to... 126 for (unsigned i = 0, e = SI->getNumSuccessors(); i != e; ++i) { 127 // Found case matching a constant operand? 128 BasicBlock *Succ = SI->getSuccessor(i); 129 if (Succ == TheOnlyDest) 130 TheOnlyDest = 0; // Don't modify the first branch to TheOnlyDest 131 else 132 Succ->removePredecessor(BB); 133 } 134 135 // Delete the old switch... 136 BB->getInstList().erase(SI); 137 return true; 138 } else if (SI->getNumSuccessors() == 2) { 139 // Otherwise, we can fold this switch into a conditional branch 140 // instruction if it has only one non-default destination. 141 Value *Cond = new ICmpInst(ICmpInst::ICMP_EQ, SI->getCondition(), 142 SI->getSuccessorValue(1), "cond", SI); 143 // Insert the new branch... 144 BranchInst::Create(SI->getSuccessor(1), SI->getSuccessor(0), Cond, SI); 145 146 // Delete the old switch... 147 SI->eraseFromParent(); 148 return true; 149 } 150 } 151 return false; 152} 153 154 155//===----------------------------------------------------------------------===// 156// Local dead code elimination... 157// 158 159/// isInstructionTriviallyDead - Return true if the result produced by the 160/// instruction is not used, and the instruction has no side effects. 161/// 162bool llvm::isInstructionTriviallyDead(Instruction *I) { 163 if (!I->use_empty() || isa<TerminatorInst>(I)) return false; 164 165 // We don't want debug info removed by anything this general. 166 if (isa<DbgInfoIntrinsic>(I)) return false; 167 168 if (!I->mayHaveSideEffects()) return true; 169 170 // Special case intrinsics that "may have side effects" but can be deleted 171 // when dead. 172 if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) 173 // Safe to delete llvm.stacksave if dead. 174 if (II->getIntrinsicID() == Intrinsic::stacksave) 175 return true; 176 return false; 177} 178 179/// RecursivelyDeleteTriviallyDeadInstructions - If the specified value is a 180/// trivially dead instruction, delete it. If that makes any of its operands 181/// trivially dead, delete them too, recursively. 182void llvm::RecursivelyDeleteTriviallyDeadInstructions(Value *V) { 183 Instruction *I = dyn_cast<Instruction>(V); 184 if (!I || !I->use_empty() || !isInstructionTriviallyDead(I)) 185 return; 186 187 SmallVector<Instruction*, 16> DeadInsts; 188 DeadInsts.push_back(I); 189 190 while (!DeadInsts.empty()) { 191 I = DeadInsts.back(); 192 DeadInsts.pop_back(); 193 194 // Null out all of the instruction's operands to see if any operand becomes 195 // dead as we go. 196 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) { 197 Value *OpV = I->getOperand(i); 198 I->setOperand(i, 0); 199 200 if (!OpV->use_empty()) continue; 201 202 // If the operand is an instruction that became dead as we nulled out the 203 // operand, and if it is 'trivially' dead, delete it in a future loop 204 // iteration. 205 if (Instruction *OpI = dyn_cast<Instruction>(OpV)) 206 if (isInstructionTriviallyDead(OpI)) 207 DeadInsts.push_back(OpI); 208 } 209 210 I->eraseFromParent(); 211 } 212} 213 214/// RecursivelyDeleteDeadPHINode - If the specified value is an effectively 215/// dead PHI node, due to being a def-use chain of single-use nodes that 216/// either forms a cycle or is terminated by a trivially dead instruction, 217/// delete it. If that makes any of its operands trivially dead, delete them 218/// too, recursively. 219void 220llvm::RecursivelyDeleteDeadPHINode(PHINode *PN) { 221 222 // We can remove a PHI if it is on a cycle in the def-use graph 223 // where each node in the cycle has degree one, i.e. only one use, 224 // and is an instruction with no side effects. 225 if (!PN->hasOneUse()) 226 return; 227 228 SmallPtrSet<PHINode *, 4> PHIs; 229 PHIs.insert(PN); 230 for (Instruction *J = cast<Instruction>(*PN->use_begin()); 231 J->hasOneUse() && !J->mayHaveSideEffects(); 232 J = cast<Instruction>(*J->use_begin())) 233 // If we find a PHI more than once, we're on a cycle that 234 // won't prove fruitful. 235 if (PHINode *JP = dyn_cast<PHINode>(J)) 236 if (!PHIs.insert(cast<PHINode>(JP))) { 237 // Break the cycle and delete the PHI and its operands. 238 JP->replaceAllUsesWith(UndefValue::get(JP->getType())); 239 RecursivelyDeleteTriviallyDeadInstructions(JP); 240 break; 241 } 242} 243 244//===----------------------------------------------------------------------===// 245// Control Flow Graph Restructuring... 246// 247 248/// MergeBasicBlockIntoOnlyPred - DestBB is a block with one predecessor and its 249/// predecessor is known to have one successor (DestBB!). Eliminate the edge 250/// between them, moving the instructions in the predecessor into DestBB and 251/// deleting the predecessor block. 252/// 253void llvm::MergeBasicBlockIntoOnlyPred(BasicBlock *DestBB) { 254 // If BB has single-entry PHI nodes, fold them. 255 while (PHINode *PN = dyn_cast<PHINode>(DestBB->begin())) { 256 Value *NewVal = PN->getIncomingValue(0); 257 // Replace self referencing PHI with undef, it must be dead. 258 if (NewVal == PN) NewVal = UndefValue::get(PN->getType()); 259 PN->replaceAllUsesWith(NewVal); 260 PN->eraseFromParent(); 261 } 262 263 BasicBlock *PredBB = DestBB->getSinglePredecessor(); 264 assert(PredBB && "Block doesn't have a single predecessor!"); 265 266 // Splice all the instructions from PredBB to DestBB. 267 PredBB->getTerminator()->eraseFromParent(); 268 DestBB->getInstList().splice(DestBB->begin(), PredBB->getInstList()); 269 270 // Anything that branched to PredBB now branches to DestBB. 271 PredBB->replaceAllUsesWith(DestBB); 272 273 // Nuke BB. 274 PredBB->eraseFromParent(); 275} 276 277/// OnlyUsedByDbgIntrinsics - Return true if the instruction I is only used 278/// by DbgIntrinsics. If DbgInUses is specified then the vector is filled 279/// with the DbgInfoIntrinsic that use the instruction I. 280bool llvm::OnlyUsedByDbgInfoIntrinsics(Instruction *I, 281 SmallVectorImpl<DbgInfoIntrinsic *> *DbgInUses) { 282 if (DbgInUses) 283 DbgInUses->clear(); 284 285 for (Value::use_iterator UI = I->use_begin(), UE = I->use_end(); UI != UE; 286 ++UI) { 287 if (DbgInfoIntrinsic *DI = dyn_cast<DbgInfoIntrinsic>(*UI)) { 288 if (DbgInUses) 289 DbgInUses->push_back(DI); 290 } else { 291 if (DbgInUses) 292 DbgInUses->clear(); 293 return false; 294 } 295 } 296 return true; 297} 298 299/// UserIsDebugInfo - Return true if U is a constant expr used by 300/// llvm.dbg.variable or llvm.dbg.global_variable 301bool llvm::UserIsDebugInfo(User *U) { 302 ConstantExpr *CE = dyn_cast<ConstantExpr>(U); 303 304 if (!CE || CE->getNumUses() != 1) 305 return false; 306 307 Constant *Init = dyn_cast<Constant>(CE->use_back()); 308 if (!Init || Init->getNumUses() != 1) 309 return false; 310 311 GlobalVariable *GV = dyn_cast<GlobalVariable>(Init->use_back()); 312 if (!GV || !GV->hasInitializer() || GV->getInitializer() != Init) 313 return false; 314 315 DIVariable DV(GV); 316 if (!DV.isNull()) 317 return true; // User is llvm.dbg.variable 318 319 DIGlobalVariable DGV(GV); 320 if (!DGV.isNull()) 321 return true; // User is llvm.dbg.global_variable 322 323 return false; 324} 325 326/// RemoveDbgInfoUser - Remove an User which is representing debug info. 327void llvm::RemoveDbgInfoUser(User *U) { 328 assert (UserIsDebugInfo(U) && "Unexpected User!"); 329 ConstantExpr *CE = cast<ConstantExpr>(U); 330 while (!CE->use_empty()) { 331 Constant *C = cast<Constant>(CE->use_back()); 332 while (!C->use_empty()) { 333 GlobalVariable *GV = cast<GlobalVariable>(C->use_back()); 334 GV->eraseFromParent(); 335 } 336 C->destroyConstant(); 337 } 338 CE->destroyConstant(); 339} 340