LLVM API Documentation
00001 //===-- LoopUnswitch.cpp - Hoist loop-invariant conditionals in loop ------===// 00002 // 00003 // The LLVM Compiler Infrastructure 00004 // 00005 // This file was developed by the LLVM research group and is distributed under 00006 // the University of Illinois Open Source License. See LICENSE.TXT for details. 00007 // 00008 //===----------------------------------------------------------------------===// 00009 // 00010 // This pass transforms loops that contain branches on loop-invariant conditions 00011 // to have multiple loops. For example, it turns the left into the right code: 00012 // 00013 // for (...) if (lic) 00014 // A for (...) 00015 // if (lic) A; B; C 00016 // B else 00017 // C for (...) 00018 // A; C 00019 // 00020 // This can increase the size of the code exponentially (doubling it every time 00021 // a loop is unswitched) so we only unswitch if the resultant code will be 00022 // smaller than a threshold. 00023 // 00024 // This pass expects LICM to be run before it to hoist invariant conditions out 00025 // of the loop, to make the unswitching opportunity obvious. 00026 // 00027 //===----------------------------------------------------------------------===// 00028 00029 #define DEBUG_TYPE "loop-unswitch" 00030 #include "llvm/Transforms/Scalar.h" 00031 #include "llvm/Constants.h" 00032 #include "llvm/Function.h" 00033 #include "llvm/Instructions.h" 00034 #include "llvm/Analysis/LoopInfo.h" 00035 #include "llvm/Transforms/Utils/Cloning.h" 00036 #include "llvm/Transforms/Utils/Local.h" 00037 #include "llvm/Transforms/Utils/BasicBlockUtils.h" 00038 #include "llvm/ADT/Statistic.h" 00039 #include "llvm/ADT/PostOrderIterator.h" 00040 #include "llvm/Support/Debug.h" 00041 #include "llvm/Support/CommandLine.h" 00042 #include <algorithm> 00043 #include <iostream> 00044 #include <set> 00045 using namespace llvm; 00046 00047 namespace { 00048 Statistic<> NumBranches("loop-unswitch", "Number of branches unswitched"); 00049 Statistic<> NumSwitches("loop-unswitch", "Number of switches unswitched"); 00050 Statistic<> NumSelects ("loop-unswitch", "Number of selects unswitched"); 00051 Statistic<> NumTrivial ("loop-unswitch", 00052 "Number of unswitches that are trivial"); 00053 Statistic<> NumSimplify("loop-unswitch", 00054 "Number of simplifications of unswitched code"); 00055 cl::opt<unsigned> 00056 Threshold("loop-unswitch-threshold", cl::desc("Max loop size to unswitch"), 00057 cl::init(10), cl::Hidden); 00058 00059 class LoopUnswitch : public FunctionPass { 00060 LoopInfo *LI; // Loop information 00061 00062 // LoopProcessWorklist - List of loops we need to process. 00063 std::vector<Loop*> LoopProcessWorklist; 00064 public: 00065 virtual bool runOnFunction(Function &F); 00066 bool visitLoop(Loop *L); 00067 00068 /// This transformation requires natural loop information & requires that 00069 /// loop preheaders be inserted into the CFG... 00070 /// 00071 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 00072 AU.addRequiredID(LoopSimplifyID); 00073 AU.addPreservedID(LoopSimplifyID); 00074 AU.addRequired<LoopInfo>(); 00075 AU.addPreserved<LoopInfo>(); 00076 AU.addRequiredID(LCSSAID); 00077 AU.addPreservedID(LCSSAID); 00078 } 00079 00080 private: 00081 /// RemoveLoopFromWorklist - If the specified loop is on the loop worklist, 00082 /// remove it. 00083 void RemoveLoopFromWorklist(Loop *L) { 00084 std::vector<Loop*>::iterator I = std::find(LoopProcessWorklist.begin(), 00085 LoopProcessWorklist.end(), L); 00086 if (I != LoopProcessWorklist.end()) 00087 LoopProcessWorklist.erase(I); 00088 } 00089 00090 bool UnswitchIfProfitable(Value *LoopCond, Constant *Val,Loop *L); 00091 unsigned getLoopUnswitchCost(Loop *L, Value *LIC); 00092 void UnswitchTrivialCondition(Loop *L, Value *Cond, Constant *Val, 00093 BasicBlock *ExitBlock); 00094 void UnswitchNontrivialCondition(Value *LIC, Constant *OnVal, Loop *L); 00095 BasicBlock *SplitEdge(BasicBlock *From, BasicBlock *To); 00096 BasicBlock *SplitBlock(BasicBlock *Old, Instruction *SplitPt); 00097 00098 void RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC, 00099 Constant *Val, bool isEqual); 00100 00101 void SimplifyCode(std::vector<Instruction*> &Worklist); 00102 void RemoveBlockIfDead(BasicBlock *BB, 00103 std::vector<Instruction*> &Worklist); 00104 void RemoveLoopFromHierarchy(Loop *L); 00105 }; 00106 RegisterOpt<LoopUnswitch> X("loop-unswitch", "Unswitch loops"); 00107 } 00108 00109 FunctionPass *llvm::createLoopUnswitchPass() { return new LoopUnswitch(); } 00110 00111 bool LoopUnswitch::runOnFunction(Function &F) { 00112 bool Changed = false; 00113 LI = &getAnalysis<LoopInfo>(); 00114 00115 // Populate the worklist of loops to process in post-order. 00116 for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I) 00117 for (po_iterator<Loop*> LI = po_begin(*I), E = po_end(*I); LI != E; ++LI) 00118 LoopProcessWorklist.push_back(*LI); 00119 00120 // Process the loops in worklist order, this is a post-order visitation of 00121 // the loops. We use a worklist of loops so that loops can be removed at any 00122 // time if they are deleted (e.g. the backedge of a loop is removed). 00123 while (!LoopProcessWorklist.empty()) { 00124 Loop *L = LoopProcessWorklist.back(); 00125 LoopProcessWorklist.pop_back(); 00126 Changed |= visitLoop(L); 00127 } 00128 00129 return Changed; 00130 } 00131 00132 /// FindLIVLoopCondition - Cond is a condition that occurs in L. If it is 00133 /// invariant in the loop, or has an invariant piece, return the invariant. 00134 /// Otherwise, return null. 00135 static Value *FindLIVLoopCondition(Value *Cond, Loop *L, bool &Changed) { 00136 // Constants should be folded, not unswitched on! 00137 if (isa<Constant>(Cond)) return false; 00138 00139 // TODO: Handle: br (VARIANT|INVARIANT). 00140 // TODO: Hoist simple expressions out of loops. 00141 if (L->isLoopInvariant(Cond)) return Cond; 00142 00143 if (BinaryOperator *BO = dyn_cast<BinaryOperator>(Cond)) 00144 if (BO->getOpcode() == Instruction::And || 00145 BO->getOpcode() == Instruction::Or) { 00146 // If either the left or right side is invariant, we can unswitch on this, 00147 // which will cause the branch to go away in one loop and the condition to 00148 // simplify in the other one. 00149 if (Value *LHS = FindLIVLoopCondition(BO->getOperand(0), L, Changed)) 00150 return LHS; 00151 if (Value *RHS = FindLIVLoopCondition(BO->getOperand(1), L, Changed)) 00152 return RHS; 00153 } 00154 00155 return 0; 00156 } 00157 00158 bool LoopUnswitch::visitLoop(Loop *L) { 00159 assert(L->isLCSSAForm()); 00160 00161 bool Changed = false; 00162 00163 // Loop over all of the basic blocks in the loop. If we find an interior 00164 // block that is branching on a loop-invariant condition, we can unswitch this 00165 // loop. 00166 for (Loop::block_iterator I = L->block_begin(), E = L->block_end(); 00167 I != E; ++I) { 00168 TerminatorInst *TI = (*I)->getTerminator(); 00169 if (BranchInst *BI = dyn_cast<BranchInst>(TI)) { 00170 // If this isn't branching on an invariant condition, we can't unswitch 00171 // it. 00172 if (BI->isConditional()) { 00173 // See if this, or some part of it, is loop invariant. If so, we can 00174 // unswitch on it if we desire. 00175 Value *LoopCond = FindLIVLoopCondition(BI->getCondition(), L, Changed); 00176 if (LoopCond && UnswitchIfProfitable(LoopCond, ConstantBool::True, L)) { 00177 ++NumBranches; 00178 return true; 00179 } 00180 } 00181 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) { 00182 Value *LoopCond = FindLIVLoopCondition(SI->getCondition(), L, Changed); 00183 if (LoopCond && SI->getNumCases() > 1) { 00184 // Find a value to unswitch on: 00185 // FIXME: this should chose the most expensive case! 00186 Constant *UnswitchVal = SI->getCaseValue(1); 00187 if (UnswitchIfProfitable(LoopCond, UnswitchVal, L)) { 00188 ++NumSwitches; 00189 return true; 00190 } 00191 } 00192 } 00193 00194 // Scan the instructions to check for unswitchable values. 00195 for (BasicBlock::iterator BBI = (*I)->begin(), E = (*I)->end(); 00196 BBI != E; ++BBI) 00197 if (SelectInst *SI = dyn_cast<SelectInst>(BBI)) { 00198 Value *LoopCond = FindLIVLoopCondition(SI->getCondition(), L, Changed); 00199 if (LoopCond && UnswitchIfProfitable(LoopCond, ConstantBool::True, L)) { 00200 ++NumSelects; 00201 return true; 00202 } 00203 } 00204 } 00205 00206 assert(L->isLCSSAForm()); 00207 00208 return Changed; 00209 } 00210 00211 /// isTrivialLoopExitBlock - Check to see if all paths from BB either: 00212 /// 1. Exit the loop with no side effects. 00213 /// 2. Branch to the latch block with no side-effects. 00214 /// 00215 /// If these conditions are true, we return true and set ExitBB to the block we 00216 /// exit through. 00217 /// 00218 static bool isTrivialLoopExitBlockHelper(Loop *L, BasicBlock *BB, 00219 BasicBlock *&ExitBB, 00220 std::set<BasicBlock*> &Visited) { 00221 if (!Visited.insert(BB).second) { 00222 // Already visited and Ok, end of recursion. 00223 return true; 00224 } else if (!L->contains(BB)) { 00225 // Otherwise, this is a loop exit, this is fine so long as this is the 00226 // first exit. 00227 if (ExitBB != 0) return false; 00228 ExitBB = BB; 00229 return true; 00230 } 00231 00232 // Otherwise, this is an unvisited intra-loop node. Check all successors. 00233 for (succ_iterator SI = succ_begin(BB), E = succ_end(BB); SI != E; ++SI) { 00234 // Check to see if the successor is a trivial loop exit. 00235 if (!isTrivialLoopExitBlockHelper(L, *SI, ExitBB, Visited)) 00236 return false; 00237 } 00238 00239 // Okay, everything after this looks good, check to make sure that this block 00240 // doesn't include any side effects. 00241 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) 00242 if (I->mayWriteToMemory()) 00243 return false; 00244 00245 return true; 00246 } 00247 00248 /// isTrivialLoopExitBlock - Return true if the specified block unconditionally 00249 /// leads to an exit from the specified loop, and has no side-effects in the 00250 /// process. If so, return the block that is exited to, otherwise return null. 00251 static BasicBlock *isTrivialLoopExitBlock(Loop *L, BasicBlock *BB) { 00252 std::set<BasicBlock*> Visited; 00253 Visited.insert(L->getHeader()); // Branches to header are ok. 00254 BasicBlock *ExitBB = 0; 00255 if (isTrivialLoopExitBlockHelper(L, BB, ExitBB, Visited)) 00256 return ExitBB; 00257 return 0; 00258 } 00259 00260 /// IsTrivialUnswitchCondition - Check to see if this unswitch condition is 00261 /// trivial: that is, that the condition controls whether or not the loop does 00262 /// anything at all. If this is a trivial condition, unswitching produces no 00263 /// code duplications (equivalently, it produces a simpler loop and a new empty 00264 /// loop, which gets deleted). 00265 /// 00266 /// If this is a trivial condition, return true, otherwise return false. When 00267 /// returning true, this sets Cond and Val to the condition that controls the 00268 /// trivial condition: when Cond dynamically equals Val, the loop is known to 00269 /// exit. Finally, this sets LoopExit to the BB that the loop exits to when 00270 /// Cond == Val. 00271 /// 00272 static bool IsTrivialUnswitchCondition(Loop *L, Value *Cond, Constant **Val = 0, 00273 BasicBlock **LoopExit = 0) { 00274 BasicBlock *Header = L->getHeader(); 00275 TerminatorInst *HeaderTerm = Header->getTerminator(); 00276 00277 BasicBlock *LoopExitBB = 0; 00278 if (BranchInst *BI = dyn_cast<BranchInst>(HeaderTerm)) { 00279 // If the header block doesn't end with a conditional branch on Cond, we 00280 // can't handle it. 00281 if (!BI->isConditional() || BI->getCondition() != Cond) 00282 return false; 00283 00284 // Check to see if a successor of the branch is guaranteed to go to the 00285 // latch block or exit through a one exit block without having any 00286 // side-effects. If so, determine the value of Cond that causes it to do 00287 // this. 00288 if ((LoopExitBB = isTrivialLoopExitBlock(L, BI->getSuccessor(0)))) { 00289 if (Val) *Val = ConstantBool::True; 00290 } else if ((LoopExitBB = isTrivialLoopExitBlock(L, BI->getSuccessor(1)))) { 00291 if (Val) *Val = ConstantBool::False; 00292 } 00293 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(HeaderTerm)) { 00294 // If this isn't a switch on Cond, we can't handle it. 00295 if (SI->getCondition() != Cond) return false; 00296 00297 // Check to see if a successor of the switch is guaranteed to go to the 00298 // latch block or exit through a one exit block without having any 00299 // side-effects. If so, determine the value of Cond that causes it to do 00300 // this. Note that we can't trivially unswitch on the default case. 00301 for (unsigned i = 1, e = SI->getNumSuccessors(); i != e; ++i) 00302 if ((LoopExitBB = isTrivialLoopExitBlock(L, SI->getSuccessor(i)))) { 00303 // Okay, we found a trivial case, remember the value that is trivial. 00304 if (Val) *Val = SI->getCaseValue(i); 00305 break; 00306 } 00307 } 00308 00309 // If we didn't find a single unique LoopExit block, or if the loop exit block 00310 // contains phi nodes, this isn't trivial. 00311 if (!LoopExitBB || isa<PHINode>(LoopExitBB->begin())) 00312 return false; // Can't handle this. 00313 00314 if (LoopExit) *LoopExit = LoopExitBB; 00315 00316 // We already know that nothing uses any scalar values defined inside of this 00317 // loop. As such, we just have to check to see if this loop will execute any 00318 // side-effecting instructions (e.g. stores, calls, volatile loads) in the 00319 // part of the loop that the code *would* execute. We already checked the 00320 // tail, check the header now. 00321 for (BasicBlock::iterator I = Header->begin(), E = Header->end(); I != E; ++I) 00322 if (I->mayWriteToMemory()) 00323 return false; 00324 return true; 00325 } 00326 00327 /// getLoopUnswitchCost - Return the cost (code size growth) that will happen if 00328 /// we choose to unswitch the specified loop on the specified value. 00329 /// 00330 unsigned LoopUnswitch::getLoopUnswitchCost(Loop *L, Value *LIC) { 00331 // If the condition is trivial, always unswitch. There is no code growth for 00332 // this case. 00333 if (IsTrivialUnswitchCondition(L, LIC)) 00334 return 0; 00335 00336 // FIXME: This is really overly conservative. However, more liberal 00337 // estimations have thus far resulted in excessive unswitching, which is bad 00338 // both in compile time and in code size. This should be replaced once 00339 // someone figures out how a good estimation. 00340 return L->getBlocks().size(); 00341 00342 unsigned Cost = 0; 00343 // FIXME: this is brain dead. It should take into consideration code 00344 // shrinkage. 00345 for (Loop::block_iterator I = L->block_begin(), E = L->block_end(); 00346 I != E; ++I) { 00347 BasicBlock *BB = *I; 00348 // Do not include empty blocks in the cost calculation. This happen due to 00349 // loop canonicalization and will be removed. 00350 if (BB->begin() == BasicBlock::iterator(BB->getTerminator())) 00351 continue; 00352 00353 // Count basic blocks. 00354 ++Cost; 00355 } 00356 00357 return Cost; 00358 } 00359 00360 /// UnswitchIfProfitable - We have found that we can unswitch L when 00361 /// LoopCond == Val to simplify the loop. If we decide that this is profitable, 00362 /// unswitch the loop, reprocess the pieces, then return true. 00363 bool LoopUnswitch::UnswitchIfProfitable(Value *LoopCond, Constant *Val,Loop *L){ 00364 // Check to see if it would be profitable to unswitch this loop. 00365 unsigned Cost = getLoopUnswitchCost(L, LoopCond); 00366 if (Cost > Threshold) { 00367 // FIXME: this should estimate growth by the amount of code shared by the 00368 // resultant unswitched loops. 00369 // 00370 DEBUG(std::cerr << "NOT unswitching loop %" 00371 << L->getHeader()->getName() << ", cost too high: " 00372 << L->getBlocks().size() << "\n"); 00373 return false; 00374 } 00375 00376 // If this is a trivial condition to unswitch (which results in no code 00377 // duplication), do it now. 00378 Constant *CondVal; 00379 BasicBlock *ExitBlock; 00380 if (IsTrivialUnswitchCondition(L, LoopCond, &CondVal, &ExitBlock)) { 00381 UnswitchTrivialCondition(L, LoopCond, CondVal, ExitBlock); 00382 } else { 00383 UnswitchNontrivialCondition(LoopCond, Val, L); 00384 } 00385 00386 return true; 00387 } 00388 00389 /// SplitBlock - Split the specified block at the specified instruction - every 00390 /// thing before SplitPt stays in Old and everything starting with SplitPt moves 00391 /// to a new block. The two blocks are joined by an unconditional branch and 00392 /// the loop info is updated. 00393 /// 00394 BasicBlock *LoopUnswitch::SplitBlock(BasicBlock *Old, Instruction *SplitPt) { 00395 BasicBlock::iterator SplitIt = SplitPt; 00396 while (isa<PHINode>(SplitIt)) 00397 ++SplitIt; 00398 BasicBlock *New = Old->splitBasicBlock(SplitIt, Old->getName()+".split"); 00399 00400 // The new block lives in whichever loop the old one did. 00401 if (Loop *L = LI->getLoopFor(Old)) 00402 L->addBasicBlockToLoop(New, *LI); 00403 00404 return New; 00405 } 00406 00407 00408 BasicBlock *LoopUnswitch::SplitEdge(BasicBlock *BB, BasicBlock *Succ) { 00409 TerminatorInst *LatchTerm = BB->getTerminator(); 00410 unsigned SuccNum = 0; 00411 for (unsigned i = 0, e = LatchTerm->getNumSuccessors(); ; ++i) { 00412 assert(i != e && "Didn't find edge?"); 00413 if (LatchTerm->getSuccessor(i) == Succ) { 00414 SuccNum = i; 00415 break; 00416 } 00417 } 00418 00419 // If this is a critical edge, let SplitCriticalEdge do it. 00420 if (SplitCriticalEdge(BB->getTerminator(), SuccNum, this)) 00421 return LatchTerm->getSuccessor(SuccNum); 00422 00423 // If the edge isn't critical, then BB has a single successor or Succ has a 00424 // single pred. Split the block. 00425 BasicBlock::iterator SplitPoint; 00426 if (BasicBlock *SP = Succ->getSinglePredecessor()) { 00427 // If the successor only has a single pred, split the top of the successor 00428 // block. 00429 assert(SP == BB && "CFG broken"); 00430 return SplitBlock(Succ, Succ->begin()); 00431 } else { 00432 // Otherwise, if BB has a single successor, split it at the bottom of the 00433 // block. 00434 assert(BB->getTerminator()->getNumSuccessors() == 1 && 00435 "Should have a single succ!"); 00436 return SplitBlock(BB, BB->getTerminator()); 00437 } 00438 } 00439 00440 00441 00442 // RemapInstruction - Convert the instruction operands from referencing the 00443 // current values into those specified by ValueMap. 00444 // 00445 static inline void RemapInstruction(Instruction *I, 00446 std::map<const Value *, Value*> &ValueMap) { 00447 for (unsigned op = 0, E = I->getNumOperands(); op != E; ++op) { 00448 Value *Op = I->getOperand(op); 00449 std::map<const Value *, Value*>::iterator It = ValueMap.find(Op); 00450 if (It != ValueMap.end()) Op = It->second; 00451 I->setOperand(op, Op); 00452 } 00453 } 00454 00455 /// CloneLoop - Recursively clone the specified loop and all of its children, 00456 /// mapping the blocks with the specified map. 00457 static Loop *CloneLoop(Loop *L, Loop *PL, std::map<const Value*, Value*> &VM, 00458 LoopInfo *LI) { 00459 Loop *New = new Loop(); 00460 00461 if (PL) 00462 PL->addChildLoop(New); 00463 else 00464 LI->addTopLevelLoop(New); 00465 00466 // Add all of the blocks in L to the new loop. 00467 for (Loop::block_iterator I = L->block_begin(), E = L->block_end(); 00468 I != E; ++I) 00469 if (LI->getLoopFor(*I) == L) 00470 New->addBasicBlockToLoop(cast<BasicBlock>(VM[*I]), *LI); 00471 00472 // Add all of the subloops to the new loop. 00473 for (Loop::iterator I = L->begin(), E = L->end(); I != E; ++I) 00474 CloneLoop(*I, New, VM, LI); 00475 00476 return New; 00477 } 00478 00479 /// EmitPreheaderBranchOnCondition - Emit a conditional branch on two values 00480 /// if LIC == Val, branch to TrueDst, otherwise branch to FalseDest. Insert the 00481 /// code immediately before InsertPt. 00482 static void EmitPreheaderBranchOnCondition(Value *LIC, Constant *Val, 00483 BasicBlock *TrueDest, 00484 BasicBlock *FalseDest, 00485 Instruction *InsertPt) { 00486 // Insert a conditional branch on LIC to the two preheaders. The original 00487 // code is the true version and the new code is the false version. 00488 Value *BranchVal = LIC; 00489 if (!isa<ConstantBool>(Val)) { 00490 BranchVal = BinaryOperator::createSetEQ(LIC, Val, "tmp", InsertPt); 00491 } else if (Val != ConstantBool::True) { 00492 // We want to enter the new loop when the condition is true. 00493 std::swap(TrueDest, FalseDest); 00494 } 00495 00496 // Insert the new branch. 00497 new BranchInst(TrueDest, FalseDest, BranchVal, InsertPt); 00498 } 00499 00500 00501 /// UnswitchTrivialCondition - Given a loop that has a trivial unswitchable 00502 /// condition in it (a cond branch from its header block to its latch block, 00503 /// where the path through the loop that doesn't execute its body has no 00504 /// side-effects), unswitch it. This doesn't involve any code duplication, just 00505 /// moving the conditional branch outside of the loop and updating loop info. 00506 void LoopUnswitch::UnswitchTrivialCondition(Loop *L, Value *Cond, 00507 Constant *Val, 00508 BasicBlock *ExitBlock) { 00509 DEBUG(std::cerr << "loop-unswitch: Trivial-Unswitch loop %" 00510 << L->getHeader()->getName() << " [" << L->getBlocks().size() 00511 << " blocks] in Function " << L->getHeader()->getParent()->getName() 00512 << " on cond: " << *Val << " == " << *Cond << "\n"); 00513 00514 // First step, split the preheader, so that we know that there is a safe place 00515 // to insert the conditional branch. We will change 'OrigPH' to have a 00516 // conditional branch on Cond. 00517 BasicBlock *OrigPH = L->getLoopPreheader(); 00518 BasicBlock *NewPH = SplitEdge(OrigPH, L->getHeader()); 00519 00520 // Now that we have a place to insert the conditional branch, create a place 00521 // to branch to: this is the exit block out of the loop that we should 00522 // short-circuit to. 00523 00524 // Split this block now, so that the loop maintains its exit block, and so 00525 // that the jump from the preheader can execute the contents of the exit block 00526 // without actually branching to it (the exit block should be dominated by the 00527 // loop header, not the preheader). 00528 assert(!L->contains(ExitBlock) && "Exit block is in the loop?"); 00529 BasicBlock *NewExit = SplitBlock(ExitBlock, ExitBlock->begin()); 00530 00531 // Okay, now we have a position to branch from and a position to branch to, 00532 // insert the new conditional branch. 00533 EmitPreheaderBranchOnCondition(Cond, Val, NewExit, NewPH, 00534 OrigPH->getTerminator()); 00535 OrigPH->getTerminator()->eraseFromParent(); 00536 00537 // We need to reprocess this loop, it could be unswitched again. 00538 LoopProcessWorklist.push_back(L); 00539 00540 // Now that we know that the loop is never entered when this condition is a 00541 // particular value, rewrite the loop with this info. We know that this will 00542 // at least eliminate the old branch. 00543 RewriteLoopBodyWithConditionConstant(L, Cond, Val, false); 00544 ++NumTrivial; 00545 } 00546 00547 00548 /// VersionLoop - We determined that the loop is profitable to unswitch when LIC 00549 /// equal Val. Split it into loop versions and test the condition outside of 00550 /// either loop. Return the loops created as Out1/Out2. 00551 void LoopUnswitch::UnswitchNontrivialCondition(Value *LIC, Constant *Val, 00552 Loop *L) { 00553 Function *F = L->getHeader()->getParent(); 00554 DEBUG(std::cerr << "loop-unswitch: Unswitching loop %" 00555 << L->getHeader()->getName() << " [" << L->getBlocks().size() 00556 << " blocks] in Function " << F->getName() 00557 << " when '" << *Val << "' == " << *LIC << "\n"); 00558 00559 // LoopBlocks contains all of the basic blocks of the loop, including the 00560 // preheader of the loop, the body of the loop, and the exit blocks of the 00561 // loop, in that order. 00562 std::vector<BasicBlock*> LoopBlocks; 00563 00564 // First step, split the preheader and exit blocks, and add these blocks to 00565 // the LoopBlocks list. 00566 BasicBlock *OrigPreheader = L->getLoopPreheader(); 00567 LoopBlocks.push_back(SplitEdge(OrigPreheader, L->getHeader())); 00568 00569 // We want the loop to come after the preheader, but before the exit blocks. 00570 LoopBlocks.insert(LoopBlocks.end(), L->block_begin(), L->block_end()); 00571 00572 std::vector<BasicBlock*> ExitBlocks; 00573 L->getExitBlocks(ExitBlocks); 00574 std::sort(ExitBlocks.begin(), ExitBlocks.end()); 00575 ExitBlocks.erase(std::unique(ExitBlocks.begin(), ExitBlocks.end()), 00576 ExitBlocks.end()); 00577 00578 // Split all of the edges from inside the loop to their exit blocks. Update 00579 // the appropriate Phi nodes as we do so. 00580 unsigned NumBlocks = L->getBlocks().size(); 00581 00582 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) { 00583 BasicBlock *ExitBlock = ExitBlocks[i]; 00584 std::vector<BasicBlock*> Preds(pred_begin(ExitBlock), pred_end(ExitBlock)); 00585 00586 for (unsigned j = 0, e = Preds.size(); j != e; ++j) { 00587 assert(L->contains(Preds[j]) && 00588 "All preds of loop exit blocks must be the same loop!"); 00589 BasicBlock* MiddleBlock = SplitEdge(Preds[j], ExitBlock); 00590 BasicBlock* StartBlock = Preds[j]; 00591 BasicBlock* EndBlock; 00592 if (MiddleBlock->getSinglePredecessor() == ExitBlock) { 00593 EndBlock = MiddleBlock; 00594 MiddleBlock = EndBlock->getSinglePredecessor();; 00595 } else { 00596 EndBlock = ExitBlock; 00597 } 00598 00599 std::set<PHINode*> InsertedPHIs; 00600 PHINode* OldLCSSA = 0; 00601 for (BasicBlock::iterator I = EndBlock->begin(); 00602 (OldLCSSA = dyn_cast<PHINode>(I)); ++I) { 00603 Value* OldValue = OldLCSSA->getIncomingValueForBlock(MiddleBlock); 00604 PHINode* NewLCSSA = new PHINode(OldLCSSA->getType(), 00605 OldLCSSA->getName() + ".us-lcssa", 00606 MiddleBlock->getTerminator()); 00607 NewLCSSA->addIncoming(OldValue, StartBlock); 00608 OldLCSSA->setIncomingValue(OldLCSSA->getBasicBlockIndex(MiddleBlock), 00609 NewLCSSA); 00610 InsertedPHIs.insert(NewLCSSA); 00611 } 00612 00613 BasicBlock::iterator InsertPt = EndBlock->begin(); 00614 while (dyn_cast<PHINode>(InsertPt)) ++InsertPt; 00615 for (BasicBlock::iterator I = MiddleBlock->begin(); 00616 (OldLCSSA = dyn_cast<PHINode>(I)) && InsertedPHIs.count(OldLCSSA) == 0; 00617 ++I) { 00618 PHINode *NewLCSSA = new PHINode(OldLCSSA->getType(), 00619 OldLCSSA->getName() + ".us-lcssa", 00620 InsertPt); 00621 OldLCSSA->replaceAllUsesWith(NewLCSSA); 00622 NewLCSSA->addIncoming(OldLCSSA, MiddleBlock); 00623 } 00624 } 00625 } 00626 00627 // The exit blocks may have been changed due to edge splitting, recompute. 00628 ExitBlocks.clear(); 00629 L->getExitBlocks(ExitBlocks); 00630 std::sort(ExitBlocks.begin(), ExitBlocks.end()); 00631 ExitBlocks.erase(std::unique(ExitBlocks.begin(), ExitBlocks.end()), 00632 ExitBlocks.end()); 00633 00634 // Add exit blocks to the loop blocks. 00635 LoopBlocks.insert(LoopBlocks.end(), ExitBlocks.begin(), ExitBlocks.end()); 00636 00637 // Next step, clone all of the basic blocks that make up the loop (including 00638 // the loop preheader and exit blocks), keeping track of the mapping between 00639 // the instructions and blocks. 00640 std::vector<BasicBlock*> NewBlocks; 00641 NewBlocks.reserve(LoopBlocks.size()); 00642 std::map<const Value*, Value*> ValueMap; 00643 for (unsigned i = 0, e = LoopBlocks.size(); i != e; ++i) { 00644 BasicBlock *New = CloneBasicBlock(LoopBlocks[i], ValueMap, ".us", F); 00645 NewBlocks.push_back(New); 00646 ValueMap[LoopBlocks[i]] = New; // Keep the BB mapping. 00647 } 00648 00649 // Splice the newly inserted blocks into the function right before the 00650 // original preheader. 00651 F->getBasicBlockList().splice(LoopBlocks[0], F->getBasicBlockList(), 00652 NewBlocks[0], F->end()); 00653 00654 // Now we create the new Loop object for the versioned loop. 00655 Loop *NewLoop = CloneLoop(L, L->getParentLoop(), ValueMap, LI); 00656 Loop *ParentLoop = L->getParentLoop(); 00657 if (ParentLoop) { 00658 // Make sure to add the cloned preheader and exit blocks to the parent loop 00659 // as well. 00660 ParentLoop->addBasicBlockToLoop(NewBlocks[0], *LI); 00661 } 00662 00663 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) { 00664 BasicBlock *NewExit = cast<BasicBlock>(ValueMap[ExitBlocks[i]]); 00665 // The new exit block should be in the same loop as the old one. 00666 if (Loop *ExitBBLoop = LI->getLoopFor(ExitBlocks[i])) 00667 ExitBBLoop->addBasicBlockToLoop(NewExit, *LI); 00668 00669 assert(NewExit->getTerminator()->getNumSuccessors() == 1 && 00670 "Exit block should have been split to have one successor!"); 00671 BasicBlock *ExitSucc = NewExit->getTerminator()->getSuccessor(0); 00672 00673 // If the successor of the exit block had PHI nodes, add an entry for 00674 // NewExit. 00675 PHINode *PN; 00676 for (BasicBlock::iterator I = ExitSucc->begin(); 00677 (PN = dyn_cast<PHINode>(I)); ++I) { 00678 Value *V = PN->getIncomingValueForBlock(ExitBlocks[i]); 00679 std::map<const Value *, Value*>::iterator It = ValueMap.find(V); 00680 if (It != ValueMap.end()) V = It->second; 00681 PN->addIncoming(V, NewExit); 00682 } 00683 } 00684 00685 // Rewrite the code to refer to itself. 00686 for (unsigned i = 0, e = NewBlocks.size(); i != e; ++i) 00687 for (BasicBlock::iterator I = NewBlocks[i]->begin(), 00688 E = NewBlocks[i]->end(); I != E; ++I) 00689 RemapInstruction(I, ValueMap); 00690 00691 // Rewrite the original preheader to select between versions of the loop. 00692 BranchInst *OldBR = cast<BranchInst>(OrigPreheader->getTerminator()); 00693 assert(OldBR->isUnconditional() && OldBR->getSuccessor(0) == LoopBlocks[0] && 00694 "Preheader splitting did not work correctly!"); 00695 00696 // Emit the new branch that selects between the two versions of this loop. 00697 EmitPreheaderBranchOnCondition(LIC, Val, NewBlocks[0], LoopBlocks[0], OldBR); 00698 OldBR->eraseFromParent(); 00699 00700 LoopProcessWorklist.push_back(L); 00701 LoopProcessWorklist.push_back(NewLoop); 00702 00703 // Now we rewrite the original code to know that the condition is true and the 00704 // new code to know that the condition is false. 00705 RewriteLoopBodyWithConditionConstant(L , LIC, Val, false); 00706 00707 // It's possible that simplifying one loop could cause the other to be 00708 // deleted. If so, don't simplify it. 00709 if (!LoopProcessWorklist.empty() && LoopProcessWorklist.back() == NewLoop) 00710 RewriteLoopBodyWithConditionConstant(NewLoop, LIC, Val, true); 00711 } 00712 00713 /// RemoveFromWorklist - Remove all instances of I from the worklist vector 00714 /// specified. 00715 static void RemoveFromWorklist(Instruction *I, 00716 std::vector<Instruction*> &Worklist) { 00717 std::vector<Instruction*>::iterator WI = std::find(Worklist.begin(), 00718 Worklist.end(), I); 00719 while (WI != Worklist.end()) { 00720 unsigned Offset = WI-Worklist.begin(); 00721 Worklist.erase(WI); 00722 WI = std::find(Worklist.begin()+Offset, Worklist.end(), I); 00723 } 00724 } 00725 00726 /// ReplaceUsesOfWith - When we find that I really equals V, remove I from the 00727 /// program, replacing all uses with V and update the worklist. 00728 static void ReplaceUsesOfWith(Instruction *I, Value *V, 00729 std::vector<Instruction*> &Worklist) { 00730 DEBUG(std::cerr << "Replace with '" << *V << "': " << *I); 00731 00732 // Add uses to the worklist, which may be dead now. 00733 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) 00734 if (Instruction *Use = dyn_cast<Instruction>(I->getOperand(i))) 00735 Worklist.push_back(Use); 00736 00737 // Add users to the worklist which may be simplified now. 00738 for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); 00739 UI != E; ++UI) 00740 Worklist.push_back(cast<Instruction>(*UI)); 00741 I->replaceAllUsesWith(V); 00742 I->eraseFromParent(); 00743 RemoveFromWorklist(I, Worklist); 00744 ++NumSimplify; 00745 } 00746 00747 /// RemoveBlockIfDead - If the specified block is dead, remove it, update loop 00748 /// information, and remove any dead successors it has. 00749 /// 00750 void LoopUnswitch::RemoveBlockIfDead(BasicBlock *BB, 00751 std::vector<Instruction*> &Worklist) { 00752 if (pred_begin(BB) != pred_end(BB)) { 00753 // This block isn't dead, since an edge to BB was just removed, see if there 00754 // are any easy simplifications we can do now. 00755 if (BasicBlock *Pred = BB->getSinglePredecessor()) { 00756 // If it has one pred, fold phi nodes in BB. 00757 while (isa<PHINode>(BB->begin())) 00758 ReplaceUsesOfWith(BB->begin(), 00759 cast<PHINode>(BB->begin())->getIncomingValue(0), 00760 Worklist); 00761 00762 // If this is the header of a loop and the only pred is the latch, we now 00763 // have an unreachable loop. 00764 if (Loop *L = LI->getLoopFor(BB)) 00765 if (L->getHeader() == BB && L->contains(Pred)) { 00766 // Remove the branch from the latch to the header block, this makes 00767 // the header dead, which will make the latch dead (because the header 00768 // dominates the latch). 00769 Pred->getTerminator()->eraseFromParent(); 00770 new UnreachableInst(Pred); 00771 00772 // The loop is now broken, remove it from LI. 00773 RemoveLoopFromHierarchy(L); 00774 00775 // Reprocess the header, which now IS dead. 00776 RemoveBlockIfDead(BB, Worklist); 00777 return; 00778 } 00779 00780 // If pred ends in a uncond branch, add uncond branch to worklist so that 00781 // the two blocks will get merged. 00782 if (BranchInst *BI = dyn_cast<BranchInst>(Pred->getTerminator())) 00783 if (BI->isUnconditional()) 00784 Worklist.push_back(BI); 00785 } 00786 return; 00787 } 00788 00789 DEBUG(std::cerr << "Nuking dead block: " << *BB); 00790 00791 // Remove the instructions in the basic block from the worklist. 00792 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) { 00793 RemoveFromWorklist(I, Worklist); 00794 00795 // Anything that uses the instructions in this basic block should have their 00796 // uses replaced with undefs. 00797 if (!I->use_empty()) 00798 I->replaceAllUsesWith(UndefValue::get(I->getType())); 00799 } 00800 00801 // If this is the edge to the header block for a loop, remove the loop and 00802 // promote all subloops. 00803 if (Loop *BBLoop = LI->getLoopFor(BB)) { 00804 if (BBLoop->getLoopLatch() == BB) 00805 RemoveLoopFromHierarchy(BBLoop); 00806 } 00807 00808 // Remove the block from the loop info, which removes it from any loops it 00809 // was in. 00810 LI->removeBlock(BB); 00811 00812 00813 // Remove phi node entries in successors for this block. 00814 TerminatorInst *TI = BB->getTerminator(); 00815 std::vector<BasicBlock*> Succs; 00816 for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i) { 00817 Succs.push_back(TI->getSuccessor(i)); 00818 TI->getSuccessor(i)->removePredecessor(BB); 00819 } 00820 00821 // Unique the successors, remove anything with multiple uses. 00822 std::sort(Succs.begin(), Succs.end()); 00823 Succs.erase(std::unique(Succs.begin(), Succs.end()), Succs.end()); 00824 00825 // Remove the basic block, including all of the instructions contained in it. 00826 BB->eraseFromParent(); 00827 00828 // Remove successor blocks here that are not dead, so that we know we only 00829 // have dead blocks in this list. Nondead blocks have a way of becoming dead, 00830 // then getting removed before we revisit them, which is badness. 00831 // 00832 for (unsigned i = 0; i != Succs.size(); ++i) 00833 if (pred_begin(Succs[i]) != pred_end(Succs[i])) { 00834 // One exception is loop headers. If this block was the preheader for a 00835 // loop, then we DO want to visit the loop so the loop gets deleted. 00836 // We know that if the successor is a loop header, that this loop had to 00837 // be the preheader: the case where this was the latch block was handled 00838 // above and headers can only have two predecessors. 00839 if (!LI->isLoopHeader(Succs[i])) { 00840 Succs.erase(Succs.begin()+i); 00841 --i; 00842 } 00843 } 00844 00845 for (unsigned i = 0, e = Succs.size(); i != e; ++i) 00846 RemoveBlockIfDead(Succs[i], Worklist); 00847 } 00848 00849 /// RemoveLoopFromHierarchy - We have discovered that the specified loop has 00850 /// become unwrapped, either because the backedge was deleted, or because the 00851 /// edge into the header was removed. If the edge into the header from the 00852 /// latch block was removed, the loop is unwrapped but subloops are still alive, 00853 /// so they just reparent loops. If the loops are actually dead, they will be 00854 /// removed later. 00855 void LoopUnswitch::RemoveLoopFromHierarchy(Loop *L) { 00856 if (Loop *ParentLoop = L->getParentLoop()) { // Not a top-level loop. 00857 // Reparent all of the blocks in this loop. Since BBLoop had a parent, 00858 // they are now all in it. 00859 for (Loop::block_iterator I = L->block_begin(), E = L->block_end(); 00860 I != E; ++I) 00861 if (LI->getLoopFor(*I) == L) // Don't change blocks in subloops. 00862 LI->changeLoopFor(*I, ParentLoop); 00863 00864 // Remove the loop from its parent loop. 00865 for (Loop::iterator I = ParentLoop->begin(), E = ParentLoop->end();; 00866 ++I) { 00867 assert(I != E && "Couldn't find loop"); 00868 if (*I == L) { 00869 ParentLoop->removeChildLoop(I); 00870 break; 00871 } 00872 } 00873 00874 // Move all subloops into the parent loop. 00875 while (L->begin() != L->end()) 00876 ParentLoop->addChildLoop(L->removeChildLoop(L->end()-1)); 00877 } else { 00878 // Reparent all of the blocks in this loop. Since BBLoop had no parent, 00879 // they no longer in a loop at all. 00880 00881 for (unsigned i = 0; i != L->getBlocks().size(); ++i) { 00882 // Don't change blocks in subloops. 00883 if (LI->getLoopFor(L->getBlocks()[i]) == L) { 00884 LI->removeBlock(L->getBlocks()[i]); 00885 --i; 00886 } 00887 } 00888 00889 // Remove the loop from the top-level LoopInfo object. 00890 for (LoopInfo::iterator I = LI->begin(), E = LI->end();; ++I) { 00891 assert(I != E && "Couldn't find loop"); 00892 if (*I == L) { 00893 LI->removeLoop(I); 00894 break; 00895 } 00896 } 00897 00898 // Move all of the subloops to the top-level. 00899 while (L->begin() != L->end()) 00900 LI->addTopLevelLoop(L->removeChildLoop(L->end()-1)); 00901 } 00902 00903 delete L; 00904 RemoveLoopFromWorklist(L); 00905 } 00906 00907 00908 00909 // RewriteLoopBodyWithConditionConstant - We know either that the value LIC has 00910 // the value specified by Val in the specified loop, or we know it does NOT have 00911 // that value. Rewrite any uses of LIC or of properties correlated to it. 00912 void LoopUnswitch::RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC, 00913 Constant *Val, 00914 bool IsEqual) { 00915 assert(!isa<Constant>(LIC) && "Why are we unswitching on a constant?"); 00916 00917 // FIXME: Support correlated properties, like: 00918 // for (...) 00919 // if (li1 < li2) 00920 // ... 00921 // if (li1 > li2) 00922 // ... 00923 00924 // FOLD boolean conditions (X|LIC), (X&LIC). Fold conditional branches, 00925 // selects, switches. 00926 std::vector<User*> Users(LIC->use_begin(), LIC->use_end()); 00927 std::vector<Instruction*> Worklist; 00928 00929 // If we know that LIC == Val, or that LIC == NotVal, just replace uses of LIC 00930 // in the loop with the appropriate one directly. 00931 if (IsEqual || isa<ConstantBool>(Val)) { 00932 Value *Replacement; 00933 if (IsEqual) 00934 Replacement = Val; 00935 else 00936 Replacement = ConstantBool::get(!cast<ConstantBool>(Val)->getValue()); 00937 00938 for (unsigned i = 0, e = Users.size(); i != e; ++i) 00939 if (Instruction *U = cast<Instruction>(Users[i])) { 00940 if (!L->contains(U->getParent())) 00941 continue; 00942 U->replaceUsesOfWith(LIC, Replacement); 00943 Worklist.push_back(U); 00944 } 00945 } else { 00946 // Otherwise, we don't know the precise value of LIC, but we do know that it 00947 // is certainly NOT "Val". As such, simplify any uses in the loop that we 00948 // can. This case occurs when we unswitch switch statements. 00949 for (unsigned i = 0, e = Users.size(); i != e; ++i) 00950 if (Instruction *U = cast<Instruction>(Users[i])) { 00951 if (!L->contains(U->getParent())) 00952 continue; 00953 00954 Worklist.push_back(U); 00955 00956 // If we know that LIC is not Val, use this info to simplify code. 00957 if (SwitchInst *SI = dyn_cast<SwitchInst>(U)) { 00958 for (unsigned i = 1, e = SI->getNumCases(); i != e; ++i) { 00959 if (SI->getCaseValue(i) == Val) { 00960 // Found a dead case value. Don't remove PHI nodes in the 00961 // successor if they become single-entry, those PHI nodes may 00962 // be in the Users list. 00963 00964 // FIXME: This is a hack. We need to keep the successor around 00965 // and hooked up so as to preserve the loop structure, because 00966 // trying to update it is complicated. So instead we preserve the 00967 // loop structure and put the block on an dead code path. 00968 00969 BasicBlock* Old = SI->getParent(); 00970 BasicBlock* Split = SplitBlock(Old, SI); 00971 00972 Instruction* OldTerm = Old->getTerminator(); 00973 BranchInst* Branch = new BranchInst(Split, 00974 SI->getSuccessor(i), 00975 ConstantBool::True, 00976 OldTerm); 00977 00978 Old->getTerminator()->eraseFromParent(); 00979 00980 00981 PHINode *PN; 00982 for (BasicBlock::iterator II = SI->getSuccessor(i)->begin(); 00983 (PN = dyn_cast<PHINode>(II)); ++II) { 00984 Value *InVal = PN->removeIncomingValue(Split, false); 00985 PN->addIncoming(InVal, Old); 00986 } 00987 00988 SI->removeCase(i); 00989 break; 00990 } 00991 } 00992 } 00993 00994 // TODO: We could do other simplifications, for example, turning 00995 // LIC == Val -> false. 00996 } 00997 } 00998 00999 SimplifyCode(Worklist); 01000 } 01001 01002 /// SimplifyCode - Okay, now that we have simplified some instructions in the 01003 /// loop, walk over it and constant prop, dce, and fold control flow where 01004 /// possible. Note that this is effectively a very simple loop-structure-aware 01005 /// optimizer. During processing of this loop, L could very well be deleted, so 01006 /// it must not be used. 01007 /// 01008 /// FIXME: When the loop optimizer is more mature, separate this out to a new 01009 /// pass. 01010 /// 01011 void LoopUnswitch::SimplifyCode(std::vector<Instruction*> &Worklist) { 01012 while (!Worklist.empty()) { 01013 Instruction *I = Worklist.back(); 01014 Worklist.pop_back(); 01015 01016 // Simple constant folding. 01017 if (Constant *C = ConstantFoldInstruction(I)) { 01018 ReplaceUsesOfWith(I, C, Worklist); 01019 continue; 01020 } 01021 01022 // Simple DCE. 01023 if (isInstructionTriviallyDead(I)) { 01024 DEBUG(std::cerr << "Remove dead instruction '" << *I); 01025 01026 // Add uses to the worklist, which may be dead now. 01027 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) 01028 if (Instruction *Use = dyn_cast<Instruction>(I->getOperand(i))) 01029 Worklist.push_back(Use); 01030 I->eraseFromParent(); 01031 RemoveFromWorklist(I, Worklist); 01032 ++NumSimplify; 01033 continue; 01034 } 01035 01036 // Special case hacks that appear commonly in unswitched code. 01037 switch (I->getOpcode()) { 01038 case Instruction::Select: 01039 if (ConstantBool *CB = dyn_cast<ConstantBool>(I->getOperand(0))) { 01040 ReplaceUsesOfWith(I, I->getOperand(!CB->getValue()+1), Worklist); 01041 continue; 01042 } 01043 break; 01044 case Instruction::And: 01045 if (isa<ConstantBool>(I->getOperand(0))) // constant -> RHS 01046 cast<BinaryOperator>(I)->swapOperands(); 01047 if (ConstantBool *CB = dyn_cast<ConstantBool>(I->getOperand(1))) { 01048 if (CB->getValue()) // X & 1 -> X 01049 ReplaceUsesOfWith(I, I->getOperand(0), Worklist); 01050 else // X & 0 -> 0 01051 ReplaceUsesOfWith(I, I->getOperand(1), Worklist); 01052 continue; 01053 } 01054 break; 01055 case Instruction::Or: 01056 if (isa<ConstantBool>(I->getOperand(0))) // constant -> RHS 01057 cast<BinaryOperator>(I)->swapOperands(); 01058 if (ConstantBool *CB = dyn_cast<ConstantBool>(I->getOperand(1))) { 01059 if (CB->getValue()) // X | 1 -> 1 01060 ReplaceUsesOfWith(I, I->getOperand(1), Worklist); 01061 else // X | 0 -> X 01062 ReplaceUsesOfWith(I, I->getOperand(0), Worklist); 01063 continue; 01064 } 01065 break; 01066 case Instruction::Br: { 01067 BranchInst *BI = cast<BranchInst>(I); 01068 if (BI->isUnconditional()) { 01069 // If BI's parent is the only pred of the successor, fold the two blocks 01070 // together. 01071 BasicBlock *Pred = BI->getParent(); 01072 BasicBlock *Succ = BI->getSuccessor(0); 01073 BasicBlock *SinglePred = Succ->getSinglePredecessor(); 01074 if (!SinglePred) continue; // Nothing to do. 01075 assert(SinglePred == Pred && "CFG broken"); 01076 01077 DEBUG(std::cerr << "Merging blocks: " << Pred->getName() << " <- " 01078 << Succ->getName() << "\n"); 01079 01080 // Resolve any single entry PHI nodes in Succ. 01081 while (PHINode *PN = dyn_cast<PHINode>(Succ->begin())) 01082 ReplaceUsesOfWith(PN, PN->getIncomingValue(0), Worklist); 01083 01084 // Move all of the successor contents from Succ to Pred. 01085 Pred->getInstList().splice(BI, Succ->getInstList(), Succ->begin(), 01086 Succ->end()); 01087 BI->eraseFromParent(); 01088 RemoveFromWorklist(BI, Worklist); 01089 01090 // If Succ has any successors with PHI nodes, update them to have 01091 // entries coming from Pred instead of Succ. 01092 Succ->replaceAllUsesWith(Pred); 01093 01094 // Remove Succ from the loop tree. 01095 LI->removeBlock(Succ); 01096 Succ->eraseFromParent(); 01097 ++NumSimplify; 01098 } else if (ConstantBool *CB = dyn_cast<ConstantBool>(BI->getCondition())){ 01099 // Conditional branch. Turn it into an unconditional branch, then 01100 // remove dead blocks. 01101 break; // FIXME: Enable. 01102 01103 DEBUG(std::cerr << "Folded branch: " << *BI); 01104 BasicBlock *DeadSucc = BI->getSuccessor(CB->getValue()); 01105 BasicBlock *LiveSucc = BI->getSuccessor(!CB->getValue()); 01106 DeadSucc->removePredecessor(BI->getParent(), true); 01107 Worklist.push_back(new BranchInst(LiveSucc, BI)); 01108 BI->eraseFromParent(); 01109 RemoveFromWorklist(BI, Worklist); 01110 ++NumSimplify; 01111 01112 RemoveBlockIfDead(DeadSucc, Worklist); 01113 } 01114 break; 01115 } 01116 } 01117 } 01118 }