1 //===- BreakCriticalEdges.cpp - Critical Edge Elimination Pass ------------===// 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 // BreakCriticalEdges pass - Break all of the critical edges in the CFG by 11 // inserting a dummy basic block. This pass may be "required" by passes that 12 // cannot deal with critical edges. For this usage, the structure type is 13 // forward declared. This pass obviously invalidates the CFG, but can update 14 // dominator trees. 15 // 16 //===----------------------------------------------------------------------===// 17 18 #include "llvm/Transforms/Scalar.h" 19 #include "llvm/ADT/SmallVector.h" 20 #include "llvm/ADT/Statistic.h" 21 #include "llvm/Analysis/AliasAnalysis.h" 22 #include "llvm/Analysis/CFG.h" 23 #include "llvm/Analysis/LoopInfo.h" 24 #include "llvm/IR/CFG.h" 25 #include "llvm/IR/Dominators.h" 26 #include "llvm/IR/Function.h" 27 #include "llvm/IR/Instructions.h" 28 #include "llvm/IR/Type.h" 29 #include "llvm/Support/ErrorHandling.h" 30 #include "llvm/Transforms/Utils/BasicBlockUtils.h" 31 using namespace llvm; 32 33 #define DEBUG_TYPE "break-crit-edges" 34 35 STATISTIC(NumBroken, "Number of blocks inserted"); 36 37 namespace { 38 struct BreakCriticalEdges : public FunctionPass { 39 static char ID; // Pass identification, replacement for typeid 40 BreakCriticalEdges() : FunctionPass(ID) { 41 initializeBreakCriticalEdgesPass(*PassRegistry::getPassRegistry()); 42 } 43 44 bool runOnFunction(Function &F) override { 45 auto *DTWP = getAnalysisIfAvailable<DominatorTreeWrapperPass>(); 46 auto *DT = DTWP ? &DTWP->getDomTree() : nullptr; 47 auto *LIWP = getAnalysisIfAvailable<LoopInfoWrapperPass>(); 48 auto *LI = LIWP ? &LIWP->getLoopInfo() : nullptr; 49 unsigned N = 50 SplitAllCriticalEdges(F, CriticalEdgeSplittingOptions(DT, LI)); 51 NumBroken += N; 52 return N > 0; 53 } 54 55 void getAnalysisUsage(AnalysisUsage &AU) const override { 56 AU.addPreserved<DominatorTreeWrapperPass>(); 57 AU.addPreserved<LoopInfoWrapperPass>(); 58 59 // No loop canonicalization guarantees are broken by this pass. 60 AU.addPreservedID(LoopSimplifyID); 61 } 62 }; 63 } 64 65 char BreakCriticalEdges::ID = 0; 66 INITIALIZE_PASS(BreakCriticalEdges, "break-crit-edges", 67 "Break critical edges in CFG", false, false) 68 69 // Publicly exposed interface to pass... 70 char &llvm::BreakCriticalEdgesID = BreakCriticalEdges::ID; 71 FunctionPass *llvm::createBreakCriticalEdgesPass() { 72 return new BreakCriticalEdges(); 73 } 74 75 //===----------------------------------------------------------------------===// 76 // Implementation of the external critical edge manipulation functions 77 //===----------------------------------------------------------------------===// 78 79 /// When a loop exit edge is split, LCSSA form may require new PHIs in the new 80 /// exit block. This function inserts the new PHIs, as needed. Preds is a list 81 /// of preds inside the loop, SplitBB is the new loop exit block, and DestBB is 82 /// the old loop exit, now the successor of SplitBB. 83 static void createPHIsForSplitLoopExit(ArrayRef<BasicBlock *> Preds, 84 BasicBlock *SplitBB, 85 BasicBlock *DestBB) { 86 // SplitBB shouldn't have anything non-trivial in it yet. 87 assert((SplitBB->getFirstNonPHI() == SplitBB->getTerminator() || 88 SplitBB->isLandingPad()) && "SplitBB has non-PHI nodes!"); 89 90 // For each PHI in the destination block. 91 for (BasicBlock::iterator I = DestBB->begin(); 92 PHINode *PN = dyn_cast<PHINode>(I); ++I) { 93 unsigned Idx = PN->getBasicBlockIndex(SplitBB); 94 Value *V = PN->getIncomingValue(Idx); 95 96 // If the input is a PHI which already satisfies LCSSA, don't create 97 // a new one. 98 if (const PHINode *VP = dyn_cast<PHINode>(V)) 99 if (VP->getParent() == SplitBB) 100 continue; 101 102 // Otherwise a new PHI is needed. Create one and populate it. 103 PHINode *NewPN = PHINode::Create( 104 PN->getType(), Preds.size(), "split", 105 SplitBB->isLandingPad() ? &SplitBB->front() : SplitBB->getTerminator()); 106 for (unsigned i = 0, e = Preds.size(); i != e; ++i) 107 NewPN->addIncoming(V, Preds[i]); 108 109 // Update the original PHI. 110 PN->setIncomingValue(Idx, NewPN); 111 } 112 } 113 114 BasicBlock * 115 llvm::SplitCriticalEdge(TerminatorInst *TI, unsigned SuccNum, 116 const CriticalEdgeSplittingOptions &Options) { 117 if (!isCriticalEdge(TI, SuccNum, Options.MergeIdenticalEdges)) 118 return nullptr; 119 120 assert(!isa<IndirectBrInst>(TI) && 121 "Cannot split critical edge from IndirectBrInst"); 122 123 BasicBlock *TIBB = TI->getParent(); 124 BasicBlock *DestBB = TI->getSuccessor(SuccNum); 125 126 // Splitting the critical edge to a pad block is non-trivial. Don't do 127 // it in this generic function. 128 if (DestBB->isEHPad()) return nullptr; 129 130 // Create a new basic block, linking it into the CFG. 131 BasicBlock *NewBB = BasicBlock::Create(TI->getContext(), 132 TIBB->getName() + "." + DestBB->getName() + "_crit_edge"); 133 // Create our unconditional branch. 134 BranchInst *NewBI = BranchInst::Create(DestBB, NewBB); 135 NewBI->setDebugLoc(TI->getDebugLoc()); 136 137 // Branch to the new block, breaking the edge. 138 TI->setSuccessor(SuccNum, NewBB); 139 140 // Insert the block into the function... right after the block TI lives in. 141 Function &F = *TIBB->getParent(); 142 Function::iterator FBBI = TIBB->getIterator(); 143 F.getBasicBlockList().insert(++FBBI, NewBB); 144 145 // If there are any PHI nodes in DestBB, we need to update them so that they 146 // merge incoming values from NewBB instead of from TIBB. 147 { 148 unsigned BBIdx = 0; 149 for (BasicBlock::iterator I = DestBB->begin(); isa<PHINode>(I); ++I) { 150 // We no longer enter through TIBB, now we come in through NewBB. 151 // Revector exactly one entry in the PHI node that used to come from 152 // TIBB to come from NewBB. 153 PHINode *PN = cast<PHINode>(I); 154 155 // Reuse the previous value of BBIdx if it lines up. In cases where we 156 // have multiple phi nodes with *lots* of predecessors, this is a speed 157 // win because we don't have to scan the PHI looking for TIBB. This 158 // happens because the BB list of PHI nodes are usually in the same 159 // order. 160 if (PN->getIncomingBlock(BBIdx) != TIBB) 161 BBIdx = PN->getBasicBlockIndex(TIBB); 162 PN->setIncomingBlock(BBIdx, NewBB); 163 } 164 } 165 166 // If there are any other edges from TIBB to DestBB, update those to go 167 // through the split block, making those edges non-critical as well (and 168 // reducing the number of phi entries in the DestBB if relevant). 169 if (Options.MergeIdenticalEdges) { 170 for (unsigned i = SuccNum+1, e = TI->getNumSuccessors(); i != e; ++i) { 171 if (TI->getSuccessor(i) != DestBB) continue; 172 173 // Remove an entry for TIBB from DestBB phi nodes. 174 DestBB->removePredecessor(TIBB, Options.DontDeleteUselessPHIs); 175 176 // We found another edge to DestBB, go to NewBB instead. 177 TI->setSuccessor(i, NewBB); 178 } 179 } 180 181 // If we have nothing to update, just return. 182 auto *DT = Options.DT; 183 auto *LI = Options.LI; 184 if (!DT && !LI) 185 return NewBB; 186 187 // Now update analysis information. Since the only predecessor of NewBB is 188 // the TIBB, TIBB clearly dominates NewBB. TIBB usually doesn't dominate 189 // anything, as there are other successors of DestBB. However, if all other 190 // predecessors of DestBB are already dominated by DestBB (e.g. DestBB is a 191 // loop header) then NewBB dominates DestBB. 192 SmallVector<BasicBlock*, 8> OtherPreds; 193 194 // If there is a PHI in the block, loop over predecessors with it, which is 195 // faster than iterating pred_begin/end. 196 if (PHINode *PN = dyn_cast<PHINode>(DestBB->begin())) { 197 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) 198 if (PN->getIncomingBlock(i) != NewBB) 199 OtherPreds.push_back(PN->getIncomingBlock(i)); 200 } else { 201 for (pred_iterator I = pred_begin(DestBB), E = pred_end(DestBB); 202 I != E; ++I) { 203 BasicBlock *P = *I; 204 if (P != NewBB) 205 OtherPreds.push_back(P); 206 } 207 } 208 209 bool NewBBDominatesDestBB = true; 210 211 // Should we update DominatorTree information? 212 if (DT) { 213 DomTreeNode *TINode = DT->getNode(TIBB); 214 215 // The new block is not the immediate dominator for any other nodes, but 216 // TINode is the immediate dominator for the new node. 217 // 218 if (TINode) { // Don't break unreachable code! 219 DomTreeNode *NewBBNode = DT->addNewBlock(NewBB, TIBB); 220 DomTreeNode *DestBBNode = nullptr; 221 222 // If NewBBDominatesDestBB hasn't been computed yet, do so with DT. 223 if (!OtherPreds.empty()) { 224 DestBBNode = DT->getNode(DestBB); 225 while (!OtherPreds.empty() && NewBBDominatesDestBB) { 226 if (DomTreeNode *OPNode = DT->getNode(OtherPreds.back())) 227 NewBBDominatesDestBB = DT->dominates(DestBBNode, OPNode); 228 OtherPreds.pop_back(); 229 } 230 OtherPreds.clear(); 231 } 232 233 // If NewBBDominatesDestBB, then NewBB dominates DestBB, otherwise it 234 // doesn't dominate anything. 235 if (NewBBDominatesDestBB) { 236 if (!DestBBNode) DestBBNode = DT->getNode(DestBB); 237 DT->changeImmediateDominator(DestBBNode, NewBBNode); 238 } 239 } 240 } 241 242 // Update LoopInfo if it is around. 243 if (LI) { 244 if (Loop *TIL = LI->getLoopFor(TIBB)) { 245 // If one or the other blocks were not in a loop, the new block is not 246 // either, and thus LI doesn't need to be updated. 247 if (Loop *DestLoop = LI->getLoopFor(DestBB)) { 248 if (TIL == DestLoop) { 249 // Both in the same loop, the NewBB joins loop. 250 DestLoop->addBasicBlockToLoop(NewBB, *LI); 251 } else if (TIL->contains(DestLoop)) { 252 // Edge from an outer loop to an inner loop. Add to the outer loop. 253 TIL->addBasicBlockToLoop(NewBB, *LI); 254 } else if (DestLoop->contains(TIL)) { 255 // Edge from an inner loop to an outer loop. Add to the outer loop. 256 DestLoop->addBasicBlockToLoop(NewBB, *LI); 257 } else { 258 // Edge from two loops with no containment relation. Because these 259 // are natural loops, we know that the destination block must be the 260 // header of its loop (adding a branch into a loop elsewhere would 261 // create an irreducible loop). 262 assert(DestLoop->getHeader() == DestBB && 263 "Should not create irreducible loops!"); 264 if (Loop *P = DestLoop->getParentLoop()) 265 P->addBasicBlockToLoop(NewBB, *LI); 266 } 267 } 268 269 // If TIBB is in a loop and DestBB is outside of that loop, we may need 270 // to update LoopSimplify form and LCSSA form. 271 if (!TIL->contains(DestBB)) { 272 assert(!TIL->contains(NewBB) && 273 "Split point for loop exit is contained in loop!"); 274 275 // Update LCSSA form in the newly created exit block. 276 if (Options.PreserveLCSSA) { 277 createPHIsForSplitLoopExit(TIBB, NewBB, DestBB); 278 } 279 280 // The only that we can break LoopSimplify form by splitting a critical 281 // edge is if after the split there exists some edge from TIL to DestBB 282 // *and* the only edge into DestBB from outside of TIL is that of 283 // NewBB. If the first isn't true, then LoopSimplify still holds, NewBB 284 // is the new exit block and it has no non-loop predecessors. If the 285 // second isn't true, then DestBB was not in LoopSimplify form prior to 286 // the split as it had a non-loop predecessor. In both of these cases, 287 // the predecessor must be directly in TIL, not in a subloop, or again 288 // LoopSimplify doesn't hold. 289 SmallVector<BasicBlock *, 4> LoopPreds; 290 for (pred_iterator I = pred_begin(DestBB), E = pred_end(DestBB); I != E; 291 ++I) { 292 BasicBlock *P = *I; 293 if (P == NewBB) 294 continue; // The new block is known. 295 if (LI->getLoopFor(P) != TIL) { 296 // No need to re-simplify, it wasn't to start with. 297 LoopPreds.clear(); 298 break; 299 } 300 LoopPreds.push_back(P); 301 } 302 if (!LoopPreds.empty()) { 303 assert(!DestBB->isEHPad() && "We don't split edges to EH pads!"); 304 BasicBlock *NewExitBB = SplitBlockPredecessors( 305 DestBB, LoopPreds, "split", DT, LI, Options.PreserveLCSSA); 306 if (Options.PreserveLCSSA) 307 createPHIsForSplitLoopExit(LoopPreds, NewExitBB, DestBB); 308 } 309 } 310 } 311 } 312 313 return NewBB; 314 } 315