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 /// createPHIsForSplitLoopExit - When a loop exit edge is split, LCSSA form 80 /// may require new PHIs in the new exit block. This function inserts the 81 /// new PHIs, as needed. Preds is a list of preds inside the loop, SplitBB 82 /// is the new loop exit block, and DestBB is the old loop exit, now the 83 /// successor of SplitBB. 84 static void createPHIsForSplitLoopExit(ArrayRef<BasicBlock *> Preds, 85 BasicBlock *SplitBB, 86 BasicBlock *DestBB) { 87 // SplitBB shouldn't have anything non-trivial in it yet. 88 assert((SplitBB->getFirstNonPHI() == SplitBB->getTerminator() || 89 SplitBB->isLandingPad()) && "SplitBB has non-PHI nodes!"); 90 91 // For each PHI in the destination block. 92 for (BasicBlock::iterator I = DestBB->begin(); 93 PHINode *PN = dyn_cast<PHINode>(I); ++I) { 94 unsigned Idx = PN->getBasicBlockIndex(SplitBB); 95 Value *V = PN->getIncomingValue(Idx); 96 97 // If the input is a PHI which already satisfies LCSSA, don't create 98 // a new one. 99 if (const PHINode *VP = dyn_cast<PHINode>(V)) 100 if (VP->getParent() == SplitBB) 101 continue; 102 103 // Otherwise a new PHI is needed. Create one and populate it. 104 PHINode *NewPN = 105 PHINode::Create(PN->getType(), Preds.size(), "split", 106 SplitBB->isLandingPad() ? 107 SplitBB->begin() : SplitBB->getTerminator()); 108 for (unsigned i = 0, e = Preds.size(); i != e; ++i) 109 NewPN->addIncoming(V, Preds[i]); 110 111 // Update the original PHI. 112 PN->setIncomingValue(Idx, NewPN); 113 } 114 } 115 116 /// SplitCriticalEdge - If this edge is a critical edge, insert a new node to 117 /// split the critical edge. This will update DominatorTree information if it 118 /// is available, thus calling this pass will not invalidate either of them. 119 /// This returns the new block if the edge was split, null otherwise. 120 /// 121 /// If MergeIdenticalEdges is true (not the default), *all* edges from TI to the 122 /// specified successor will be merged into the same critical edge block. 123 /// This is most commonly interesting with switch instructions, which may 124 /// have many edges to any one destination. This ensures that all edges to that 125 /// dest go to one block instead of each going to a different block, but isn't 126 /// the standard definition of a "critical edge". 127 /// 128 /// It is invalid to call this function on a critical edge that starts at an 129 /// IndirectBrInst. Splitting these edges will almost always create an invalid 130 /// program because the address of the new block won't be the one that is jumped 131 /// to. 132 /// 133 BasicBlock *llvm::SplitCriticalEdge(TerminatorInst *TI, unsigned SuccNum, 134 const CriticalEdgeSplittingOptions &Options) { 135 if (!isCriticalEdge(TI, SuccNum, Options.MergeIdenticalEdges)) 136 return nullptr; 137 138 assert(!isa<IndirectBrInst>(TI) && 139 "Cannot split critical edge from IndirectBrInst"); 140 141 BasicBlock *TIBB = TI->getParent(); 142 BasicBlock *DestBB = TI->getSuccessor(SuccNum); 143 144 // Splitting the critical edge to a landing pad block is non-trivial. Don't do 145 // it in this generic function. 146 if (DestBB->isLandingPad()) return nullptr; 147 148 // Create a new basic block, linking it into the CFG. 149 BasicBlock *NewBB = BasicBlock::Create(TI->getContext(), 150 TIBB->getName() + "." + DestBB->getName() + "_crit_edge"); 151 // Create our unconditional branch. 152 BranchInst *NewBI = BranchInst::Create(DestBB, NewBB); 153 NewBI->setDebugLoc(TI->getDebugLoc()); 154 155 // Branch to the new block, breaking the edge. 156 TI->setSuccessor(SuccNum, NewBB); 157 158 // Insert the block into the function... right after the block TI lives in. 159 Function &F = *TIBB->getParent(); 160 Function::iterator FBBI = TIBB; 161 F.getBasicBlockList().insert(++FBBI, NewBB); 162 163 // If there are any PHI nodes in DestBB, we need to update them so that they 164 // merge incoming values from NewBB instead of from TIBB. 165 { 166 unsigned BBIdx = 0; 167 for (BasicBlock::iterator I = DestBB->begin(); isa<PHINode>(I); ++I) { 168 // We no longer enter through TIBB, now we come in through NewBB. 169 // Revector exactly one entry in the PHI node that used to come from 170 // TIBB to come from NewBB. 171 PHINode *PN = cast<PHINode>(I); 172 173 // Reuse the previous value of BBIdx if it lines up. In cases where we 174 // have multiple phi nodes with *lots* of predecessors, this is a speed 175 // win because we don't have to scan the PHI looking for TIBB. This 176 // happens because the BB list of PHI nodes are usually in the same 177 // order. 178 if (PN->getIncomingBlock(BBIdx) != TIBB) 179 BBIdx = PN->getBasicBlockIndex(TIBB); 180 PN->setIncomingBlock(BBIdx, NewBB); 181 } 182 } 183 184 // If there are any other edges from TIBB to DestBB, update those to go 185 // through the split block, making those edges non-critical as well (and 186 // reducing the number of phi entries in the DestBB if relevant). 187 if (Options.MergeIdenticalEdges) { 188 for (unsigned i = SuccNum+1, e = TI->getNumSuccessors(); i != e; ++i) { 189 if (TI->getSuccessor(i) != DestBB) continue; 190 191 // Remove an entry for TIBB from DestBB phi nodes. 192 DestBB->removePredecessor(TIBB, Options.DontDeleteUselessPHIs); 193 194 // We found another edge to DestBB, go to NewBB instead. 195 TI->setSuccessor(i, NewBB); 196 } 197 } 198 199 // If we have nothing to update, just return. 200 auto *AA = Options.AA; 201 auto *DT = Options.DT; 202 auto *LI = Options.LI; 203 if (!DT && !LI) 204 return NewBB; 205 206 // Now update analysis information. Since the only predecessor of NewBB is 207 // the TIBB, TIBB clearly dominates NewBB. TIBB usually doesn't dominate 208 // anything, as there are other successors of DestBB. However, if all other 209 // predecessors of DestBB are already dominated by DestBB (e.g. DestBB is a 210 // loop header) then NewBB dominates DestBB. 211 SmallVector<BasicBlock*, 8> OtherPreds; 212 213 // If there is a PHI in the block, loop over predecessors with it, which is 214 // faster than iterating pred_begin/end. 215 if (PHINode *PN = dyn_cast<PHINode>(DestBB->begin())) { 216 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) 217 if (PN->getIncomingBlock(i) != NewBB) 218 OtherPreds.push_back(PN->getIncomingBlock(i)); 219 } else { 220 for (pred_iterator I = pred_begin(DestBB), E = pred_end(DestBB); 221 I != E; ++I) { 222 BasicBlock *P = *I; 223 if (P != NewBB) 224 OtherPreds.push_back(P); 225 } 226 } 227 228 bool NewBBDominatesDestBB = true; 229 230 // Should we update DominatorTree information? 231 if (DT) { 232 DomTreeNode *TINode = DT->getNode(TIBB); 233 234 // The new block is not the immediate dominator for any other nodes, but 235 // TINode is the immediate dominator for the new node. 236 // 237 if (TINode) { // Don't break unreachable code! 238 DomTreeNode *NewBBNode = DT->addNewBlock(NewBB, TIBB); 239 DomTreeNode *DestBBNode = nullptr; 240 241 // If NewBBDominatesDestBB hasn't been computed yet, do so with DT. 242 if (!OtherPreds.empty()) { 243 DestBBNode = DT->getNode(DestBB); 244 while (!OtherPreds.empty() && NewBBDominatesDestBB) { 245 if (DomTreeNode *OPNode = DT->getNode(OtherPreds.back())) 246 NewBBDominatesDestBB = DT->dominates(DestBBNode, OPNode); 247 OtherPreds.pop_back(); 248 } 249 OtherPreds.clear(); 250 } 251 252 // If NewBBDominatesDestBB, then NewBB dominates DestBB, otherwise it 253 // doesn't dominate anything. 254 if (NewBBDominatesDestBB) { 255 if (!DestBBNode) DestBBNode = DT->getNode(DestBB); 256 DT->changeImmediateDominator(DestBBNode, NewBBNode); 257 } 258 } 259 } 260 261 // Update LoopInfo if it is around. 262 if (LI) { 263 if (Loop *TIL = LI->getLoopFor(TIBB)) { 264 // If one or the other blocks were not in a loop, the new block is not 265 // either, and thus LI doesn't need to be updated. 266 if (Loop *DestLoop = LI->getLoopFor(DestBB)) { 267 if (TIL == DestLoop) { 268 // Both in the same loop, the NewBB joins loop. 269 DestLoop->addBasicBlockToLoop(NewBB, *LI); 270 } else if (TIL->contains(DestLoop)) { 271 // Edge from an outer loop to an inner loop. Add to the outer loop. 272 TIL->addBasicBlockToLoop(NewBB, *LI); 273 } else if (DestLoop->contains(TIL)) { 274 // Edge from an inner loop to an outer loop. Add to the outer loop. 275 DestLoop->addBasicBlockToLoop(NewBB, *LI); 276 } else { 277 // Edge from two loops with no containment relation. Because these 278 // are natural loops, we know that the destination block must be the 279 // header of its loop (adding a branch into a loop elsewhere would 280 // create an irreducible loop). 281 assert(DestLoop->getHeader() == DestBB && 282 "Should not create irreducible loops!"); 283 if (Loop *P = DestLoop->getParentLoop()) 284 P->addBasicBlockToLoop(NewBB, *LI); 285 } 286 } 287 288 // If TIBB is in a loop and DestBB is outside of that loop, we may need 289 // to update LoopSimplify form and LCSSA form. 290 if (!TIL->contains(DestBB)) { 291 assert(!TIL->contains(NewBB) && 292 "Split point for loop exit is contained in loop!"); 293 294 // Update LCSSA form in the newly created exit block. 295 if (Options.PreserveLCSSA) { 296 createPHIsForSplitLoopExit(TIBB, NewBB, DestBB); 297 } 298 299 // The only that we can break LoopSimplify form by splitting a critical 300 // edge is if after the split there exists some edge from TIL to DestBB 301 // *and* the only edge into DestBB from outside of TIL is that of 302 // NewBB. If the first isn't true, then LoopSimplify still holds, NewBB 303 // is the new exit block and it has no non-loop predecessors. If the 304 // second isn't true, then DestBB was not in LoopSimplify form prior to 305 // the split as it had a non-loop predecessor. In both of these cases, 306 // the predecessor must be directly in TIL, not in a subloop, or again 307 // LoopSimplify doesn't hold. 308 SmallVector<BasicBlock *, 4> LoopPreds; 309 for (pred_iterator I = pred_begin(DestBB), E = pred_end(DestBB); I != E; 310 ++I) { 311 BasicBlock *P = *I; 312 if (P == NewBB) 313 continue; // The new block is known. 314 if (LI->getLoopFor(P) != TIL) { 315 // No need to re-simplify, it wasn't to start with. 316 LoopPreds.clear(); 317 break; 318 } 319 LoopPreds.push_back(P); 320 } 321 if (!LoopPreds.empty()) { 322 assert(!DestBB->isLandingPad() && 323 "We don't split edges to landing pads!"); 324 BasicBlock *NewExitBB = SplitBlockPredecessors( 325 DestBB, LoopPreds, "split", AA, DT, LI, Options.PreserveLCSSA); 326 if (Options.PreserveLCSSA) 327 createPHIsForSplitLoopExit(LoopPreds, NewExitBB, DestBB); 328 } 329 } 330 } 331 } 332 333 return NewBB; 334 } 335