1 //===- Transform/Utils/BasicBlockUtils.h - BasicBlock Utils -----*- C++ -*-===// 2 // 3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. 4 // See https://llvm.org/LICENSE.txt for license information. 5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception 6 // 7 //===----------------------------------------------------------------------===// 8 // 9 // This family of functions perform manipulations on basic blocks, and 10 // instructions contained within basic blocks. 11 // 12 //===----------------------------------------------------------------------===// 13 14 #ifndef LLVM_TRANSFORMS_UTILS_BASICBLOCKUTILS_H 15 #define LLVM_TRANSFORMS_UTILS_BASICBLOCKUTILS_H 16 17 // FIXME: Move to this file: BasicBlock::removePredecessor, BB::splitBasicBlock 18 19 #include "llvm/ADT/ArrayRef.h" 20 #include "llvm/Analysis/DomTreeUpdater.h" 21 #include "llvm/IR/BasicBlock.h" 22 #include "llvm/IR/CFG.h" 23 #include "llvm/IR/InstrTypes.h" 24 #include <cassert> 25 26 namespace llvm { 27 28 class BlockFrequencyInfo; 29 class BranchProbabilityInfo; 30 class DominatorTree; 31 class DomTreeUpdater; 32 class Function; 33 class Instruction; 34 class LoopInfo; 35 class MDNode; 36 class MemoryDependenceResults; 37 class MemorySSAUpdater; 38 class PostDominatorTree; 39 class ReturnInst; 40 class TargetLibraryInfo; 41 class Value; 42 43 /// Replace contents of every block in \p BBs with single unreachable 44 /// instruction. If \p Updates is specified, collect all necessary DT updates 45 /// into this vector. If \p KeepOneInputPHIs is true, one-input Phis in 46 /// successors of blocks being deleted will be preserved. 47 void DetatchDeadBlocks(ArrayRef <BasicBlock *> BBs, 48 SmallVectorImpl<DominatorTree::UpdateType> *Updates, 49 bool KeepOneInputPHIs = false); 50 51 /// Delete the specified block, which must have no predecessors. 52 void DeleteDeadBlock(BasicBlock *BB, DomTreeUpdater *DTU = nullptr, 53 bool KeepOneInputPHIs = false); 54 55 /// Delete the specified blocks from \p BB. The set of deleted blocks must have 56 /// no predecessors that are not being deleted themselves. \p BBs must have no 57 /// duplicating blocks. If there are loops among this set of blocks, all 58 /// relevant loop info updates should be done before this function is called. 59 /// If \p KeepOneInputPHIs is true, one-input Phis in successors of blocks 60 /// being deleted will be preserved. 61 void DeleteDeadBlocks(ArrayRef <BasicBlock *> BBs, 62 DomTreeUpdater *DTU = nullptr, 63 bool KeepOneInputPHIs = false); 64 65 /// Delete all basic blocks from \p F that are not reachable from its entry 66 /// node. If \p KeepOneInputPHIs is true, one-input Phis in successors of 67 /// blocks being deleted will be preserved. 68 bool EliminateUnreachableBlocks(Function &F, DomTreeUpdater *DTU = nullptr, 69 bool KeepOneInputPHIs = false); 70 71 /// We know that BB has one predecessor. If there are any single-entry PHI nodes 72 /// in it, fold them away. This handles the case when all entries to the PHI 73 /// nodes in a block are guaranteed equal, such as when the block has exactly 74 /// one predecessor. 75 void FoldSingleEntryPHINodes(BasicBlock *BB, 76 MemoryDependenceResults *MemDep = nullptr); 77 78 /// Examine each PHI in the given block and delete it if it is dead. Also 79 /// recursively delete any operands that become dead as a result. This includes 80 /// tracing the def-use list from the PHI to see if it is ultimately unused or 81 /// if it reaches an unused cycle. Return true if any PHIs were deleted. 82 bool DeleteDeadPHIs(BasicBlock *BB, const TargetLibraryInfo *TLI = nullptr); 83 84 /// Attempts to merge a block into its predecessor, if possible. The return 85 /// value indicates success or failure. 86 /// By default do not merge blocks if BB's predecessor has multiple successors. 87 /// If PredecessorWithTwoSuccessors = true, the blocks can only be merged 88 /// if BB's Pred has a branch to BB and to AnotherBB, and BB has a single 89 /// successor Sing. In this case the branch will be updated with Sing instead of 90 /// BB, and BB will still be merged into its predecessor and removed. 91 bool MergeBlockIntoPredecessor(BasicBlock *BB, DomTreeUpdater *DTU = nullptr, 92 LoopInfo *LI = nullptr, 93 MemorySSAUpdater *MSSAU = nullptr, 94 MemoryDependenceResults *MemDep = nullptr, 95 bool PredecessorWithTwoSuccessors = false); 96 97 /// Try to remove redundant dbg.value instructions from given basic block. 98 /// Returns true if at least one instruction was removed. 99 bool RemoveRedundantDbgInstrs(BasicBlock *BB); 100 101 /// Replace all uses of an instruction (specified by BI) with a value, then 102 /// remove and delete the original instruction. 103 void ReplaceInstWithValue(BasicBlock::InstListType &BIL, 104 BasicBlock::iterator &BI, Value *V); 105 106 /// Replace the instruction specified by BI with the instruction specified by I. 107 /// Copies DebugLoc from BI to I, if I doesn't already have a DebugLoc. The 108 /// original instruction is deleted and BI is updated to point to the new 109 /// instruction. 110 void ReplaceInstWithInst(BasicBlock::InstListType &BIL, 111 BasicBlock::iterator &BI, Instruction *I); 112 113 /// Replace the instruction specified by From with the instruction specified by 114 /// To. Copies DebugLoc from BI to I, if I doesn't already have a DebugLoc. 115 void ReplaceInstWithInst(Instruction *From, Instruction *To); 116 117 /// Option class for critical edge splitting. 118 /// 119 /// This provides a builder interface for overriding the default options used 120 /// during critical edge splitting. 121 struct CriticalEdgeSplittingOptions { 122 DominatorTree *DT; 123 PostDominatorTree *PDT; 124 LoopInfo *LI; 125 MemorySSAUpdater *MSSAU; 126 bool MergeIdenticalEdges = false; 127 bool KeepOneInputPHIs = false; 128 bool PreserveLCSSA = false; 129 bool IgnoreUnreachableDests = false; 130 131 CriticalEdgeSplittingOptions(DominatorTree *DT = nullptr, 132 LoopInfo *LI = nullptr, 133 MemorySSAUpdater *MSSAU = nullptr, 134 PostDominatorTree *PDT = nullptr) DTCriticalEdgeSplittingOptions135 : DT(DT), PDT(PDT), LI(LI), MSSAU(MSSAU) {} 136 setMergeIdenticalEdgesCriticalEdgeSplittingOptions137 CriticalEdgeSplittingOptions &setMergeIdenticalEdges() { 138 MergeIdenticalEdges = true; 139 return *this; 140 } 141 setKeepOneInputPHIsCriticalEdgeSplittingOptions142 CriticalEdgeSplittingOptions &setKeepOneInputPHIs() { 143 KeepOneInputPHIs = true; 144 return *this; 145 } 146 setPreserveLCSSACriticalEdgeSplittingOptions147 CriticalEdgeSplittingOptions &setPreserveLCSSA() { 148 PreserveLCSSA = true; 149 return *this; 150 } 151 setIgnoreUnreachableDestsCriticalEdgeSplittingOptions152 CriticalEdgeSplittingOptions &setIgnoreUnreachableDests() { 153 IgnoreUnreachableDests = true; 154 return *this; 155 } 156 }; 157 158 /// If this edge is a critical edge, insert a new node to split the critical 159 /// edge. This will update the analyses passed in through the option struct. 160 /// This returns the new block if the edge was split, null otherwise. 161 /// 162 /// If MergeIdenticalEdges in the options struct is true (not the default), 163 /// *all* edges from TI to the specified successor will be merged into the same 164 /// critical edge block. This is most commonly interesting with switch 165 /// instructions, which may have many edges to any one destination. This 166 /// ensures that all edges to that dest go to one block instead of each going 167 /// to a different block, but isn't the standard definition of a "critical 168 /// edge". 169 /// 170 /// It is invalid to call this function on a critical edge that starts at an 171 /// IndirectBrInst. Splitting these edges will almost always create an invalid 172 /// program because the address of the new block won't be the one that is jumped 173 /// to. 174 BasicBlock *SplitCriticalEdge(Instruction *TI, unsigned SuccNum, 175 const CriticalEdgeSplittingOptions &Options = 176 CriticalEdgeSplittingOptions()); 177 178 inline BasicBlock * 179 SplitCriticalEdge(BasicBlock *BB, succ_iterator SI, 180 const CriticalEdgeSplittingOptions &Options = 181 CriticalEdgeSplittingOptions()) { 182 return SplitCriticalEdge(BB->getTerminator(), SI.getSuccessorIndex(), 183 Options); 184 } 185 186 /// If the edge from *PI to BB is not critical, return false. Otherwise, split 187 /// all edges between the two blocks and return true. This updates all of the 188 /// same analyses as the other SplitCriticalEdge function. If P is specified, it 189 /// updates the analyses described above. 190 inline bool SplitCriticalEdge(BasicBlock *Succ, pred_iterator PI, 191 const CriticalEdgeSplittingOptions &Options = 192 CriticalEdgeSplittingOptions()) { 193 bool MadeChange = false; 194 Instruction *TI = (*PI)->getTerminator(); 195 for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i) 196 if (TI->getSuccessor(i) == Succ) 197 MadeChange |= !!SplitCriticalEdge(TI, i, Options); 198 return MadeChange; 199 } 200 201 /// If an edge from Src to Dst is critical, split the edge and return true, 202 /// otherwise return false. This method requires that there be an edge between 203 /// the two blocks. It updates the analyses passed in the options struct 204 inline BasicBlock * 205 SplitCriticalEdge(BasicBlock *Src, BasicBlock *Dst, 206 const CriticalEdgeSplittingOptions &Options = 207 CriticalEdgeSplittingOptions()) { 208 Instruction *TI = Src->getTerminator(); 209 unsigned i = 0; 210 while (true) { 211 assert(i != TI->getNumSuccessors() && "Edge doesn't exist!"); 212 if (TI->getSuccessor(i) == Dst) 213 return SplitCriticalEdge(TI, i, Options); 214 ++i; 215 } 216 } 217 218 /// Loop over all of the edges in the CFG, breaking critical edges as they are 219 /// found. Returns the number of broken edges. 220 unsigned SplitAllCriticalEdges(Function &F, 221 const CriticalEdgeSplittingOptions &Options = 222 CriticalEdgeSplittingOptions()); 223 224 /// Split the edge connecting specified block. 225 BasicBlock *SplitEdge(BasicBlock *From, BasicBlock *To, 226 DominatorTree *DT = nullptr, LoopInfo *LI = nullptr, 227 MemorySSAUpdater *MSSAU = nullptr); 228 229 /// Split the specified block at the specified instruction - everything before 230 /// SplitPt stays in Old and everything starting with SplitPt moves to a new 231 /// block. The two blocks are joined by an unconditional branch and the loop 232 /// info is updated. 233 BasicBlock *SplitBlock(BasicBlock *Old, Instruction *SplitPt, 234 DominatorTree *DT = nullptr, LoopInfo *LI = nullptr, 235 MemorySSAUpdater *MSSAU = nullptr, 236 const Twine &BBName = ""); 237 238 /// This method introduces at least one new basic block into the function and 239 /// moves some of the predecessors of BB to be predecessors of the new block. 240 /// The new predecessors are indicated by the Preds array. The new block is 241 /// given a suffix of 'Suffix'. Returns new basic block to which predecessors 242 /// from Preds are now pointing. 243 /// 244 /// If BB is a landingpad block then additional basicblock might be introduced. 245 /// It will have Suffix+".split_lp". See SplitLandingPadPredecessors for more 246 /// details on this case. 247 /// 248 /// This currently updates the LLVM IR, DominatorTree, LoopInfo, and LCCSA but 249 /// no other analyses. In particular, it does not preserve LoopSimplify 250 /// (because it's complicated to handle the case where one of the edges being 251 /// split is an exit of a loop with other exits). 252 BasicBlock *SplitBlockPredecessors(BasicBlock *BB, ArrayRef<BasicBlock *> Preds, 253 const char *Suffix, 254 DominatorTree *DT = nullptr, 255 LoopInfo *LI = nullptr, 256 MemorySSAUpdater *MSSAU = nullptr, 257 bool PreserveLCSSA = false); 258 259 /// This method transforms the landing pad, OrigBB, by introducing two new basic 260 /// blocks into the function. One of those new basic blocks gets the 261 /// predecessors listed in Preds. The other basic block gets the remaining 262 /// predecessors of OrigBB. The landingpad instruction OrigBB is clone into both 263 /// of the new basic blocks. The new blocks are given the suffixes 'Suffix1' and 264 /// 'Suffix2', and are returned in the NewBBs vector. 265 /// 266 /// This currently updates the LLVM IR, DominatorTree, LoopInfo, and LCCSA but 267 /// no other analyses. In particular, it does not preserve LoopSimplify 268 /// (because it's complicated to handle the case where one of the edges being 269 /// split is an exit of a loop with other exits). 270 void SplitLandingPadPredecessors( 271 BasicBlock *OrigBB, ArrayRef<BasicBlock *> Preds, const char *Suffix, 272 const char *Suffix2, SmallVectorImpl<BasicBlock *> &NewBBs, 273 DominatorTree *DT = nullptr, LoopInfo *LI = nullptr, 274 MemorySSAUpdater *MSSAU = nullptr, bool PreserveLCSSA = false); 275 276 /// This method duplicates the specified return instruction into a predecessor 277 /// which ends in an unconditional branch. If the return instruction returns a 278 /// value defined by a PHI, propagate the right value into the return. It 279 /// returns the new return instruction in the predecessor. 280 ReturnInst *FoldReturnIntoUncondBranch(ReturnInst *RI, BasicBlock *BB, 281 BasicBlock *Pred, 282 DomTreeUpdater *DTU = nullptr); 283 284 /// Split the containing block at the specified instruction - everything before 285 /// SplitBefore stays in the old basic block, and the rest of the instructions 286 /// in the BB are moved to a new block. The two blocks are connected by a 287 /// conditional branch (with value of Cmp being the condition). 288 /// Before: 289 /// Head 290 /// SplitBefore 291 /// Tail 292 /// After: 293 /// Head 294 /// if (Cond) 295 /// ThenBlock 296 /// SplitBefore 297 /// Tail 298 /// 299 /// If \p ThenBlock is not specified, a new block will be created for it. 300 /// If \p Unreachable is true, the newly created block will end with 301 /// UnreachableInst, otherwise it branches to Tail. 302 /// Returns the NewBasicBlock's terminator. 303 /// 304 /// Updates DT and LI if given. 305 Instruction *SplitBlockAndInsertIfThen(Value *Cond, Instruction *SplitBefore, 306 bool Unreachable, 307 MDNode *BranchWeights = nullptr, 308 DominatorTree *DT = nullptr, 309 LoopInfo *LI = nullptr, 310 BasicBlock *ThenBlock = nullptr); 311 312 /// SplitBlockAndInsertIfThenElse is similar to SplitBlockAndInsertIfThen, 313 /// but also creates the ElseBlock. 314 /// Before: 315 /// Head 316 /// SplitBefore 317 /// Tail 318 /// After: 319 /// Head 320 /// if (Cond) 321 /// ThenBlock 322 /// else 323 /// ElseBlock 324 /// SplitBefore 325 /// Tail 326 void SplitBlockAndInsertIfThenElse(Value *Cond, Instruction *SplitBefore, 327 Instruction **ThenTerm, 328 Instruction **ElseTerm, 329 MDNode *BranchWeights = nullptr); 330 331 /// Check whether BB is the merge point of a if-region. 332 /// If so, return the boolean condition that determines which entry into 333 /// BB will be taken. Also, return by references the block that will be 334 /// entered from if the condition is true, and the block that will be 335 /// entered if the condition is false. 336 /// 337 /// This does no checking to see if the true/false blocks have large or unsavory 338 /// instructions in them. 339 Value *GetIfCondition(BasicBlock *BB, BasicBlock *&IfTrue, 340 BasicBlock *&IfFalse); 341 342 // Split critical edges where the source of the edge is an indirectbr 343 // instruction. This isn't always possible, but we can handle some easy cases. 344 // This is useful because MI is unable to split such critical edges, 345 // which means it will not be able to sink instructions along those edges. 346 // This is especially painful for indirect branches with many successors, where 347 // we end up having to prepare all outgoing values in the origin block. 348 // 349 // Our normal algorithm for splitting critical edges requires us to update 350 // the outgoing edges of the edge origin block, but for an indirectbr this 351 // is hard, since it would require finding and updating the block addresses 352 // the indirect branch uses. But if a block only has a single indirectbr 353 // predecessor, with the others being regular branches, we can do it in a 354 // different way. 355 // Say we have A -> D, B -> D, I -> D where only I -> D is an indirectbr. 356 // We can split D into D0 and D1, where D0 contains only the PHIs from D, 357 // and D1 is the D block body. We can then duplicate D0 as D0A and D0B, and 358 // create the following structure: 359 // A -> D0A, B -> D0A, I -> D0B, D0A -> D1, D0B -> D1 360 // If BPI and BFI aren't non-null, BPI/BFI will be updated accordingly. 361 bool SplitIndirectBrCriticalEdges(Function &F, 362 BranchProbabilityInfo *BPI = nullptr, 363 BlockFrequencyInfo *BFI = nullptr); 364 365 } // end namespace llvm 366 367 #endif // LLVM_TRANSFORMS_UTILS_BASICBLOCKUTILS_H 368