Eugene Zelenko | a369a45 | 2017-05-16 23:10:25 +0000 | [diff] [blame] | 1 | //===- SimpleLoopUnswitch.cpp - Hoist loop-invariant control flow ---------===// |
Chandler Carruth | 1353f9a | 2017-04-27 18:45:20 +0000 | [diff] [blame] | 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 | |
Eugene Zelenko | a369a45 | 2017-05-16 23:10:25 +0000 | [diff] [blame] | 10 | #include "llvm/ADT/DenseMap.h" |
| 11 | #include "llvm/ADT/Sequence.h" |
| 12 | #include "llvm/ADT/SetVector.h" |
Chandler Carruth | 1353f9a | 2017-04-27 18:45:20 +0000 | [diff] [blame] | 13 | #include "llvm/ADT/SmallPtrSet.h" |
Eugene Zelenko | a369a45 | 2017-05-16 23:10:25 +0000 | [diff] [blame] | 14 | #include "llvm/ADT/SmallVector.h" |
Chandler Carruth | 1353f9a | 2017-04-27 18:45:20 +0000 | [diff] [blame] | 15 | #include "llvm/ADT/Statistic.h" |
Eugene Zelenko | a369a45 | 2017-05-16 23:10:25 +0000 | [diff] [blame] | 16 | #include "llvm/ADT/STLExtras.h" |
| 17 | #include "llvm/ADT/Twine.h" |
Chandler Carruth | 1353f9a | 2017-04-27 18:45:20 +0000 | [diff] [blame] | 18 | #include "llvm/Analysis/AssumptionCache.h" |
Eugene Zelenko | a369a45 | 2017-05-16 23:10:25 +0000 | [diff] [blame] | 19 | #include "llvm/Analysis/LoopAnalysisManager.h" |
Chandler Carruth | 1353f9a | 2017-04-27 18:45:20 +0000 | [diff] [blame] | 20 | #include "llvm/Analysis/LoopInfo.h" |
| 21 | #include "llvm/Analysis/LoopPass.h" |
Eugene Zelenko | a369a45 | 2017-05-16 23:10:25 +0000 | [diff] [blame] | 22 | #include "llvm/IR/BasicBlock.h" |
| 23 | #include "llvm/IR/Constant.h" |
Chandler Carruth | 1353f9a | 2017-04-27 18:45:20 +0000 | [diff] [blame] | 24 | #include "llvm/IR/Constants.h" |
| 25 | #include "llvm/IR/Dominators.h" |
| 26 | #include "llvm/IR/Function.h" |
Eugene Zelenko | a369a45 | 2017-05-16 23:10:25 +0000 | [diff] [blame] | 27 | #include "llvm/IR/InstrTypes.h" |
| 28 | #include "llvm/IR/Instruction.h" |
Chandler Carruth | 1353f9a | 2017-04-27 18:45:20 +0000 | [diff] [blame] | 29 | #include "llvm/IR/Instructions.h" |
Eugene Zelenko | a369a45 | 2017-05-16 23:10:25 +0000 | [diff] [blame] | 30 | #include "llvm/IR/Use.h" |
| 31 | #include "llvm/IR/Value.h" |
| 32 | #include "llvm/Pass.h" |
| 33 | #include "llvm/Support/Casting.h" |
Chandler Carruth | 1353f9a | 2017-04-27 18:45:20 +0000 | [diff] [blame] | 34 | #include "llvm/Support/Debug.h" |
Eugene Zelenko | a369a45 | 2017-05-16 23:10:25 +0000 | [diff] [blame] | 35 | #include "llvm/Support/ErrorHandling.h" |
| 36 | #include "llvm/Support/GenericDomTree.h" |
Chandler Carruth | 1353f9a | 2017-04-27 18:45:20 +0000 | [diff] [blame] | 37 | #include "llvm/Support/raw_ostream.h" |
| 38 | #include "llvm/Transforms/Utils/BasicBlockUtils.h" |
Chandler Carruth | 1353f9a | 2017-04-27 18:45:20 +0000 | [diff] [blame] | 39 | #include "llvm/Transforms/Utils/LoopUtils.h" |
Eugene Zelenko | a369a45 | 2017-05-16 23:10:25 +0000 | [diff] [blame] | 40 | #include "llvm/Transforms/Scalar/SimpleLoopUnswitch.h" |
| 41 | #include <algorithm> |
| 42 | #include <cassert> |
| 43 | #include <iterator> |
| 44 | #include <utility> |
Chandler Carruth | 1353f9a | 2017-04-27 18:45:20 +0000 | [diff] [blame] | 45 | |
| 46 | #define DEBUG_TYPE "simple-loop-unswitch" |
| 47 | |
| 48 | using namespace llvm; |
| 49 | |
| 50 | STATISTIC(NumBranches, "Number of branches unswitched"); |
| 51 | STATISTIC(NumSwitches, "Number of switches unswitched"); |
| 52 | STATISTIC(NumTrivial, "Number of unswitches that are trivial"); |
| 53 | |
| 54 | static void replaceLoopUsesWithConstant(Loop &L, Value &LIC, |
| 55 | Constant &Replacement) { |
| 56 | assert(!isa<Constant>(LIC) && "Why are we unswitching on a constant?"); |
| 57 | |
| 58 | // Replace uses of LIC in the loop with the given constant. |
| 59 | for (auto UI = LIC.use_begin(), UE = LIC.use_end(); UI != UE;) { |
| 60 | // Grab the use and walk past it so we can clobber it in the use list. |
| 61 | Use *U = &*UI++; |
| 62 | Instruction *UserI = dyn_cast<Instruction>(U->getUser()); |
| 63 | if (!UserI || !L.contains(UserI)) |
| 64 | continue; |
| 65 | |
| 66 | // Replace this use within the loop body. |
| 67 | *U = &Replacement; |
| 68 | } |
| 69 | } |
| 70 | |
| 71 | /// Update the dominator tree after removing one exiting predecessor of a loop |
| 72 | /// exit block. |
| 73 | static void updateLoopExitIDom(BasicBlock *LoopExitBB, Loop &L, |
Chandler Carruth | d869b18 | 2017-05-12 02:19:59 +0000 | [diff] [blame] | 74 | DominatorTree &DT) { |
Chandler Carruth | 1353f9a | 2017-04-27 18:45:20 +0000 | [diff] [blame] | 75 | assert(pred_begin(LoopExitBB) != pred_end(LoopExitBB) && |
| 76 | "Cannot have empty predecessors of the loop exit block if we split " |
| 77 | "off a block to unswitch!"); |
| 78 | |
| 79 | BasicBlock *IDom = *pred_begin(LoopExitBB); |
| 80 | // Walk all of the other predecessors finding the nearest common dominator |
| 81 | // until all predecessors are covered or we reach the loop header. The loop |
| 82 | // header necessarily dominates all loop exit blocks in loop simplified form |
| 83 | // so we can early-exit the moment we hit that block. |
| 84 | for (auto PI = std::next(pred_begin(LoopExitBB)), PE = pred_end(LoopExitBB); |
| 85 | PI != PE && IDom != L.getHeader(); ++PI) |
| 86 | IDom = DT.findNearestCommonDominator(IDom, *PI); |
| 87 | |
| 88 | DT.changeImmediateDominator(LoopExitBB, IDom); |
| 89 | } |
| 90 | |
| 91 | /// Update the dominator tree after unswitching a particular former exit block. |
| 92 | /// |
| 93 | /// This handles the full update of the dominator tree after hoisting a block |
| 94 | /// that previously was an exit block (or split off of an exit block) up to be |
| 95 | /// reached from the new immediate dominator of the preheader. |
| 96 | /// |
| 97 | /// The common case is simple -- we just move the unswitched block to have an |
| 98 | /// immediate dominator of the old preheader. But in complex cases, there may |
| 99 | /// be other blocks reachable from the unswitched block that are immediately |
| 100 | /// dominated by some node between the unswitched one and the old preheader. |
| 101 | /// All of these also need to be hoisted in the dominator tree. We also want to |
| 102 | /// minimize queries to the dominator tree because each step of this |
| 103 | /// invalidates any DFS numbers that would make queries fast. |
| 104 | static void updateDTAfterUnswitch(BasicBlock *UnswitchedBB, BasicBlock *OldPH, |
| 105 | DominatorTree &DT) { |
| 106 | DomTreeNode *OldPHNode = DT[OldPH]; |
| 107 | DomTreeNode *UnswitchedNode = DT[UnswitchedBB]; |
| 108 | // If the dominator tree has already been updated for this unswitched node, |
| 109 | // we're done. This makes it easier to use this routine if there are multiple |
| 110 | // paths to the same unswitched destination. |
| 111 | if (UnswitchedNode->getIDom() == OldPHNode) |
| 112 | return; |
| 113 | |
| 114 | // First collect the domtree nodes that we are hoisting over. These are the |
| 115 | // set of nodes which may have children that need to be hoisted as well. |
| 116 | SmallPtrSet<DomTreeNode *, 4> DomChain; |
| 117 | for (auto *IDom = UnswitchedNode->getIDom(); IDom != OldPHNode; |
| 118 | IDom = IDom->getIDom()) |
| 119 | DomChain.insert(IDom); |
| 120 | |
| 121 | // The unswitched block ends up immediately dominated by the old preheader -- |
| 122 | // regardless of whether it is the loop exit block or split off of the loop |
| 123 | // exit block. |
| 124 | DT.changeImmediateDominator(UnswitchedNode, OldPHNode); |
| 125 | |
| 126 | // Blocks reachable from the unswitched block may need to change their IDom |
| 127 | // as well. |
| 128 | SmallSetVector<BasicBlock *, 4> Worklist; |
| 129 | for (auto *SuccBB : successors(UnswitchedBB)) |
| 130 | Worklist.insert(SuccBB); |
| 131 | |
| 132 | // Walk the worklist. We grow the list in the loop and so must recompute size. |
| 133 | for (int i = 0; i < (int)Worklist.size(); ++i) { |
| 134 | auto *BB = Worklist[i]; |
| 135 | |
| 136 | DomTreeNode *Node = DT[BB]; |
| 137 | assert(!DomChain.count(Node) && |
| 138 | "Cannot be dominated by a block you can reach!"); |
| 139 | // If this block doesn't have an immediate dominator somewhere in the chain |
| 140 | // we hoisted over, then its position in the domtree hasn't changed. Either |
| 141 | // it is above the region hoisted and still valid, or it is below the |
| 142 | // hoisted block and so was trivially updated. This also applies to |
| 143 | // everything reachable from this block so we're completely done with the |
| 144 | // it. |
| 145 | if (!DomChain.count(Node->getIDom())) |
| 146 | continue; |
| 147 | |
| 148 | // We need to change the IDom for this node but also walk its successors |
| 149 | // which could have similar dominance position. |
| 150 | DT.changeImmediateDominator(Node, OldPHNode); |
| 151 | for (auto *SuccBB : successors(BB)) |
| 152 | Worklist.insert(SuccBB); |
| 153 | } |
| 154 | } |
| 155 | |
Chandler Carruth | d869b18 | 2017-05-12 02:19:59 +0000 | [diff] [blame] | 156 | /// Check that all the LCSSA PHI nodes in the loop exit block have trivial |
| 157 | /// incoming values along this edge. |
| 158 | static bool areLoopExitPHIsLoopInvariant(Loop &L, BasicBlock &ExitingBB, |
| 159 | BasicBlock &ExitBB) { |
| 160 | for (Instruction &I : ExitBB) { |
| 161 | auto *PN = dyn_cast<PHINode>(&I); |
| 162 | if (!PN) |
| 163 | // No more PHIs to check. |
| 164 | return true; |
| 165 | |
| 166 | // If the incoming value for this edge isn't loop invariant the unswitch |
| 167 | // won't be trivial. |
| 168 | if (!L.isLoopInvariant(PN->getIncomingValueForBlock(&ExitingBB))) |
| 169 | return false; |
| 170 | } |
| 171 | llvm_unreachable("Basic blocks should never be empty!"); |
| 172 | } |
| 173 | |
| 174 | /// Rewrite the PHI nodes in an unswitched loop exit basic block. |
| 175 | /// |
| 176 | /// Requires that the loop exit and unswitched basic block are the same, and |
| 177 | /// that the exiting block was a unique predecessor of that block. Rewrites the |
| 178 | /// PHI nodes in that block such that what were LCSSA PHI nodes become trivial |
| 179 | /// PHI nodes from the old preheader that now contains the unswitched |
| 180 | /// terminator. |
| 181 | static void rewritePHINodesForUnswitchedExitBlock(BasicBlock &UnswitchedBB, |
| 182 | BasicBlock &OldExitingBB, |
| 183 | BasicBlock &OldPH) { |
| 184 | for (Instruction &I : UnswitchedBB) { |
| 185 | auto *PN = dyn_cast<PHINode>(&I); |
| 186 | if (!PN) |
| 187 | // No more PHIs to check. |
| 188 | break; |
| 189 | |
| 190 | // When the loop exit is directly unswitched we just need to update the |
| 191 | // incoming basic block. We loop to handle weird cases with repeated |
| 192 | // incoming blocks, but expect to typically only have one operand here. |
Eugene Zelenko | a369a45 | 2017-05-16 23:10:25 +0000 | [diff] [blame] | 193 | for (auto i : seq<int>(0, PN->getNumOperands())) { |
Chandler Carruth | d869b18 | 2017-05-12 02:19:59 +0000 | [diff] [blame] | 194 | assert(PN->getIncomingBlock(i) == &OldExitingBB && |
| 195 | "Found incoming block different from unique predecessor!"); |
| 196 | PN->setIncomingBlock(i, &OldPH); |
| 197 | } |
| 198 | } |
| 199 | } |
| 200 | |
| 201 | /// Rewrite the PHI nodes in the loop exit basic block and the split off |
| 202 | /// unswitched block. |
| 203 | /// |
| 204 | /// Because the exit block remains an exit from the loop, this rewrites the |
| 205 | /// LCSSA PHI nodes in it to remove the unswitched edge and introduces PHI |
| 206 | /// nodes into the unswitched basic block to select between the value in the |
| 207 | /// old preheader and the loop exit. |
| 208 | static void rewritePHINodesForExitAndUnswitchedBlocks(BasicBlock &ExitBB, |
| 209 | BasicBlock &UnswitchedBB, |
| 210 | BasicBlock &OldExitingBB, |
| 211 | BasicBlock &OldPH) { |
| 212 | assert(&ExitBB != &UnswitchedBB && |
| 213 | "Must have different loop exit and unswitched blocks!"); |
| 214 | Instruction *InsertPt = &*UnswitchedBB.begin(); |
| 215 | for (Instruction &I : ExitBB) { |
| 216 | auto *PN = dyn_cast<PHINode>(&I); |
| 217 | if (!PN) |
| 218 | // No more PHIs to check. |
| 219 | break; |
| 220 | |
| 221 | auto *NewPN = PHINode::Create(PN->getType(), /*NumReservedValues*/ 2, |
| 222 | PN->getName() + ".split", InsertPt); |
| 223 | |
| 224 | // Walk backwards over the old PHI node's inputs to minimize the cost of |
| 225 | // removing each one. We have to do this weird loop manually so that we |
| 226 | // create the same number of new incoming edges in the new PHI as we expect |
| 227 | // each case-based edge to be included in the unswitched switch in some |
| 228 | // cases. |
| 229 | // FIXME: This is really, really gross. It would be much cleaner if LLVM |
| 230 | // allowed us to create a single entry for a predecessor block without |
| 231 | // having separate entries for each "edge" even though these edges are |
| 232 | // required to produce identical results. |
| 233 | for (int i = PN->getNumIncomingValues() - 1; i >= 0; --i) { |
| 234 | if (PN->getIncomingBlock(i) != &OldExitingBB) |
| 235 | continue; |
| 236 | |
| 237 | Value *Incoming = PN->removeIncomingValue(i); |
| 238 | NewPN->addIncoming(Incoming, &OldPH); |
| 239 | } |
| 240 | |
| 241 | // Now replace the old PHI with the new one and wire the old one in as an |
| 242 | // input to the new one. |
| 243 | PN->replaceAllUsesWith(NewPN); |
| 244 | NewPN->addIncoming(PN, &ExitBB); |
| 245 | } |
| 246 | } |
| 247 | |
Chandler Carruth | 1353f9a | 2017-04-27 18:45:20 +0000 | [diff] [blame] | 248 | /// Unswitch a trivial branch if the condition is loop invariant. |
| 249 | /// |
| 250 | /// This routine should only be called when loop code leading to the branch has |
| 251 | /// been validated as trivial (no side effects). This routine checks if the |
| 252 | /// condition is invariant and one of the successors is a loop exit. This |
| 253 | /// allows us to unswitch without duplicating the loop, making it trivial. |
| 254 | /// |
| 255 | /// If this routine fails to unswitch the branch it returns false. |
| 256 | /// |
| 257 | /// If the branch can be unswitched, this routine splits the preheader and |
| 258 | /// hoists the branch above that split. Preserves loop simplified form |
| 259 | /// (splitting the exit block as necessary). It simplifies the branch within |
| 260 | /// the loop to an unconditional branch but doesn't remove it entirely. Further |
| 261 | /// cleanup can be done with some simplify-cfg like pass. |
| 262 | static bool unswitchTrivialBranch(Loop &L, BranchInst &BI, DominatorTree &DT, |
| 263 | LoopInfo &LI) { |
| 264 | assert(BI.isConditional() && "Can only unswitch a conditional branch!"); |
| 265 | DEBUG(dbgs() << " Trying to unswitch branch: " << BI << "\n"); |
| 266 | |
| 267 | Value *LoopCond = BI.getCondition(); |
| 268 | |
| 269 | // Need a trivial loop condition to unswitch. |
| 270 | if (!L.isLoopInvariant(LoopCond)) |
| 271 | return false; |
| 272 | |
| 273 | // FIXME: We should compute this once at the start and update it! |
| 274 | SmallVector<BasicBlock *, 16> ExitBlocks; |
| 275 | L.getExitBlocks(ExitBlocks); |
| 276 | SmallPtrSet<BasicBlock *, 16> ExitBlockSet(ExitBlocks.begin(), |
| 277 | ExitBlocks.end()); |
| 278 | |
| 279 | // Check to see if a successor of the branch is guaranteed to |
| 280 | // exit through a unique exit block without having any |
| 281 | // side-effects. If so, determine the value of Cond that causes |
| 282 | // it to do this. |
| 283 | ConstantInt *CondVal = ConstantInt::getTrue(BI.getContext()); |
| 284 | ConstantInt *Replacement = ConstantInt::getFalse(BI.getContext()); |
| 285 | int LoopExitSuccIdx = 0; |
| 286 | auto *LoopExitBB = BI.getSuccessor(0); |
| 287 | if (!ExitBlockSet.count(LoopExitBB)) { |
| 288 | std::swap(CondVal, Replacement); |
| 289 | LoopExitSuccIdx = 1; |
| 290 | LoopExitBB = BI.getSuccessor(1); |
| 291 | if (!ExitBlockSet.count(LoopExitBB)) |
| 292 | return false; |
| 293 | } |
| 294 | auto *ContinueBB = BI.getSuccessor(1 - LoopExitSuccIdx); |
| 295 | assert(L.contains(ContinueBB) && |
| 296 | "Cannot have both successors exit and still be in the loop!"); |
| 297 | |
Chandler Carruth | d869b18 | 2017-05-12 02:19:59 +0000 | [diff] [blame] | 298 | auto *ParentBB = BI.getParent(); |
| 299 | if (!areLoopExitPHIsLoopInvariant(L, *ParentBB, *LoopExitBB)) |
Chandler Carruth | 1353f9a | 2017-04-27 18:45:20 +0000 | [diff] [blame] | 300 | return false; |
| 301 | |
| 302 | DEBUG(dbgs() << " unswitching trivial branch when: " << CondVal |
| 303 | << " == " << LoopCond << "\n"); |
| 304 | |
| 305 | // Split the preheader, so that we know that there is a safe place to insert |
| 306 | // the conditional branch. We will change the preheader to have a conditional |
| 307 | // branch on LoopCond. |
| 308 | BasicBlock *OldPH = L.getLoopPreheader(); |
| 309 | BasicBlock *NewPH = SplitEdge(OldPH, L.getHeader(), &DT, &LI); |
| 310 | |
| 311 | // Now that we have a place to insert the conditional branch, create a place |
| 312 | // to branch to: this is the exit block out of the loop that we are |
| 313 | // unswitching. We need to split this if there are other loop predecessors. |
| 314 | // Because the loop is in simplified form, *any* other predecessor is enough. |
| 315 | BasicBlock *UnswitchedBB; |
| 316 | if (BasicBlock *PredBB = LoopExitBB->getUniquePredecessor()) { |
| 317 | (void)PredBB; |
Chandler Carruth | d869b18 | 2017-05-12 02:19:59 +0000 | [diff] [blame] | 318 | assert(PredBB == BI.getParent() && |
| 319 | "A branch's parent isn't a predecessor!"); |
Chandler Carruth | 1353f9a | 2017-04-27 18:45:20 +0000 | [diff] [blame] | 320 | UnswitchedBB = LoopExitBB; |
| 321 | } else { |
| 322 | UnswitchedBB = SplitBlock(LoopExitBB, &LoopExitBB->front(), &DT, &LI); |
| 323 | } |
| 324 | |
Chandler Carruth | 1353f9a | 2017-04-27 18:45:20 +0000 | [diff] [blame] | 325 | // Now splice the branch to gate reaching the new preheader and re-point its |
| 326 | // successors. |
| 327 | OldPH->getInstList().splice(std::prev(OldPH->end()), |
| 328 | BI.getParent()->getInstList(), BI); |
| 329 | OldPH->getTerminator()->eraseFromParent(); |
| 330 | BI.setSuccessor(LoopExitSuccIdx, UnswitchedBB); |
| 331 | BI.setSuccessor(1 - LoopExitSuccIdx, NewPH); |
| 332 | |
| 333 | // Create a new unconditional branch that will continue the loop as a new |
| 334 | // terminator. |
| 335 | BranchInst::Create(ContinueBB, ParentBB); |
| 336 | |
Chandler Carruth | d869b18 | 2017-05-12 02:19:59 +0000 | [diff] [blame] | 337 | // Rewrite the relevant PHI nodes. |
| 338 | if (UnswitchedBB == LoopExitBB) |
| 339 | rewritePHINodesForUnswitchedExitBlock(*UnswitchedBB, *ParentBB, *OldPH); |
| 340 | else |
| 341 | rewritePHINodesForExitAndUnswitchedBlocks(*LoopExitBB, *UnswitchedBB, |
| 342 | *ParentBB, *OldPH); |
| 343 | |
Chandler Carruth | 1353f9a | 2017-04-27 18:45:20 +0000 | [diff] [blame] | 344 | // Now we need to update the dominator tree. |
| 345 | updateDTAfterUnswitch(UnswitchedBB, OldPH, DT); |
| 346 | // But if we split something off of the loop exit block then we also removed |
| 347 | // one of the predecessors for the loop exit block and may need to update its |
| 348 | // idom. |
| 349 | if (UnswitchedBB != LoopExitBB) |
| 350 | updateLoopExitIDom(LoopExitBB, L, DT); |
| 351 | |
| 352 | // Since this is an i1 condition we can also trivially replace uses of it |
| 353 | // within the loop with a constant. |
| 354 | replaceLoopUsesWithConstant(L, *LoopCond, *Replacement); |
| 355 | |
| 356 | ++NumTrivial; |
| 357 | ++NumBranches; |
| 358 | return true; |
| 359 | } |
| 360 | |
| 361 | /// Unswitch a trivial switch if the condition is loop invariant. |
| 362 | /// |
| 363 | /// This routine should only be called when loop code leading to the switch has |
| 364 | /// been validated as trivial (no side effects). This routine checks if the |
| 365 | /// condition is invariant and that at least one of the successors is a loop |
| 366 | /// exit. This allows us to unswitch without duplicating the loop, making it |
| 367 | /// trivial. |
| 368 | /// |
| 369 | /// If this routine fails to unswitch the switch it returns false. |
| 370 | /// |
| 371 | /// If the switch can be unswitched, this routine splits the preheader and |
| 372 | /// copies the switch above that split. If the default case is one of the |
| 373 | /// exiting cases, it copies the non-exiting cases and points them at the new |
| 374 | /// preheader. If the default case is not exiting, it copies the exiting cases |
| 375 | /// and points the default at the preheader. It preserves loop simplified form |
| 376 | /// (splitting the exit blocks as necessary). It simplifies the switch within |
| 377 | /// the loop by removing now-dead cases. If the default case is one of those |
| 378 | /// unswitched, it replaces its destination with a new basic block containing |
| 379 | /// only unreachable. Such basic blocks, while technically loop exits, are not |
| 380 | /// considered for unswitching so this is a stable transform and the same |
| 381 | /// switch will not be revisited. If after unswitching there is only a single |
| 382 | /// in-loop successor, the switch is further simplified to an unconditional |
| 383 | /// branch. Still more cleanup can be done with some simplify-cfg like pass. |
| 384 | static bool unswitchTrivialSwitch(Loop &L, SwitchInst &SI, DominatorTree &DT, |
| 385 | LoopInfo &LI) { |
| 386 | DEBUG(dbgs() << " Trying to unswitch switch: " << SI << "\n"); |
| 387 | Value *LoopCond = SI.getCondition(); |
| 388 | |
| 389 | // If this isn't switching on an invariant condition, we can't unswitch it. |
| 390 | if (!L.isLoopInvariant(LoopCond)) |
| 391 | return false; |
| 392 | |
Chandler Carruth | d869b18 | 2017-05-12 02:19:59 +0000 | [diff] [blame] | 393 | auto *ParentBB = SI.getParent(); |
| 394 | |
Chandler Carruth | 1353f9a | 2017-04-27 18:45:20 +0000 | [diff] [blame] | 395 | // FIXME: We should compute this once at the start and update it! |
| 396 | SmallVector<BasicBlock *, 16> ExitBlocks; |
| 397 | L.getExitBlocks(ExitBlocks); |
| 398 | SmallPtrSet<BasicBlock *, 16> ExitBlockSet(ExitBlocks.begin(), |
| 399 | ExitBlocks.end()); |
| 400 | |
| 401 | SmallVector<int, 4> ExitCaseIndices; |
| 402 | for (auto Case : SI.cases()) { |
| 403 | auto *SuccBB = Case.getCaseSuccessor(); |
Chandler Carruth | d869b18 | 2017-05-12 02:19:59 +0000 | [diff] [blame] | 404 | if (ExitBlockSet.count(SuccBB) && |
| 405 | areLoopExitPHIsLoopInvariant(L, *ParentBB, *SuccBB)) |
Chandler Carruth | 1353f9a | 2017-04-27 18:45:20 +0000 | [diff] [blame] | 406 | ExitCaseIndices.push_back(Case.getCaseIndex()); |
| 407 | } |
| 408 | BasicBlock *DefaultExitBB = nullptr; |
| 409 | if (ExitBlockSet.count(SI.getDefaultDest()) && |
Chandler Carruth | d869b18 | 2017-05-12 02:19:59 +0000 | [diff] [blame] | 410 | areLoopExitPHIsLoopInvariant(L, *ParentBB, *SI.getDefaultDest()) && |
Chandler Carruth | 1353f9a | 2017-04-27 18:45:20 +0000 | [diff] [blame] | 411 | !isa<UnreachableInst>(SI.getDefaultDest()->getTerminator())) |
| 412 | DefaultExitBB = SI.getDefaultDest(); |
| 413 | else if (ExitCaseIndices.empty()) |
| 414 | return false; |
| 415 | |
| 416 | DEBUG(dbgs() << " unswitching trivial cases...\n"); |
| 417 | |
| 418 | SmallVector<std::pair<ConstantInt *, BasicBlock *>, 4> ExitCases; |
| 419 | ExitCases.reserve(ExitCaseIndices.size()); |
| 420 | // We walk the case indices backwards so that we remove the last case first |
| 421 | // and don't disrupt the earlier indices. |
| 422 | for (unsigned Index : reverse(ExitCaseIndices)) { |
| 423 | auto CaseI = SI.case_begin() + Index; |
| 424 | // Save the value of this case. |
| 425 | ExitCases.push_back({CaseI->getCaseValue(), CaseI->getCaseSuccessor()}); |
| 426 | // Delete the unswitched cases. |
| 427 | SI.removeCase(CaseI); |
| 428 | } |
| 429 | |
| 430 | // Check if after this all of the remaining cases point at the same |
| 431 | // successor. |
| 432 | BasicBlock *CommonSuccBB = nullptr; |
| 433 | if (SI.getNumCases() > 0 && |
| 434 | std::all_of(std::next(SI.case_begin()), SI.case_end(), |
| 435 | [&SI](const SwitchInst::CaseHandle &Case) { |
| 436 | return Case.getCaseSuccessor() == |
| 437 | SI.case_begin()->getCaseSuccessor(); |
| 438 | })) |
| 439 | CommonSuccBB = SI.case_begin()->getCaseSuccessor(); |
| 440 | |
| 441 | if (DefaultExitBB) { |
| 442 | // We can't remove the default edge so replace it with an edge to either |
| 443 | // the single common remaining successor (if we have one) or an unreachable |
| 444 | // block. |
| 445 | if (CommonSuccBB) { |
| 446 | SI.setDefaultDest(CommonSuccBB); |
| 447 | } else { |
Chandler Carruth | 1353f9a | 2017-04-27 18:45:20 +0000 | [diff] [blame] | 448 | BasicBlock *UnreachableBB = BasicBlock::Create( |
| 449 | ParentBB->getContext(), |
| 450 | Twine(ParentBB->getName()) + ".unreachable_default", |
| 451 | ParentBB->getParent()); |
| 452 | new UnreachableInst(ParentBB->getContext(), UnreachableBB); |
| 453 | SI.setDefaultDest(UnreachableBB); |
| 454 | DT.addNewBlock(UnreachableBB, ParentBB); |
| 455 | } |
| 456 | } else { |
| 457 | // If we're not unswitching the default, we need it to match any cases to |
| 458 | // have a common successor or if we have no cases it is the common |
| 459 | // successor. |
| 460 | if (SI.getNumCases() == 0) |
| 461 | CommonSuccBB = SI.getDefaultDest(); |
| 462 | else if (SI.getDefaultDest() != CommonSuccBB) |
| 463 | CommonSuccBB = nullptr; |
| 464 | } |
| 465 | |
| 466 | // Split the preheader, so that we know that there is a safe place to insert |
| 467 | // the switch. |
| 468 | BasicBlock *OldPH = L.getLoopPreheader(); |
| 469 | BasicBlock *NewPH = SplitEdge(OldPH, L.getHeader(), &DT, &LI); |
| 470 | OldPH->getTerminator()->eraseFromParent(); |
| 471 | |
| 472 | // Now add the unswitched switch. |
| 473 | auto *NewSI = SwitchInst::Create(LoopCond, NewPH, ExitCases.size(), OldPH); |
| 474 | |
Chandler Carruth | d869b18 | 2017-05-12 02:19:59 +0000 | [diff] [blame] | 475 | // Rewrite the IR for the unswitched basic blocks. This requires two steps. |
| 476 | // First, we split any exit blocks with remaining in-loop predecessors. Then |
| 477 | // we update the PHIs in one of two ways depending on if there was a split. |
| 478 | // We walk in reverse so that we split in the same order as the cases |
| 479 | // appeared. This is purely for convenience of reading the resulting IR, but |
| 480 | // it doesn't cost anything really. |
| 481 | SmallPtrSet<BasicBlock *, 2> UnswitchedExitBBs; |
Chandler Carruth | 1353f9a | 2017-04-27 18:45:20 +0000 | [diff] [blame] | 482 | SmallDenseMap<BasicBlock *, BasicBlock *, 2> SplitExitBBMap; |
| 483 | // Handle the default exit if necessary. |
| 484 | // FIXME: It'd be great if we could merge this with the loop below but LLVM's |
| 485 | // ranges aren't quite powerful enough yet. |
Chandler Carruth | d869b18 | 2017-05-12 02:19:59 +0000 | [diff] [blame] | 486 | if (DefaultExitBB) { |
| 487 | if (pred_empty(DefaultExitBB)) { |
| 488 | UnswitchedExitBBs.insert(DefaultExitBB); |
| 489 | rewritePHINodesForUnswitchedExitBlock(*DefaultExitBB, *ParentBB, *OldPH); |
| 490 | } else { |
| 491 | auto *SplitBB = |
| 492 | SplitBlock(DefaultExitBB, &DefaultExitBB->front(), &DT, &LI); |
| 493 | rewritePHINodesForExitAndUnswitchedBlocks(*DefaultExitBB, *SplitBB, |
| 494 | *ParentBB, *OldPH); |
| 495 | updateLoopExitIDom(DefaultExitBB, L, DT); |
| 496 | DefaultExitBB = SplitExitBBMap[DefaultExitBB] = SplitBB; |
| 497 | } |
Chandler Carruth | 1353f9a | 2017-04-27 18:45:20 +0000 | [diff] [blame] | 498 | } |
| 499 | // Note that we must use a reference in the for loop so that we update the |
| 500 | // container. |
| 501 | for (auto &CasePair : reverse(ExitCases)) { |
| 502 | // Grab a reference to the exit block in the pair so that we can update it. |
Chandler Carruth | d869b18 | 2017-05-12 02:19:59 +0000 | [diff] [blame] | 503 | BasicBlock *ExitBB = CasePair.second; |
Chandler Carruth | 1353f9a | 2017-04-27 18:45:20 +0000 | [diff] [blame] | 504 | |
| 505 | // If this case is the last edge into the exit block, we can simply reuse it |
| 506 | // as it will no longer be a loop exit. No mapping necessary. |
Chandler Carruth | d869b18 | 2017-05-12 02:19:59 +0000 | [diff] [blame] | 507 | if (pred_empty(ExitBB)) { |
| 508 | // Only rewrite once. |
| 509 | if (UnswitchedExitBBs.insert(ExitBB).second) |
| 510 | rewritePHINodesForUnswitchedExitBlock(*ExitBB, *ParentBB, *OldPH); |
Chandler Carruth | 1353f9a | 2017-04-27 18:45:20 +0000 | [diff] [blame] | 511 | continue; |
Chandler Carruth | d869b18 | 2017-05-12 02:19:59 +0000 | [diff] [blame] | 512 | } |
Chandler Carruth | 1353f9a | 2017-04-27 18:45:20 +0000 | [diff] [blame] | 513 | |
| 514 | // Otherwise we need to split the exit block so that we retain an exit |
| 515 | // block from the loop and a target for the unswitched condition. |
| 516 | BasicBlock *&SplitExitBB = SplitExitBBMap[ExitBB]; |
| 517 | if (!SplitExitBB) { |
| 518 | // If this is the first time we see this, do the split and remember it. |
| 519 | SplitExitBB = SplitBlock(ExitBB, &ExitBB->front(), &DT, &LI); |
Chandler Carruth | d869b18 | 2017-05-12 02:19:59 +0000 | [diff] [blame] | 520 | rewritePHINodesForExitAndUnswitchedBlocks(*ExitBB, *SplitExitBB, |
| 521 | *ParentBB, *OldPH); |
Chandler Carruth | 1353f9a | 2017-04-27 18:45:20 +0000 | [diff] [blame] | 522 | updateLoopExitIDom(ExitBB, L, DT); |
| 523 | } |
Chandler Carruth | d869b18 | 2017-05-12 02:19:59 +0000 | [diff] [blame] | 524 | // Update the case pair to point to the split block. |
| 525 | CasePair.second = SplitExitBB; |
Chandler Carruth | 1353f9a | 2017-04-27 18:45:20 +0000 | [diff] [blame] | 526 | } |
| 527 | |
| 528 | // Now add the unswitched cases. We do this in reverse order as we built them |
| 529 | // in reverse order. |
| 530 | for (auto CasePair : reverse(ExitCases)) { |
| 531 | ConstantInt *CaseVal = CasePair.first; |
| 532 | BasicBlock *UnswitchedBB = CasePair.second; |
| 533 | |
| 534 | NewSI->addCase(CaseVal, UnswitchedBB); |
| 535 | updateDTAfterUnswitch(UnswitchedBB, OldPH, DT); |
| 536 | } |
| 537 | |
| 538 | // If the default was unswitched, re-point it and add explicit cases for |
| 539 | // entering the loop. |
| 540 | if (DefaultExitBB) { |
| 541 | NewSI->setDefaultDest(DefaultExitBB); |
| 542 | updateDTAfterUnswitch(DefaultExitBB, OldPH, DT); |
| 543 | |
| 544 | // We removed all the exit cases, so we just copy the cases to the |
| 545 | // unswitched switch. |
| 546 | for (auto Case : SI.cases()) |
| 547 | NewSI->addCase(Case.getCaseValue(), NewPH); |
| 548 | } |
| 549 | |
| 550 | // If we ended up with a common successor for every path through the switch |
| 551 | // after unswitching, rewrite it to an unconditional branch to make it easy |
| 552 | // to recognize. Otherwise we potentially have to recognize the default case |
| 553 | // pointing at unreachable and other complexity. |
| 554 | if (CommonSuccBB) { |
| 555 | BasicBlock *BB = SI.getParent(); |
| 556 | SI.eraseFromParent(); |
| 557 | BranchInst::Create(CommonSuccBB, BB); |
| 558 | } |
| 559 | |
| 560 | DT.verifyDomTree(); |
| 561 | ++NumTrivial; |
| 562 | ++NumSwitches; |
| 563 | return true; |
| 564 | } |
| 565 | |
| 566 | /// This routine scans the loop to find a branch or switch which occurs before |
| 567 | /// any side effects occur. These can potentially be unswitched without |
| 568 | /// duplicating the loop. If a branch or switch is successfully unswitched the |
| 569 | /// scanning continues to see if subsequent branches or switches have become |
| 570 | /// trivial. Once all trivial candidates have been unswitched, this routine |
| 571 | /// returns. |
| 572 | /// |
| 573 | /// The return value indicates whether anything was unswitched (and therefore |
| 574 | /// changed). |
| 575 | static bool unswitchAllTrivialConditions(Loop &L, DominatorTree &DT, |
| 576 | LoopInfo &LI) { |
| 577 | bool Changed = false; |
| 578 | |
| 579 | // If loop header has only one reachable successor we should keep looking for |
| 580 | // trivial condition candidates in the successor as well. An alternative is |
| 581 | // to constant fold conditions and merge successors into loop header (then we |
| 582 | // only need to check header's terminator). The reason for not doing this in |
| 583 | // LoopUnswitch pass is that it could potentially break LoopPassManager's |
| 584 | // invariants. Folding dead branches could either eliminate the current loop |
| 585 | // or make other loops unreachable. LCSSA form might also not be preserved |
| 586 | // after deleting branches. The following code keeps traversing loop header's |
| 587 | // successors until it finds the trivial condition candidate (condition that |
| 588 | // is not a constant). Since unswitching generates branches with constant |
| 589 | // conditions, this scenario could be very common in practice. |
| 590 | BasicBlock *CurrentBB = L.getHeader(); |
| 591 | SmallPtrSet<BasicBlock *, 8> Visited; |
| 592 | Visited.insert(CurrentBB); |
| 593 | do { |
| 594 | // Check if there are any side-effecting instructions (e.g. stores, calls, |
| 595 | // volatile loads) in the part of the loop that the code *would* execute |
| 596 | // without unswitching. |
| 597 | if (llvm::any_of(*CurrentBB, |
| 598 | [](Instruction &I) { return I.mayHaveSideEffects(); })) |
| 599 | return Changed; |
| 600 | |
| 601 | TerminatorInst *CurrentTerm = CurrentBB->getTerminator(); |
| 602 | |
| 603 | if (auto *SI = dyn_cast<SwitchInst>(CurrentTerm)) { |
| 604 | // Don't bother trying to unswitch past a switch with a constant |
| 605 | // condition. This should be removed prior to running this pass by |
| 606 | // simplify-cfg. |
| 607 | if (isa<Constant>(SI->getCondition())) |
| 608 | return Changed; |
| 609 | |
| 610 | if (!unswitchTrivialSwitch(L, *SI, DT, LI)) |
| 611 | // Coludn't unswitch this one so we're done. |
| 612 | return Changed; |
| 613 | |
| 614 | // Mark that we managed to unswitch something. |
| 615 | Changed = true; |
| 616 | |
| 617 | // If unswitching turned the terminator into an unconditional branch then |
| 618 | // we can continue. The unswitching logic specifically works to fold any |
| 619 | // cases it can into an unconditional branch to make it easier to |
| 620 | // recognize here. |
| 621 | auto *BI = dyn_cast<BranchInst>(CurrentBB->getTerminator()); |
| 622 | if (!BI || BI->isConditional()) |
| 623 | return Changed; |
| 624 | |
| 625 | CurrentBB = BI->getSuccessor(0); |
| 626 | continue; |
| 627 | } |
| 628 | |
| 629 | auto *BI = dyn_cast<BranchInst>(CurrentTerm); |
| 630 | if (!BI) |
| 631 | // We do not understand other terminator instructions. |
| 632 | return Changed; |
| 633 | |
| 634 | // Don't bother trying to unswitch past an unconditional branch or a branch |
| 635 | // with a constant value. These should be removed by simplify-cfg prior to |
| 636 | // running this pass. |
| 637 | if (!BI->isConditional() || isa<Constant>(BI->getCondition())) |
| 638 | return Changed; |
| 639 | |
| 640 | // Found a trivial condition candidate: non-foldable conditional branch. If |
| 641 | // we fail to unswitch this, we can't do anything else that is trivial. |
| 642 | if (!unswitchTrivialBranch(L, *BI, DT, LI)) |
| 643 | return Changed; |
| 644 | |
| 645 | // Mark that we managed to unswitch something. |
| 646 | Changed = true; |
| 647 | |
| 648 | // We unswitched the branch. This should always leave us with an |
| 649 | // unconditional branch that we can follow now. |
| 650 | BI = cast<BranchInst>(CurrentBB->getTerminator()); |
| 651 | assert(!BI->isConditional() && |
| 652 | "Cannot form a conditional branch by unswitching1"); |
| 653 | CurrentBB = BI->getSuccessor(0); |
| 654 | |
| 655 | // When continuing, if we exit the loop or reach a previous visited block, |
| 656 | // then we can not reach any trivial condition candidates (unfoldable |
| 657 | // branch instructions or switch instructions) and no unswitch can happen. |
| 658 | } while (L.contains(CurrentBB) && Visited.insert(CurrentBB).second); |
| 659 | |
| 660 | return Changed; |
| 661 | } |
| 662 | |
| 663 | /// Unswitch control flow predicated on loop invariant conditions. |
| 664 | /// |
| 665 | /// This first hoists all branches or switches which are trivial (IE, do not |
| 666 | /// require duplicating any part of the loop) out of the loop body. It then |
| 667 | /// looks at other loop invariant control flows and tries to unswitch those as |
| 668 | /// well by cloning the loop if the result is small enough. |
| 669 | static bool unswitchLoop(Loop &L, DominatorTree &DT, LoopInfo &LI, |
| 670 | AssumptionCache &AC) { |
| 671 | assert(L.isLCSSAForm(DT) && |
| 672 | "Loops must be in LCSSA form before unswitching."); |
| 673 | bool Changed = false; |
| 674 | |
| 675 | // Must be in loop simplified form: we need a preheader and dedicated exits. |
| 676 | if (!L.isLoopSimplifyForm()) |
| 677 | return false; |
| 678 | |
| 679 | // Try trivial unswitch first before loop over other basic blocks in the loop. |
| 680 | Changed |= unswitchAllTrivialConditions(L, DT, LI); |
| 681 | |
| 682 | // FIXME: Add support for non-trivial unswitching by cloning the loop. |
| 683 | |
| 684 | return Changed; |
| 685 | } |
| 686 | |
| 687 | PreservedAnalyses SimpleLoopUnswitchPass::run(Loop &L, LoopAnalysisManager &AM, |
| 688 | LoopStandardAnalysisResults &AR, |
| 689 | LPMUpdater &U) { |
| 690 | Function &F = *L.getHeader()->getParent(); |
| 691 | (void)F; |
| 692 | |
| 693 | DEBUG(dbgs() << "Unswitching loop in " << F.getName() << ": " << L << "\n"); |
| 694 | |
| 695 | if (!unswitchLoop(L, AR.DT, AR.LI, AR.AC)) |
| 696 | return PreservedAnalyses::all(); |
| 697 | |
| 698 | #ifndef NDEBUG |
| 699 | // Historically this pass has had issues with the dominator tree so verify it |
| 700 | // in asserts builds. |
| 701 | AR.DT.verifyDomTree(); |
| 702 | #endif |
| 703 | return getLoopPassPreservedAnalyses(); |
| 704 | } |
| 705 | |
| 706 | namespace { |
Eugene Zelenko | a369a45 | 2017-05-16 23:10:25 +0000 | [diff] [blame] | 707 | |
Chandler Carruth | 1353f9a | 2017-04-27 18:45:20 +0000 | [diff] [blame] | 708 | class SimpleLoopUnswitchLegacyPass : public LoopPass { |
| 709 | public: |
| 710 | static char ID; // Pass ID, replacement for typeid |
Eugene Zelenko | a369a45 | 2017-05-16 23:10:25 +0000 | [diff] [blame] | 711 | |
Chandler Carruth | 1353f9a | 2017-04-27 18:45:20 +0000 | [diff] [blame] | 712 | explicit SimpleLoopUnswitchLegacyPass() : LoopPass(ID) { |
| 713 | initializeSimpleLoopUnswitchLegacyPassPass( |
| 714 | *PassRegistry::getPassRegistry()); |
| 715 | } |
| 716 | |
| 717 | bool runOnLoop(Loop *L, LPPassManager &LPM) override; |
| 718 | |
| 719 | void getAnalysisUsage(AnalysisUsage &AU) const override { |
| 720 | AU.addRequired<AssumptionCacheTracker>(); |
| 721 | getLoopAnalysisUsage(AU); |
| 722 | } |
| 723 | }; |
Eugene Zelenko | a369a45 | 2017-05-16 23:10:25 +0000 | [diff] [blame] | 724 | |
| 725 | } // end anonymous namespace |
Chandler Carruth | 1353f9a | 2017-04-27 18:45:20 +0000 | [diff] [blame] | 726 | |
| 727 | bool SimpleLoopUnswitchLegacyPass::runOnLoop(Loop *L, LPPassManager &LPM) { |
| 728 | if (skipLoop(L)) |
| 729 | return false; |
| 730 | |
| 731 | Function &F = *L->getHeader()->getParent(); |
| 732 | |
| 733 | DEBUG(dbgs() << "Unswitching loop in " << F.getName() << ": " << *L << "\n"); |
| 734 | |
| 735 | auto &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree(); |
| 736 | auto &LI = getAnalysis<LoopInfoWrapperPass>().getLoopInfo(); |
| 737 | auto &AC = getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F); |
| 738 | |
| 739 | bool Changed = unswitchLoop(*L, DT, LI, AC); |
| 740 | |
| 741 | #ifndef NDEBUG |
| 742 | // Historically this pass has had issues with the dominator tree so verify it |
| 743 | // in asserts builds. |
| 744 | DT.verifyDomTree(); |
| 745 | #endif |
| 746 | return Changed; |
| 747 | } |
| 748 | |
| 749 | char SimpleLoopUnswitchLegacyPass::ID = 0; |
| 750 | INITIALIZE_PASS_BEGIN(SimpleLoopUnswitchLegacyPass, "simple-loop-unswitch", |
| 751 | "Simple unswitch loops", false, false) |
| 752 | INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker) |
| 753 | INITIALIZE_PASS_DEPENDENCY(LoopPass) |
| 754 | INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass) |
| 755 | INITIALIZE_PASS_END(SimpleLoopUnswitchLegacyPass, "simple-loop-unswitch", |
| 756 | "Simple unswitch loops", false, false) |
| 757 | |
| 758 | Pass *llvm::createSimpleLoopUnswitchLegacyPass() { |
| 759 | return new SimpleLoopUnswitchLegacyPass(); |
| 760 | } |