1 //===-- LCSSA.cpp - Convert loops into loop-closed SSA form ---------------===// 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 // This pass transforms loops by placing phi nodes at the end of the loops for 11 // all values that are live across the loop boundary. For example, it turns 12 // the left into the right code: 13 // 14 // for (...) for (...) 15 // if (c) if (c) 16 // X1 = ... X1 = ... 17 // else else 18 // X2 = ... X2 = ... 19 // X3 = phi(X1, X2) X3 = phi(X1, X2) 20 // ... = X3 + 4 X4 = phi(X3) 21 // ... = X4 + 4 22 // 23 // This is still valid LLVM; the extra phi nodes are purely redundant, and will 24 // be trivially eliminated by InstCombine. The major benefit of this 25 // transformation is that it makes many other loop optimizations, such as 26 // LoopUnswitching, simpler. 27 // 28 //===----------------------------------------------------------------------===// 29 30 #include "llvm/Transforms/Scalar.h" 31 #include "llvm/ADT/STLExtras.h" 32 #include "llvm/ADT/Statistic.h" 33 #include "llvm/Analysis/AliasAnalysis.h" 34 #include "llvm/Analysis/LoopPass.h" 35 #include "llvm/Analysis/ScalarEvolution.h" 36 #include "llvm/IR/Constants.h" 37 #include "llvm/IR/Dominators.h" 38 #include "llvm/IR/Function.h" 39 #include "llvm/IR/Instructions.h" 40 #include "llvm/IR/PredIteratorCache.h" 41 #include "llvm/Pass.h" 42 #include "llvm/Transforms/Utils/LoopUtils.h" 43 #include "llvm/Transforms/Utils/SSAUpdater.h" 44 using namespace llvm; 45 46 #define DEBUG_TYPE "lcssa" 47 48 STATISTIC(NumLCSSA, "Number of live out of a loop variables"); 49 50 /// Return true if the specified block is in the list. 51 static bool isExitBlock(BasicBlock *BB, 52 const SmallVectorImpl<BasicBlock *> &ExitBlocks) { 53 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) 54 if (ExitBlocks[i] == BB) 55 return true; 56 return false; 57 } 58 59 /// Given an instruction in the loop, check to see if it has any uses that are 60 /// outside the current loop. If so, insert LCSSA PHI nodes and rewrite the 61 /// uses. 62 static bool processInstruction(Loop &L, Instruction &Inst, DominatorTree &DT, 63 const SmallVectorImpl<BasicBlock *> &ExitBlocks, 64 PredIteratorCache &PredCache) { 65 SmallVector<Use *, 16> UsesToRewrite; 66 67 BasicBlock *InstBB = Inst.getParent(); 68 69 for (Use &U : Inst.uses()) { 70 Instruction *User = cast<Instruction>(U.getUser()); 71 BasicBlock *UserBB = User->getParent(); 72 if (PHINode *PN = dyn_cast<PHINode>(User)) 73 UserBB = PN->getIncomingBlock(U); 74 75 if (InstBB != UserBB && !L.contains(UserBB)) 76 UsesToRewrite.push_back(&U); 77 } 78 79 // If there are no uses outside the loop, exit with no change. 80 if (UsesToRewrite.empty()) 81 return false; 82 83 ++NumLCSSA; // We are applying the transformation 84 85 // Invoke instructions are special in that their result value is not available 86 // along their unwind edge. The code below tests to see whether DomBB 87 // dominates 88 // the value, so adjust DomBB to the normal destination block, which is 89 // effectively where the value is first usable. 90 BasicBlock *DomBB = Inst.getParent(); 91 if (InvokeInst *Inv = dyn_cast<InvokeInst>(&Inst)) 92 DomBB = Inv->getNormalDest(); 93 94 DomTreeNode *DomNode = DT.getNode(DomBB); 95 96 SmallVector<PHINode *, 16> AddedPHIs; 97 98 SSAUpdater SSAUpdate; 99 SSAUpdate.Initialize(Inst.getType(), Inst.getName()); 100 101 // Insert the LCSSA phi's into all of the exit blocks dominated by the 102 // value, and add them to the Phi's map. 103 for (SmallVectorImpl<BasicBlock *>::const_iterator BBI = ExitBlocks.begin(), 104 BBE = ExitBlocks.end(); 105 BBI != BBE; ++BBI) { 106 BasicBlock *ExitBB = *BBI; 107 if (!DT.dominates(DomNode, DT.getNode(ExitBB))) 108 continue; 109 110 // If we already inserted something for this BB, don't reprocess it. 111 if (SSAUpdate.HasValueForBlock(ExitBB)) 112 continue; 113 114 PHINode *PN = PHINode::Create(Inst.getType(), PredCache.GetNumPreds(ExitBB), 115 Inst.getName() + ".lcssa", ExitBB->begin()); 116 117 // Add inputs from inside the loop for this PHI. 118 for (BasicBlock **PI = PredCache.GetPreds(ExitBB); *PI; ++PI) { 119 PN->addIncoming(&Inst, *PI); 120 121 // If the exit block has a predecessor not within the loop, arrange for 122 // the incoming value use corresponding to that predecessor to be 123 // rewritten in terms of a different LCSSA PHI. 124 if (!L.contains(*PI)) 125 UsesToRewrite.push_back( 126 &PN->getOperandUse(PN->getOperandNumForIncomingValue( 127 PN->getNumIncomingValues() - 1))); 128 } 129 130 AddedPHIs.push_back(PN); 131 132 // Remember that this phi makes the value alive in this block. 133 SSAUpdate.AddAvailableValue(ExitBB, PN); 134 } 135 136 // Rewrite all uses outside the loop in terms of the new PHIs we just 137 // inserted. 138 for (unsigned i = 0, e = UsesToRewrite.size(); i != e; ++i) { 139 // If this use is in an exit block, rewrite to use the newly inserted PHI. 140 // This is required for correctness because SSAUpdate doesn't handle uses in 141 // the same block. It assumes the PHI we inserted is at the end of the 142 // block. 143 Instruction *User = cast<Instruction>(UsesToRewrite[i]->getUser()); 144 BasicBlock *UserBB = User->getParent(); 145 if (PHINode *PN = dyn_cast<PHINode>(User)) 146 UserBB = PN->getIncomingBlock(*UsesToRewrite[i]); 147 148 if (isa<PHINode>(UserBB->begin()) && isExitBlock(UserBB, ExitBlocks)) { 149 // Tell the VHs that the uses changed. This updates SCEV's caches. 150 if (UsesToRewrite[i]->get()->hasValueHandle()) 151 ValueHandleBase::ValueIsRAUWd(*UsesToRewrite[i], UserBB->begin()); 152 UsesToRewrite[i]->set(UserBB->begin()); 153 continue; 154 } 155 156 // Otherwise, do full PHI insertion. 157 SSAUpdate.RewriteUse(*UsesToRewrite[i]); 158 } 159 160 // Remove PHI nodes that did not have any uses rewritten. 161 for (unsigned i = 0, e = AddedPHIs.size(); i != e; ++i) { 162 if (AddedPHIs[i]->use_empty()) 163 AddedPHIs[i]->eraseFromParent(); 164 } 165 166 return true; 167 } 168 169 /// Return true if the specified block dominates at least 170 /// one of the blocks in the specified list. 171 static bool 172 blockDominatesAnExit(BasicBlock *BB, 173 DominatorTree &DT, 174 const SmallVectorImpl<BasicBlock *> &ExitBlocks) { 175 DomTreeNode *DomNode = DT.getNode(BB); 176 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) 177 if (DT.dominates(DomNode, DT.getNode(ExitBlocks[i]))) 178 return true; 179 180 return false; 181 } 182 183 bool llvm::formLCSSA(Loop &L, DominatorTree &DT, ScalarEvolution *SE) { 184 bool Changed = false; 185 186 // Get the set of exiting blocks. 187 SmallVector<BasicBlock *, 8> ExitBlocks; 188 L.getExitBlocks(ExitBlocks); 189 190 if (ExitBlocks.empty()) 191 return false; 192 193 PredIteratorCache PredCache; 194 195 // Look at all the instructions in the loop, checking to see if they have uses 196 // outside the loop. If so, rewrite those uses. 197 for (Loop::block_iterator BBI = L.block_begin(), BBE = L.block_end(); 198 BBI != BBE; ++BBI) { 199 BasicBlock *BB = *BBI; 200 201 // For large loops, avoid use-scanning by using dominance information: In 202 // particular, if a block does not dominate any of the loop exits, then none 203 // of the values defined in the block could be used outside the loop. 204 if (!blockDominatesAnExit(BB, DT, ExitBlocks)) 205 continue; 206 207 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) { 208 // Reject two common cases fast: instructions with no uses (like stores) 209 // and instructions with one use that is in the same block as this. 210 if (I->use_empty() || 211 (I->hasOneUse() && I->user_back()->getParent() == BB && 212 !isa<PHINode>(I->user_back()))) 213 continue; 214 215 Changed |= processInstruction(L, *I, DT, ExitBlocks, PredCache); 216 } 217 } 218 219 // If we modified the code, remove any caches about the loop from SCEV to 220 // avoid dangling entries. 221 // FIXME: This is a big hammer, can we clear the cache more selectively? 222 if (SE && Changed) 223 SE->forgetLoop(&L); 224 225 assert(L.isLCSSAForm(DT)); 226 227 return Changed; 228 } 229 230 /// Process a loop nest depth first. 231 bool llvm::formLCSSARecursively(Loop &L, DominatorTree &DT, 232 ScalarEvolution *SE) { 233 bool Changed = false; 234 235 // Recurse depth-first through inner loops. 236 for (Loop::iterator LI = L.begin(), LE = L.end(); LI != LE; ++LI) 237 Changed |= formLCSSARecursively(**LI, DT, SE); 238 239 Changed |= formLCSSA(L, DT, SE); 240 return Changed; 241 } 242 243 namespace { 244 struct LCSSA : public FunctionPass { 245 static char ID; // Pass identification, replacement for typeid 246 LCSSA() : FunctionPass(ID) { 247 initializeLCSSAPass(*PassRegistry::getPassRegistry()); 248 } 249 250 // Cached analysis information for the current function. 251 DominatorTree *DT; 252 LoopInfo *LI; 253 ScalarEvolution *SE; 254 255 bool runOnFunction(Function &F) override; 256 257 /// This transformation requires natural loop information & requires that 258 /// loop preheaders be inserted into the CFG. It maintains both of these, 259 /// as well as the CFG. It also requires dominator information. 260 void getAnalysisUsage(AnalysisUsage &AU) const override { 261 AU.setPreservesCFG(); 262 263 AU.addRequired<DominatorTreeWrapperPass>(); 264 AU.addRequired<LoopInfo>(); 265 AU.addPreservedID(LoopSimplifyID); 266 AU.addPreserved<AliasAnalysis>(); 267 AU.addPreserved<ScalarEvolution>(); 268 } 269 270 private: 271 void verifyAnalysis() const override; 272 }; 273 } 274 275 char LCSSA::ID = 0; 276 INITIALIZE_PASS_BEGIN(LCSSA, "lcssa", "Loop-Closed SSA Form Pass", false, false) 277 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass) 278 INITIALIZE_PASS_DEPENDENCY(LoopInfo) 279 INITIALIZE_PASS_END(LCSSA, "lcssa", "Loop-Closed SSA Form Pass", false, false) 280 281 Pass *llvm::createLCSSAPass() { return new LCSSA(); } 282 char &llvm::LCSSAID = LCSSA::ID; 283 284 285 /// Process all loops in the function, inner-most out. 286 bool LCSSA::runOnFunction(Function &F) { 287 bool Changed = false; 288 LI = &getAnalysis<LoopInfo>(); 289 DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree(); 290 SE = getAnalysisIfAvailable<ScalarEvolution>(); 291 292 // Simplify each loop nest in the function. 293 for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I) 294 Changed |= formLCSSARecursively(**I, *DT, SE); 295 296 return Changed; 297 } 298 299 static void verifyLoop(Loop &L, DominatorTree &DT) { 300 // Recurse depth-first through inner loops. 301 for (Loop::iterator LI = L.begin(), LE = L.end(); LI != LE; ++LI) 302 verifyLoop(**LI, DT); 303 304 // Check the special guarantees that LCSSA makes. 305 //assert(L.isLCSSAForm(DT) && "LCSSA form not preserved!"); 306 } 307 308 void LCSSA::verifyAnalysis() const { 309 // Verify each loop nest in the function, assuming LI still points at that 310 // function's loop info. 311 for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I) 312 verifyLoop(**I, *DT); 313 } 314