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