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