LCSSA.cpp revision bbf81d88116d23fb0776412b5916f7d0b8b3ca7e
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" 43using namespace llvm; 44 45STATISTIC(NumLCSSA, "Number of live out of a loop variables"); 46 47namespace { 48 struct LCSSA : public LoopPass { 49 static char ID; // Pass identification, replacement for typeid 50 LCSSA() : LoopPass(&ID) {} 51 52 // Cached analysis information for the current function. 53 DominatorTree *DT; 54 std::vector<BasicBlock*> LoopBlocks; 55 PredIteratorCache PredCache; 56 Loop *L; 57 58 virtual bool runOnLoop(Loop *L, LPPassManager &LPM); 59 60 /// This transformation requires natural loop information & requires that 61 /// loop preheaders be inserted into the CFG. It maintains both of these, 62 /// as well as the CFG. It also requires dominator information. 63 /// 64 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 65 AU.setPreservesCFG(); 66 67 // LCSSA doesn't actually require LoopSimplify, but the PassManager 68 // doesn't know how to schedule LoopSimplify by itself. 69 AU.addRequiredID(LoopSimplifyID); 70 AU.addPreservedID(LoopSimplifyID); 71 AU.addRequiredTransitive<LoopInfo>(); 72 AU.addPreserved<LoopInfo>(); 73 AU.addRequiredTransitive<DominatorTree>(); 74 AU.addPreserved<ScalarEvolution>(); 75 AU.addPreserved<DominatorTree>(); 76 77 // Request DominanceFrontier now, even though LCSSA does 78 // not use it. This allows Pass Manager to schedule Dominance 79 // Frontier early enough such that one LPPassManager can handle 80 // multiple loop transformation passes. 81 AU.addRequired<DominanceFrontier>(); 82 AU.addPreserved<DominanceFrontier>(); 83 } 84 private: 85 bool ProcessInstruction(Instruction *Inst, 86 const SmallVectorImpl<BasicBlock*> &ExitBlocks); 87 88 /// verifyAnalysis() - Verify loop nest. 89 virtual void verifyAnalysis() const { 90 // Check the special guarantees that LCSSA makes. 91 assert(L->isLCSSAForm(*DT) && "LCSSA form not preserved!"); 92 } 93 94 /// inLoop - returns true if the given block is within the current loop 95 bool inLoop(BasicBlock *B) const { 96 return std::binary_search(LoopBlocks.begin(), LoopBlocks.end(), B); 97 } 98 }; 99} 100 101char LCSSA::ID = 0; 102static RegisterPass<LCSSA> X("lcssa", "Loop-Closed SSA Form Pass"); 103 104Pass *llvm::createLCSSAPass() { return new LCSSA(); } 105const PassInfo *const llvm::LCSSAID = &X; 106 107 108/// BlockDominatesAnExit - Return true if the specified block dominates at least 109/// one of the blocks in the specified list. 110static bool BlockDominatesAnExit(BasicBlock *BB, 111 const SmallVectorImpl<BasicBlock*> &ExitBlocks, 112 DominatorTree *DT) { 113 DomTreeNode *DomNode = DT->getNode(BB); 114 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) 115 if (DT->dominates(DomNode, DT->getNode(ExitBlocks[i]))) 116 return true; 117 118 return false; 119} 120 121 122/// runOnFunction - Process all loops in the function, inner-most out. 123bool LCSSA::runOnLoop(Loop *TheLoop, LPPassManager &LPM) { 124 L = TheLoop; 125 126 DT = &getAnalysis<DominatorTree>(); 127 128 // Get the set of exiting blocks. 129 SmallVector<BasicBlock*, 8> ExitBlocks; 130 L->getExitBlocks(ExitBlocks); 131 132 if (ExitBlocks.empty()) 133 return false; 134 135 // Speed up queries by creating a sorted vector of blocks. 136 LoopBlocks.clear(); 137 LoopBlocks.insert(LoopBlocks.end(), L->block_begin(), L->block_end()); 138 array_pod_sort(LoopBlocks.begin(), LoopBlocks.end()); 139 140 // Look at all the instructions in the loop, checking to see if they have uses 141 // outside the loop. If so, rewrite those uses. 142 bool MadeChange = false; 143 144 for (Loop::block_iterator BBI = L->block_begin(), E = L->block_end(); 145 BBI != E; ++BBI) { 146 BasicBlock *BB = *BBI; 147 148 // For large loops, avoid use-scanning by using dominance information: In 149 // particular, if a block does not dominate any of the loop exits, then none 150 // of the values defined in the block could be used outside the loop. 151 if (!BlockDominatesAnExit(BB, ExitBlocks, DT)) 152 continue; 153 154 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); 155 I != E; ++I) { 156 // Reject two common cases fast: instructions with no uses (like stores) 157 // and instructions with one use that is in the same block as this. 158 if (I->use_empty() || 159 (I->hasOneUse() && I->use_back()->getParent() == BB && 160 !isa<PHINode>(I->use_back()))) 161 continue; 162 163 MadeChange |= ProcessInstruction(I, ExitBlocks); 164 } 165 } 166 167 assert(L->isLCSSAForm(*DT)); 168 PredCache.clear(); 169 170 return MadeChange; 171} 172 173/// isExitBlock - Return true if the specified block is in the list. 174static bool isExitBlock(BasicBlock *BB, 175 const SmallVectorImpl<BasicBlock*> &ExitBlocks) { 176 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) 177 if (ExitBlocks[i] == BB) 178 return true; 179 return false; 180} 181 182/// ProcessInstruction - Given an instruction in the loop, check to see if it 183/// has any uses that are outside the current loop. If so, insert LCSSA PHI 184/// nodes and rewrite the uses. 185bool LCSSA::ProcessInstruction(Instruction *Inst, 186 const SmallVectorImpl<BasicBlock*> &ExitBlocks) { 187 SmallVector<Use*, 16> UsesToRewrite; 188 189 BasicBlock *InstBB = Inst->getParent(); 190 191 for (Value::use_iterator UI = Inst->use_begin(), E = Inst->use_end(); 192 UI != E; ++UI) { 193 BasicBlock *UserBB = cast<Instruction>(*UI)->getParent(); 194 if (PHINode *PN = dyn_cast<PHINode>(*UI)) 195 UserBB = PN->getIncomingBlock(UI); 196 197 if (InstBB != UserBB && !inLoop(UserBB)) 198 UsesToRewrite.push_back(&UI.getUse()); 199 } 200 201 // If there are no uses outside the loop, exit with no change. 202 if (UsesToRewrite.empty()) return false; 203 204 ++NumLCSSA; // We are applying the transformation 205 206 // Invoke instructions are special in that their result value is not available 207 // along their unwind edge. The code below tests to see whether DomBB dominates 208 // the value, so adjust DomBB to the normal destination block, which is 209 // effectively where the value is first usable. 210 BasicBlock *DomBB = Inst->getParent(); 211 if (InvokeInst *Inv = dyn_cast<InvokeInst>(Inst)) 212 DomBB = Inv->getNormalDest(); 213 214 DomTreeNode *DomNode = DT->getNode(DomBB); 215 216 SSAUpdater SSAUpdate; 217 SSAUpdate.Initialize(Inst); 218 219 // Insert the LCSSA phi's into all of the exit blocks dominated by the 220 // value, and add them to the Phi's map. 221 for (SmallVectorImpl<BasicBlock*>::const_iterator BBI = ExitBlocks.begin(), 222 BBE = ExitBlocks.end(); BBI != BBE; ++BBI) { 223 BasicBlock *ExitBB = *BBI; 224 if (!DT->dominates(DomNode, DT->getNode(ExitBB))) continue; 225 226 // If we already inserted something for this BB, don't reprocess it. 227 if (SSAUpdate.HasValueForBlock(ExitBB)) continue; 228 229 PHINode *PN = PHINode::Create(Inst->getType(), Inst->getName()+".lcssa", 230 ExitBB->begin()); 231 PN->reserveOperandSpace(PredCache.GetNumPreds(ExitBB)); 232 233 // Add inputs from inside the loop for this PHI. 234 for (BasicBlock **PI = PredCache.GetPreds(ExitBB); *PI; ++PI) { 235 PN->addIncoming(Inst, *PI); 236 237 // If the exit block has a predecessor not within the loop, arrange for 238 // the incoming value use corresponding to that predecessor to be 239 // rewritten in terms of a different LCSSA PHI. 240 if (!inLoop(*PI)) 241 UsesToRewrite.push_back( 242 &PN->getOperandUse( 243 PN->getOperandNumForIncomingValue(PN->getNumIncomingValues()-1))); 244 } 245 246 // Remember that this phi makes the value alive in this block. 247 SSAUpdate.AddAvailableValue(ExitBB, PN); 248 } 249 250 // Rewrite all uses outside the loop in terms of the new PHIs we just 251 // inserted. 252 for (unsigned i = 0, e = UsesToRewrite.size(); i != e; ++i) { 253 // If this use is in an exit block, rewrite to use the newly inserted PHI. 254 // This is required for correctness because SSAUpdate doesn't handle uses in 255 // the same block. It assumes the PHI we inserted is at the end of the 256 // block. 257 Instruction *User = cast<Instruction>(UsesToRewrite[i]->getUser()); 258 BasicBlock *UserBB = User->getParent(); 259 if (PHINode *PN = dyn_cast<PHINode>(User)) 260 UserBB = PN->getIncomingBlock(*UsesToRewrite[i]); 261 262 if (isa<PHINode>(UserBB->begin()) && 263 isExitBlock(UserBB, ExitBlocks)) { 264 UsesToRewrite[i]->set(UserBB->begin()); 265 continue; 266 } 267 268 // Otherwise, do full PHI insertion. 269 SSAUpdate.RewriteUse(*UsesToRewrite[i]); 270 } 271 272 return true; 273} 274 275