LCSSA.cpp revision 36b56886974eae4f9c5ebc96befd3e7bfe5de338
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/AliasAnalysis.h" 35#include "llvm/Analysis/LoopPass.h" 36#include "llvm/Analysis/ScalarEvolution.h" 37#include "llvm/IR/Constants.h" 38#include "llvm/IR/Dominators.h" 39#include "llvm/IR/Function.h" 40#include "llvm/IR/Instructions.h" 41#include "llvm/IR/PredIteratorCache.h" 42#include "llvm/Pass.h" 43#include "llvm/Transforms/Utils/LoopUtils.h" 44#include "llvm/Transforms/Utils/SSAUpdater.h" 45using namespace llvm; 46 47STATISTIC(NumLCSSA, "Number of live out of a loop variables"); 48 49/// Return true if the specified block is in the list. 50static bool isExitBlock(BasicBlock *BB, 51 const SmallVectorImpl<BasicBlock *> &ExitBlocks) { 52 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) 53 if (ExitBlocks[i] == BB) 54 return true; 55 return false; 56} 57 58/// Given an instruction in the loop, check to see if it has any uses that are 59/// outside the current loop. If so, insert LCSSA PHI nodes and rewrite the 60/// uses. 61static bool processInstruction(Loop &L, Instruction &Inst, DominatorTree &DT, 62 const SmallVectorImpl<BasicBlock *> &ExitBlocks, 63 PredIteratorCache &PredCache) { 64 SmallVector<Use *, 16> UsesToRewrite; 65 66 BasicBlock *InstBB = Inst.getParent(); 67 68 for (Use &U : Inst.uses()) { 69 Instruction *User = cast<Instruction>(U.getUser()); 70 BasicBlock *UserBB = User->getParent(); 71 if (PHINode *PN = dyn_cast<PHINode>(User)) 72 UserBB = PN->getIncomingBlock(U); 73 74 if (InstBB != UserBB && !L.contains(UserBB)) 75 UsesToRewrite.push_back(&U); 76 } 77 78 // If there are no uses outside the loop, exit with no change. 79 if (UsesToRewrite.empty()) 80 return false; 81 82 ++NumLCSSA; // We are applying the transformation 83 84 // Invoke instructions are special in that their result value is not available 85 // along their unwind edge. The code below tests to see whether DomBB 86 // dominates 87 // the value, so adjust DomBB to the normal destination block, which is 88 // effectively where the value is first usable. 89 BasicBlock *DomBB = Inst.getParent(); 90 if (InvokeInst *Inv = dyn_cast<InvokeInst>(&Inst)) 91 DomBB = Inv->getNormalDest(); 92 93 DomTreeNode *DomNode = DT.getNode(DomBB); 94 95 SmallVector<PHINode *, 16> AddedPHIs; 96 97 SSAUpdater SSAUpdate; 98 SSAUpdate.Initialize(Inst.getType(), Inst.getName()); 99 100 // Insert the LCSSA phi's into all of the exit blocks dominated by the 101 // value, and add them to the Phi's map. 102 for (SmallVectorImpl<BasicBlock *>::const_iterator BBI = ExitBlocks.begin(), 103 BBE = ExitBlocks.end(); 104 BBI != BBE; ++BBI) { 105 BasicBlock *ExitBB = *BBI; 106 if (!DT.dominates(DomNode, DT.getNode(ExitBB))) 107 continue; 108 109 // If we already inserted something for this BB, don't reprocess it. 110 if (SSAUpdate.HasValueForBlock(ExitBB)) 111 continue; 112 113 PHINode *PN = PHINode::Create(Inst.getType(), PredCache.GetNumPreds(ExitBB), 114 Inst.getName() + ".lcssa", ExitBB->begin()); 115 116 // Add inputs from inside the loop for this PHI. 117 for (BasicBlock **PI = PredCache.GetPreds(ExitBB); *PI; ++PI) { 118 PN->addIncoming(&Inst, *PI); 119 120 // If the exit block has a predecessor not within the loop, arrange for 121 // the incoming value use corresponding to that predecessor to be 122 // rewritten in terms of a different LCSSA PHI. 123 if (!L.contains(*PI)) 124 UsesToRewrite.push_back( 125 &PN->getOperandUse(PN->getOperandNumForIncomingValue( 126 PN->getNumIncomingValues() - 1))); 127 } 128 129 AddedPHIs.push_back(PN); 130 131 // Remember that this phi makes the value alive in this block. 132 SSAUpdate.AddAvailableValue(ExitBB, PN); 133 } 134 135 // Rewrite all uses outside the loop in terms of the new PHIs we just 136 // inserted. 137 for (unsigned i = 0, e = UsesToRewrite.size(); i != e; ++i) { 138 // If this use is in an exit block, rewrite to use the newly inserted PHI. 139 // This is required for correctness because SSAUpdate doesn't handle uses in 140 // the same block. It assumes the PHI we inserted is at the end of the 141 // block. 142 Instruction *User = cast<Instruction>(UsesToRewrite[i]->getUser()); 143 BasicBlock *UserBB = User->getParent(); 144 if (PHINode *PN = dyn_cast<PHINode>(User)) 145 UserBB = PN->getIncomingBlock(*UsesToRewrite[i]); 146 147 if (isa<PHINode>(UserBB->begin()) && isExitBlock(UserBB, ExitBlocks)) { 148 // Tell the VHs that the uses changed. This updates SCEV's caches. 149 if (UsesToRewrite[i]->get()->hasValueHandle()) 150 ValueHandleBase::ValueIsRAUWd(*UsesToRewrite[i], UserBB->begin()); 151 UsesToRewrite[i]->set(UserBB->begin()); 152 continue; 153 } 154 155 // Otherwise, do full PHI insertion. 156 SSAUpdate.RewriteUse(*UsesToRewrite[i]); 157 } 158 159 // Remove PHI nodes that did not have any uses rewritten. 160 for (unsigned i = 0, e = AddedPHIs.size(); i != e; ++i) { 161 if (AddedPHIs[i]->use_empty()) 162 AddedPHIs[i]->eraseFromParent(); 163 } 164 165 return true; 166} 167 168/// Return true if the specified block dominates at least 169/// one of the blocks in the specified list. 170static bool 171blockDominatesAnExit(BasicBlock *BB, 172 DominatorTree &DT, 173 const SmallVectorImpl<BasicBlock *> &ExitBlocks) { 174 DomTreeNode *DomNode = DT.getNode(BB); 175 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) 176 if (DT.dominates(DomNode, DT.getNode(ExitBlocks[i]))) 177 return true; 178 179 return false; 180} 181 182bool llvm::formLCSSA(Loop &L, DominatorTree &DT, ScalarEvolution *SE) { 183 bool Changed = false; 184 185 // Get the set of exiting blocks. 186 SmallVector<BasicBlock *, 8> ExitBlocks; 187 L.getExitBlocks(ExitBlocks); 188 189 if (ExitBlocks.empty()) 190 return false; 191 192 PredIteratorCache PredCache; 193 194 // Look at all the instructions in the loop, checking to see if they have uses 195 // outside the loop. If so, rewrite those uses. 196 for (Loop::block_iterator BBI = L.block_begin(), BBE = L.block_end(); 197 BBI != BBE; ++BBI) { 198 BasicBlock *BB = *BBI; 199 200 // For large loops, avoid use-scanning by using dominance information: In 201 // particular, if a block does not dominate any of the loop exits, then none 202 // of the values defined in the block could be used outside the loop. 203 if (!blockDominatesAnExit(BB, DT, ExitBlocks)) 204 continue; 205 206 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) { 207 // Reject two common cases fast: instructions with no uses (like stores) 208 // and instructions with one use that is in the same block as this. 209 if (I->use_empty() || 210 (I->hasOneUse() && I->user_back()->getParent() == BB && 211 !isa<PHINode>(I->user_back()))) 212 continue; 213 214 Changed |= processInstruction(L, *I, DT, ExitBlocks, PredCache); 215 } 216 } 217 218 // If we modified the code, remove any caches about the loop from SCEV to 219 // avoid dangling entries. 220 // FIXME: This is a big hammer, can we clear the cache more selectively? 221 if (SE && Changed) 222 SE->forgetLoop(&L); 223 224 assert(L.isLCSSAForm(DT)); 225 226 return Changed; 227} 228 229/// Process a loop nest depth first. 230bool llvm::formLCSSARecursively(Loop &L, DominatorTree &DT, 231 ScalarEvolution *SE) { 232 bool Changed = false; 233 234 // Recurse depth-first through inner loops. 235 for (Loop::iterator LI = L.begin(), LE = L.end(); LI != LE; ++LI) 236 Changed |= formLCSSARecursively(**LI, DT, SE); 237 238 Changed |= formLCSSA(L, DT, SE); 239 return Changed; 240} 241 242namespace { 243struct LCSSA : public FunctionPass { 244 static char ID; // Pass identification, replacement for typeid 245 LCSSA() : FunctionPass(ID) { 246 initializeLCSSAPass(*PassRegistry::getPassRegistry()); 247 } 248 249 // Cached analysis information for the current function. 250 DominatorTree *DT; 251 LoopInfo *LI; 252 ScalarEvolution *SE; 253 254 bool runOnFunction(Function &F) override; 255 256 /// This transformation requires natural loop information & requires that 257 /// loop preheaders be inserted into the CFG. It maintains both of these, 258 /// as well as the CFG. It also requires dominator information. 259 void getAnalysisUsage(AnalysisUsage &AU) const override { 260 AU.setPreservesCFG(); 261 262 AU.addRequired<DominatorTreeWrapperPass>(); 263 AU.addRequired<LoopInfo>(); 264 AU.addPreservedID(LoopSimplifyID); 265 AU.addPreserved<AliasAnalysis>(); 266 AU.addPreserved<ScalarEvolution>(); 267 } 268 269private: 270 bool processLoop(Loop &L); 271 272 void verifyAnalysis() const override; 273}; 274} 275 276char LCSSA::ID = 0; 277INITIALIZE_PASS_BEGIN(LCSSA, "lcssa", "Loop-Closed SSA Form Pass", false, false) 278INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass) 279INITIALIZE_PASS_DEPENDENCY(LoopInfo) 280INITIALIZE_PASS_END(LCSSA, "lcssa", "Loop-Closed SSA Form Pass", false, false) 281 282Pass *llvm::createLCSSAPass() { return new LCSSA(); } 283char &llvm::LCSSAID = LCSSA::ID; 284 285 286/// Process all loops in the function, inner-most out. 287bool LCSSA::runOnFunction(Function &F) { 288 bool Changed = false; 289 LI = &getAnalysis<LoopInfo>(); 290 DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree(); 291 SE = getAnalysisIfAvailable<ScalarEvolution>(); 292 293 // Simplify each loop nest in the function. 294 for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I) 295 Changed |= formLCSSARecursively(**I, *DT, SE); 296 297 return Changed; 298} 299 300static void verifyLoop(Loop &L, DominatorTree &DT) { 301 // Recurse depth-first through inner loops. 302 for (Loop::iterator LI = L.begin(), LE = L.end(); LI != LE; ++LI) 303 verifyLoop(**LI, DT); 304 305 // Check the special guarantees that LCSSA makes. 306 //assert(L.isLCSSAForm(DT) && "LCSSA form not preserved!"); 307} 308 309void LCSSA::verifyAnalysis() const { 310 // Verify each loop nest in the function, assuming LI still points at that 311 // function's loop info. 312 for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I) 313 verifyLoop(**I, *DT); 314} 315