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