1193323Sed//===-- LCSSA.cpp - Convert loops into loop-closed SSA form ---------------===// 2193323Sed// 3193323Sed// The LLVM Compiler Infrastructure 4193323Sed// 5193323Sed// This file is distributed under the University of Illinois Open Source 6193323Sed// License. See LICENSE.TXT for details. 7193323Sed// 8193323Sed//===----------------------------------------------------------------------===// 9193323Sed// 10193323Sed// This pass transforms loops by placing phi nodes at the end of the loops for 11193323Sed// all values that are live across the loop boundary. For example, it turns 12193323Sed// the left into the right code: 13193323Sed// 14193323Sed// for (...) for (...) 15193323Sed// if (c) if (c) 16193323Sed// X1 = ... X1 = ... 17193323Sed// else else 18193323Sed// X2 = ... X2 = ... 19193323Sed// X3 = phi(X1, X2) X3 = phi(X1, X2) 20193323Sed// ... = X3 + 4 X4 = phi(X3) 21193323Sed// ... = X4 + 4 22193323Sed// 23193323Sed// This is still valid LLVM; the extra phi nodes are purely redundant, and will 24193323Sed// be trivially eliminated by InstCombine. The major benefit of this 25193323Sed// transformation is that it makes many other loop optimizations, such as 26193323Sed// LoopUnswitching, simpler. 27193323Sed// 28193323Sed//===----------------------------------------------------------------------===// 29193323Sed 30193323Sed#define DEBUG_TYPE "lcssa" 31193323Sed#include "llvm/Transforms/Scalar.h" 32252723Sdim#include "llvm/ADT/STLExtras.h" 33252723Sdim#include "llvm/ADT/Statistic.h" 34193323Sed#include "llvm/Analysis/Dominators.h" 35266759Sdim#include "llvm/Analysis/AliasAnalysis.h" 36193323Sed#include "llvm/Analysis/LoopPass.h" 37193323Sed#include "llvm/Analysis/ScalarEvolution.h" 38252723Sdim#include "llvm/IR/Constants.h" 39252723Sdim#include "llvm/IR/Function.h" 40252723Sdim#include "llvm/IR/Instructions.h" 41252723Sdim#include "llvm/Pass.h" 42252723Sdim#include "llvm/Support/PredIteratorCache.h" 43198090Srdivacky#include "llvm/Transforms/Utils/SSAUpdater.h" 44193323Sedusing namespace llvm; 45193323Sed 46193323SedSTATISTIC(NumLCSSA, "Number of live out of a loop variables"); 47193323Sed 48193323Sednamespace { 49198090Srdivacky struct LCSSA : public LoopPass { 50193323Sed static char ID; // Pass identification, replacement for typeid 51218893Sdim LCSSA() : LoopPass(ID) { 52218893Sdim initializeLCSSAPass(*PassRegistry::getPassRegistry()); 53218893Sdim } 54193323Sed 55193323Sed // Cached analysis information for the current function. 56193323Sed DominatorTree *DT; 57245431Sdim LoopInfo *LI; 58245431Sdim ScalarEvolution *SE; 59193323Sed PredIteratorCache PredCache; 60198090Srdivacky Loop *L; 61193323Sed 62193323Sed virtual bool runOnLoop(Loop *L, LPPassManager &LPM); 63193323Sed 64193323Sed /// This transformation requires natural loop information & requires that 65193323Sed /// loop preheaders be inserted into the CFG. It maintains both of these, 66193323Sed /// as well as the CFG. It also requires dominator information. 67193323Sed /// 68193323Sed virtual void getAnalysisUsage(AnalysisUsage &AU) const { 69193323Sed AU.setPreservesCFG(); 70199481Srdivacky 71212904Sdim AU.addRequired<DominatorTree>(); 72212904Sdim AU.addRequired<LoopInfo>(); 73193323Sed AU.addPreservedID(LoopSimplifyID); 74266759Sdim AU.addPreserved<AliasAnalysis>(); 75193323Sed AU.addPreserved<ScalarEvolution>(); 76193323Sed } 77193323Sed private: 78198090Srdivacky bool ProcessInstruction(Instruction *Inst, 79198090Srdivacky const SmallVectorImpl<BasicBlock*> &ExitBlocks); 80198090Srdivacky 81198090Srdivacky /// verifyAnalysis() - Verify loop nest. 82198090Srdivacky virtual void verifyAnalysis() const { 83198090Srdivacky // Check the special guarantees that LCSSA makes. 84205218Srdivacky assert(L->isLCSSAForm(*DT) && "LCSSA form not preserved!"); 85198090Srdivacky } 86193323Sed }; 87193323Sed} 88193323Sed 89193323Sedchar LCSSA::ID = 0; 90218893SdimINITIALIZE_PASS_BEGIN(LCSSA, "lcssa", "Loop-Closed SSA Form Pass", false, false) 91218893SdimINITIALIZE_PASS_DEPENDENCY(DominatorTree) 92218893SdimINITIALIZE_PASS_DEPENDENCY(LoopInfo) 93218893SdimINITIALIZE_PASS_END(LCSSA, "lcssa", "Loop-Closed SSA Form Pass", false, false) 94193323Sed 95193323SedPass *llvm::createLCSSAPass() { return new LCSSA(); } 96212904Sdimchar &llvm::LCSSAID = LCSSA::ID; 97193323Sed 98198090Srdivacky 99198090Srdivacky/// BlockDominatesAnExit - Return true if the specified block dominates at least 100198090Srdivacky/// one of the blocks in the specified list. 101198090Srdivackystatic bool BlockDominatesAnExit(BasicBlock *BB, 102198090Srdivacky const SmallVectorImpl<BasicBlock*> &ExitBlocks, 103198090Srdivacky DominatorTree *DT) { 104198090Srdivacky DomTreeNode *DomNode = DT->getNode(BB); 105198090Srdivacky for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) 106198090Srdivacky if (DT->dominates(DomNode, DT->getNode(ExitBlocks[i]))) 107198090Srdivacky return true; 108198090Srdivacky 109198090Srdivacky return false; 110198090Srdivacky} 111198090Srdivacky 112198090Srdivacky 113193323Sed/// runOnFunction - Process all loops in the function, inner-most out. 114198090Srdivackybool LCSSA::runOnLoop(Loop *TheLoop, LPPassManager &LPM) { 115198090Srdivacky L = TheLoop; 116193323Sed 117193323Sed DT = &getAnalysis<DominatorTree>(); 118245431Sdim LI = &getAnalysis<LoopInfo>(); 119245431Sdim SE = getAnalysisIfAvailable<ScalarEvolution>(); 120193323Sed 121198090Srdivacky // Get the set of exiting blocks. 122198090Srdivacky SmallVector<BasicBlock*, 8> ExitBlocks; 123198090Srdivacky L->getExitBlocks(ExitBlocks); 124198090Srdivacky 125198090Srdivacky if (ExitBlocks.empty()) 126198090Srdivacky return false; 127198090Srdivacky 128198090Srdivacky // Look at all the instructions in the loop, checking to see if they have uses 129198090Srdivacky // outside the loop. If so, rewrite those uses. 130198090Srdivacky bool MadeChange = false; 131193323Sed 132198090Srdivacky for (Loop::block_iterator BBI = L->block_begin(), E = L->block_end(); 133198090Srdivacky BBI != E; ++BBI) { 134198090Srdivacky BasicBlock *BB = *BBI; 135198090Srdivacky 136198090Srdivacky // For large loops, avoid use-scanning by using dominance information: In 137198090Srdivacky // particular, if a block does not dominate any of the loop exits, then none 138198090Srdivacky // of the values defined in the block could be used outside the loop. 139198090Srdivacky if (!BlockDominatesAnExit(BB, ExitBlocks, DT)) 140198090Srdivacky continue; 141198090Srdivacky 142198090Srdivacky for (BasicBlock::iterator I = BB->begin(), E = BB->end(); 143198090Srdivacky I != E; ++I) { 144198090Srdivacky // Reject two common cases fast: instructions with no uses (like stores) 145198090Srdivacky // and instructions with one use that is in the same block as this. 146198090Srdivacky if (I->use_empty() || 147198090Srdivacky (I->hasOneUse() && I->use_back()->getParent() == BB && 148198090Srdivacky !isa<PHINode>(I->use_back()))) 149198090Srdivacky continue; 150198090Srdivacky 151198090Srdivacky MadeChange |= ProcessInstruction(I, ExitBlocks); 152198090Srdivacky } 153198090Srdivacky } 154245431Sdim 155245431Sdim // If we modified the code, remove any caches about the loop from SCEV to 156245431Sdim // avoid dangling entries. 157245431Sdim // FIXME: This is a big hammer, can we clear the cache more selectively? 158245431Sdim if (SE && MadeChange) 159245431Sdim SE->forgetLoop(L); 160193323Sed 161205218Srdivacky assert(L->isLCSSAForm(*DT)); 162198090Srdivacky PredCache.clear(); 163198090Srdivacky 164198090Srdivacky return MadeChange; 165198090Srdivacky} 166198090Srdivacky 167198090Srdivacky/// isExitBlock - Return true if the specified block is in the list. 168198090Srdivackystatic bool isExitBlock(BasicBlock *BB, 169198090Srdivacky const SmallVectorImpl<BasicBlock*> &ExitBlocks) { 170198090Srdivacky for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) 171198090Srdivacky if (ExitBlocks[i] == BB) 172198090Srdivacky return true; 173198090Srdivacky return false; 174198090Srdivacky} 175198090Srdivacky 176198090Srdivacky/// ProcessInstruction - Given an instruction in the loop, check to see if it 177198090Srdivacky/// has any uses that are outside the current loop. If so, insert LCSSA PHI 178198090Srdivacky/// nodes and rewrite the uses. 179198090Srdivackybool LCSSA::ProcessInstruction(Instruction *Inst, 180198090Srdivacky const SmallVectorImpl<BasicBlock*> &ExitBlocks) { 181198090Srdivacky SmallVector<Use*, 16> UsesToRewrite; 182193323Sed 183198090Srdivacky BasicBlock *InstBB = Inst->getParent(); 184193323Sed 185198090Srdivacky for (Value::use_iterator UI = Inst->use_begin(), E = Inst->use_end(); 186198090Srdivacky UI != E; ++UI) { 187210299Sed User *U = *UI; 188210299Sed BasicBlock *UserBB = cast<Instruction>(U)->getParent(); 189210299Sed if (PHINode *PN = dyn_cast<PHINode>(U)) 190198090Srdivacky UserBB = PN->getIncomingBlock(UI); 191198090Srdivacky 192263509Sdim if (InstBB != UserBB && !L->contains(UserBB)) 193198090Srdivacky UsesToRewrite.push_back(&UI.getUse()); 194198090Srdivacky } 195210299Sed 196198090Srdivacky // If there are no uses outside the loop, exit with no change. 197198090Srdivacky if (UsesToRewrite.empty()) return false; 198193323Sed 199193323Sed ++NumLCSSA; // We are applying the transformation 200193323Sed 201195098Sed // Invoke instructions are special in that their result value is not available 202195098Sed // along their unwind edge. The code below tests to see whether DomBB dominates 203195098Sed // the value, so adjust DomBB to the normal destination block, which is 204195098Sed // effectively where the value is first usable. 205198090Srdivacky BasicBlock *DomBB = Inst->getParent(); 206198090Srdivacky if (InvokeInst *Inv = dyn_cast<InvokeInst>(Inst)) 207195098Sed DomBB = Inv->getNormalDest(); 208195098Sed 209195098Sed DomTreeNode *DomNode = DT->getNode(DomBB); 210195098Sed 211221345Sdim SmallVector<PHINode*, 16> AddedPHIs; 212221345Sdim 213198090Srdivacky SSAUpdater SSAUpdate; 214212904Sdim SSAUpdate.Initialize(Inst->getType(), Inst->getName()); 215198090Srdivacky 216198090Srdivacky // Insert the LCSSA phi's into all of the exit blocks dominated by the 217199481Srdivacky // value, and add them to the Phi's map. 218198090Srdivacky for (SmallVectorImpl<BasicBlock*>::const_iterator BBI = ExitBlocks.begin(), 219198090Srdivacky BBE = ExitBlocks.end(); BBI != BBE; ++BBI) { 220198090Srdivacky BasicBlock *ExitBB = *BBI; 221198090Srdivacky if (!DT->dominates(DomNode, DT->getNode(ExitBB))) continue; 222198090Srdivacky 223198090Srdivacky // If we already inserted something for this BB, don't reprocess it. 224198090Srdivacky if (SSAUpdate.HasValueForBlock(ExitBB)) continue; 225198090Srdivacky 226221345Sdim PHINode *PN = PHINode::Create(Inst->getType(), 227221345Sdim PredCache.GetNumPreds(ExitBB), 228221345Sdim Inst->getName()+".lcssa", 229198090Srdivacky ExitBB->begin()); 230193323Sed 231198090Srdivacky // Add inputs from inside the loop for this PHI. 232199481Srdivacky for (BasicBlock **PI = PredCache.GetPreds(ExitBB); *PI; ++PI) { 233198090Srdivacky PN->addIncoming(Inst, *PI); 234199481Srdivacky 235199481Srdivacky // If the exit block has a predecessor not within the loop, arrange for 236199481Srdivacky // the incoming value use corresponding to that predecessor to be 237199481Srdivacky // rewritten in terms of a different LCSSA PHI. 238263509Sdim if (!L->contains(*PI)) 239199481Srdivacky UsesToRewrite.push_back( 240199481Srdivacky &PN->getOperandUse( 241199481Srdivacky PN->getOperandNumForIncomingValue(PN->getNumIncomingValues()-1))); 242199481Srdivacky } 243221345Sdim 244221345Sdim AddedPHIs.push_back(PN); 245198090Srdivacky 246198090Srdivacky // Remember that this phi makes the value alive in this block. 247198090Srdivacky SSAUpdate.AddAvailableValue(ExitBB, PN); 248193323Sed } 249245431Sdim 250198090Srdivacky // Rewrite all uses outside the loop in terms of the new PHIs we just 251198090Srdivacky // inserted. 252198090Srdivacky for (unsigned i = 0, e = UsesToRewrite.size(); i != e; ++i) { 253198090Srdivacky // If this use is in an exit block, rewrite to use the newly inserted PHI. 254198090Srdivacky // This is required for correctness because SSAUpdate doesn't handle uses in 255198090Srdivacky // the same block. It assumes the PHI we inserted is at the end of the 256198090Srdivacky // block. 257198090Srdivacky Instruction *User = cast<Instruction>(UsesToRewrite[i]->getUser()); 258198090Srdivacky BasicBlock *UserBB = User->getParent(); 259198090Srdivacky if (PHINode *PN = dyn_cast<PHINode>(User)) 260198090Srdivacky UserBB = PN->getIncomingBlock(*UsesToRewrite[i]); 261198090Srdivacky 262198090Srdivacky if (isa<PHINode>(UserBB->begin()) && 263198090Srdivacky isExitBlock(UserBB, ExitBlocks)) { 264245431Sdim // Tell the VHs that the uses changed. This updates SCEV's caches. 265245431Sdim if (UsesToRewrite[i]->get()->hasValueHandle()) 266245431Sdim ValueHandleBase::ValueIsRAUWd(*UsesToRewrite[i], UserBB->begin()); 267198090Srdivacky UsesToRewrite[i]->set(UserBB->begin()); 268193323Sed continue; 269193323Sed } 270193323Sed 271198090Srdivacky // Otherwise, do full PHI insertion. 272198090Srdivacky SSAUpdate.RewriteUse(*UsesToRewrite[i]); 273193323Sed } 274221345Sdim 275221345Sdim // Remove PHI nodes that did not have any uses rewritten. 276221345Sdim for (unsigned i = 0, e = AddedPHIs.size(); i != e; ++i) { 277221345Sdim if (AddedPHIs[i]->use_empty()) 278221345Sdim AddedPHIs[i]->eraseFromParent(); 279221345Sdim } 280193323Sed 281198090Srdivacky return true; 282193323Sed} 283193323Sed 284