1 //===-- LCSSA.cpp - Convert loops into loop-closed SSA form ---------------===// 2 // 3 // The LLVM Compiler Infrastructure 4 // 5 // This file was developed by Owen Anderson and is distributed under the 6 // University of Illinois Open Source 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 #include "llvm/Transforms/Scalar.h" 31 #include "llvm/Pass.h" 32 #include "llvm/Function.h" 33 #include "llvm/Instructions.h" 34 #include "llvm/ADT/Statistic.h" 35 #include "llvm/Analysis/Dominators.h" 36 #include "llvm/Analysis/LoopInfo.h" 37 #include "llvm/Support/CFG.h" 38 #include <algorithm> 39 #include <vector> 40 41 using namespace llvm; 42 43 namespace { 44 static Statistic<> NumLCSSA("lcssa", "Number of times LCSSA was applied"); 45 46 class LCSSA : public FunctionPass { 47 public: 48 49 50 LoopInfo *LI; // Loop information 51 DominatorTree *DT; // Dominator Tree for the current Loop... 52 DominanceFrontier *DF; // Current Dominance Frontier 53 54 virtual bool runOnFunction(Function &F); 55 bool visitSubloop(Loop* L); 56 57 /// This transformation requires natural loop information & requires that 58 /// loop preheaders be inserted into the CFG. It maintains both of these, 59 /// as well as the CFG. It also requires dominator information. 60 /// 61 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 62 AU.setPreservesCFG(); 63 AU.addRequiredID(LoopSimplifyID); 64 AU.addPreservedID(LoopSimplifyID); 65 AU.addRequired<LoopInfo>(); 66 AU.addPreserved<LoopInfo>(); 67 AU.addRequired<DominatorTree>(); // Not sure if this one will actually 68 // be needed. 69 AU.addRequired<DominanceFrontier>(); 70 } 71 private: 72 std::set<Instruction*> getLoopValuesUsedOutsideLoop(Loop *L, 73 std::vector<BasicBlock*> LoopBlocks); 74 }; 75 76 RegisterOpt<LCSSA> X("lcssa", "Loop-Closed SSA Form Pass"); 77 } 78 79 FunctionPass *llvm::createLCSSAPass() { return new LCSSA(); } 80 81 bool LCSSA::runOnFunction(Function &F) { 82 bool changed = false; 83 LI = &getAnalysis<LoopInfo>(); 84 DF = &getAnalysis<DominanceFrontier>(); 85 DT = &getAnalysis<DominatorTree>(); 86 87 for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I) { 88 changed |= visitSubloop(*I); 89 } 90 91 return changed; 92 } 93 94 bool LCSSA::visitSubloop(Loop* L) { 95 for (Loop::iterator I = L->begin(), E = L->end(); I != E; ++I) 96 visitSubloop(*I); 97 98 // Speed up queries by creating a sorted list of blocks 99 std::vector<BasicBlock*> LoopBlocks(L->block_begin(), L->block_end()); 100 std::sort(LoopBlocks.begin(), LoopBlocks.end()); 101 102 std::set<Instruction*> AffectedValues = getLoopValuesUsedOutsideLoop(L, 103 LoopBlocks); 104 105 std::vector<BasicBlock*> exitBlocks; 106 L->getExitBlocks(exitBlocks); 107 108 for (std::set<Instruction*>::iterator I = AffectedValues.begin(), 109 E = AffectedValues.end(); I != E; ++I) { 110 ++NumLCSSA; // We are applying the transformation 111 for (std::vector<BasicBlock*>::iterator BBI = exitBlocks.begin(), 112 BBE = exitBlocks.end(); BBI != BBE; ++BBI) { 113 PHINode *phi = new PHINode((*I)->getType(), "lcssa"); 114 (*BBI)->getInstList().insert((*BBI)->front(), phi); 115 116 for (pred_iterator PI = pred_begin(*BBI), PE = pred_end(*BBI); PI != PE; 117 ++PI) 118 phi->addIncoming(*I, *PI); 119 } 120 121 for (Value::use_iterator UI = (*I)->use_begin(), UE = (*I)->use_end(); 122 UI != UE; ++UI) { 123 BasicBlock *UserBB = cast<Instruction>(*UI)->getParent(); 124 if (!std::binary_search(LoopBlocks.begin(), LoopBlocks.end(), UserBB)) 125 ; // FIXME: This should update the SSA form. 126 } 127 } 128 129 return true; // FIXME: Should be more intelligent in our return value. 130 } 131 132 /// getLoopValuesUsedOutsideLoop - Return any values defined in the loop that 133 /// are used by instructions outside of it. 134 std::set<Instruction*> LCSSA::getLoopValuesUsedOutsideLoop(Loop *L, 135 std::vector<BasicBlock*> LoopBlocks) { 136 137 std::set<Instruction*> AffectedValues; 138 for (Loop::block_iterator BB = L->block_begin(), E = L->block_end(); 139 BB != E; ++BB) { 140 for (BasicBlock::iterator I = (*BB)->begin(), E = (*BB)->end(); I != E; ++I) 141 for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E; 142 ++UI) { 143 BasicBlock *UserBB = cast<Instruction>(*UI)->getParent(); 144 if (!std::binary_search(LoopBlocks.begin(), LoopBlocks.end(), UserBB)) 145 AffectedValues.insert(I); 146 } 147 } 148 return AffectedValues; 149 } 150