LICM.cpp revision 0513e9fe03f6e3c7d0272b8f4f82359e8d1a2e44
1//===-- LICM.cpp - Loop Invariant Code Motion Pass ------------------------===// 2// 3// This pass is a simple loop invariant code motion pass. 4// 5// Note that this pass does NOT require pre-headers to exist on loops in the 6// CFG, but if there is not distinct preheader for a loop, the hoisted code will 7// be *DUPLICATED* in every basic block, outside of the loop, that preceeds the 8// loop header. Additionally, any use of one of these hoisted expressions 9// cannot be loop invariant itself, because the expression hoisted gets a PHI 10// node that is loop variant. 11// 12// For these reasons, and many more, it makes sense to run a pass before this 13// that ensures that there are preheaders on all loops. That said, we don't 14// REQUIRE it. :) 15// 16//===----------------------------------------------------------------------===// 17 18#include "llvm/Transforms/Scalar.h" 19#include "llvm/Transforms/Utils/Local.h" 20#include "llvm/Analysis/LoopInfo.h" 21#include "llvm/iOperators.h" 22#include "llvm/iPHINode.h" 23#include "llvm/Support/InstVisitor.h" 24#include "llvm/Support/CFG.h" 25#include "Support/STLExtras.h" 26#include "Support/StatisticReporter.h" 27#include <algorithm> 28using std::string; 29 30static Statistic<> NumHoistedNPH("licm\t\t- Number of insts hoisted to multiple" 31 " loop preds (bad, no loop pre-header)"); 32static Statistic<> NumHoistedPH("licm\t\t- Number of insts hoisted to a loop " 33 "pre-header"); 34 35namespace { 36 struct LICM : public FunctionPass, public InstVisitor<LICM> { 37 virtual bool runOnFunction(Function &F); 38 39 // This transformation requires natural loop information... 40 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 41 AU.preservesCFG(); 42 AU.addRequired<LoopInfo>(); 43 } 44 45 private: 46 // List of predecessor blocks for the current loop - These blocks are where 47 // we hoist loop invariants to for the current loop. 48 // 49 std::vector<BasicBlock*> LoopPreds, LoopBackEdges; 50 51 Loop *CurLoop; // The current loop we are working on... 52 bool Changed; // Set to true when we change anything. 53 54 // visitLoop - Hoist expressions out of the specified loop... 55 void visitLoop(Loop *L); 56 57 // notInCurrentLoop - Little predicate that returns true if the specified 58 // basic block is in a subloop of the current one, not the current one 59 // itself. 60 // 61 bool notInCurrentLoop(BasicBlock *BB) { 62 for (unsigned i = 0, e = CurLoop->getSubLoops().size(); i != e; ++i) 63 if (CurLoop->getSubLoops()[i]->contains(BB)) 64 return true; // A subloop actually contains this block! 65 return false; 66 } 67 68 // hoist - When an instruction is found to only use loop invariant operands 69 // that is safe to hoist, this instruction is called to do the dirty work. 70 // 71 void hoist(Instruction &I); 72 73 // isLoopInvariant - Return true if the specified value is loop invariant 74 inline bool isLoopInvariant(Value *V) { 75 if (Instruction *I = dyn_cast<Instruction>(V)) 76 return !CurLoop->contains(I->getParent()); 77 return true; // All non-instructions are loop invariant 78 } 79 80 // visitBasicBlock - Run LICM on a particular block. 81 void visitBasicBlock(BasicBlock *BB); 82 83 // Instruction visitation handlers... these basically control whether or not 84 // the specified instruction types are hoisted. 85 // 86 friend class InstVisitor<LICM>; 87 void visitBinaryOperator(Instruction &I) { 88 if (isLoopInvariant(I.getOperand(0)) && isLoopInvariant(I.getOperand(1))) 89 hoist(I); 90 } 91 void visitCastInst(CastInst &I) { 92 if (isLoopInvariant(I.getOperand(0))) hoist((Instruction&)I); 93 } 94 void visitShiftInst(ShiftInst &I) { visitBinaryOperator((Instruction&)I); } 95 96 void visitGetElementPtrInst(GetElementPtrInst &GEPI) { 97 Instruction &I = (Instruction&)GEPI; 98 for (unsigned i = 0, e = I.getNumOperands(); i != e; ++i) 99 if (!isLoopInvariant(I.getOperand(i))) return; 100 hoist(I); 101 } 102 }; 103 104 RegisterOpt<LICM> X("licm", "Loop Invariant Code Motion"); 105} 106 107Pass *createLICMPass() { return new LICM(); } 108 109bool LICM::runOnFunction(Function &) { 110 // get our loop information... 111 const std::vector<Loop*> &TopLevelLoops = 112 getAnalysis<LoopInfo>().getTopLevelLoops(); 113 114 // Traverse loops in postorder, hoisting expressions out of the deepest loops 115 // first. 116 // 117 Changed = false; 118 std::for_each(TopLevelLoops.begin(), TopLevelLoops.end(), 119 bind_obj(this, &LICM::visitLoop)); 120 return Changed; 121} 122 123void LICM::visitLoop(Loop *L) { 124 // Recurse through all subloops before we process this loop... 125 std::for_each(L->getSubLoops().begin(), L->getSubLoops().end(), 126 bind_obj(this, &LICM::visitLoop)); 127 CurLoop = L; 128 129 // Calculate the set of predecessors for this loop. The predecessors for this 130 // loop are equal to the predecessors for the header node of the loop that are 131 // not themselves in the loop. 132 // 133 BasicBlock *Header = L->getHeader(); 134 135 // Calculate the sets of predecessors and backedges of the loop... 136 LoopBackEdges.insert(LoopBackEdges.end(),pred_begin(Header),pred_end(Header)); 137 138 std::vector<BasicBlock*>::iterator LPI = 139 std::partition(LoopBackEdges.begin(), LoopBackEdges.end(), 140 bind_obj(CurLoop, &Loop::contains)); 141 142 // Move all predecessors to the LoopPreds vector... 143 LoopPreds.insert(LoopPreds.end(), LPI, LoopBackEdges.end()); 144 145 // Remove predecessors from backedges list... 146 LoopBackEdges.erase(LPI, LoopBackEdges.end()); 147 148 149 // The only way that there could be no predecessors to a loop is if the loop 150 // is not reachable. Since we don't care about optimizing dead loops, 151 // summarily ignore them. 152 // 153 if (LoopPreds.empty()) return; 154 155 // We want to visit all of the instructions in this loop... that are not parts 156 // of our subloops (they have already had their invariants hoisted out of 157 // their loop, into this loop, so there is no need to process the BODIES of 158 // the subloops). 159 // 160 std::vector<BasicBlock*> BBs(L->getBlocks().begin(), L->getBlocks().end()); 161 162 // Remove blocks that are actually in subloops... 163 BBs.erase(std::remove_if(BBs.begin(), BBs.end(), 164 bind_obj(this, &LICM::notInCurrentLoop)), BBs.end()); 165 166 // Visit all of the basic blocks we have chosen, hoisting out the instructions 167 // as neccesary. This leaves dead copies of the instruction in the loop 168 // unfortunately... 169 // 170 for_each(BBs.begin(), BBs.end(), bind_obj(this, &LICM::visitBasicBlock)); 171 172 // Clear out loops state information for the next iteration 173 CurLoop = 0; 174 LoopPreds.clear(); 175 LoopBackEdges.clear(); 176} 177 178void LICM::visitBasicBlock(BasicBlock *BB) { 179 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ) { 180 visit(*I); 181 182 if (dceInstruction(I)) 183 Changed = true; 184 else 185 ++I; 186 } 187} 188 189 190void LICM::hoist(Instruction &Inst) { 191 if (Inst.use_empty()) return; // Don't (re) hoist dead instructions! 192 //cerr << "Hoisting " << Inst; 193 194 BasicBlock *Header = CurLoop->getHeader(); 195 196 // Old instruction will be removed, so take it's name... 197 string InstName = Inst.getName(); 198 Inst.setName(""); 199 200 // The common case is that we have a pre-header. Generate special case code 201 // that is faster if that is the case. 202 // 203 if (LoopPreds.size() == 1) { 204 BasicBlock *Pred = LoopPreds[0]; 205 206 // Create a new copy of the instruction, for insertion into Pred. 207 Instruction *New = Inst.clone(); 208 New->setName(InstName); 209 210 // Insert the new node in Pred, before the terminator. 211 Pred->getInstList().insert(--Pred->end(), New); 212 213 // Kill the old instruction... 214 Inst.replaceAllUsesWith(New); 215 ++NumHoistedPH; 216 217 } else { 218 // No loop pre-header, insert a PHI node into header to capture all of the 219 // incoming versions of the value. 220 // 221 PHINode *LoopVal = new PHINode(Inst.getType(), InstName+".phi"); 222 223 // Insert the new PHI node into the loop header... 224 Header->getInstList().push_front(LoopVal); 225 226 // Insert cloned versions of the instruction into all of the loop preds. 227 for (unsigned i = 0, e = LoopPreds.size(); i != e; ++i) { 228 BasicBlock *Pred = LoopPreds[i]; 229 230 // Create a new copy of the instruction, for insertion into Pred. 231 Instruction *New = Inst.clone(); 232 New->setName(InstName); 233 234 // Insert the new node in Pred, before the terminator. 235 Pred->getInstList().insert(--Pred->end(), New); 236 237 // Add the incoming value to the PHI node. 238 LoopVal->addIncoming(New, Pred); 239 } 240 241 // Add incoming values to the PHI node for all backedges in the loop... 242 for (unsigned i = 0, e = LoopBackEdges.size(); i != e; ++i) 243 LoopVal->addIncoming(LoopVal, LoopBackEdges[i]); 244 245 // Replace all uses of the old version of the instruction in the loop with 246 // the new version that is out of the loop. We know that this is ok, 247 // because the new definition is in the loop header, which dominates the 248 // entire loop body. The old definition was defined _inside_ of the loop, 249 // so the scope cannot extend outside of the loop, so we're ok. 250 // 251 Inst.replaceAllUsesWith(LoopVal); 252 ++NumHoistedNPH; 253 } 254 255 Changed = true; 256} 257 258