MachineLICM.cpp revision ac69582664714c2656a28ed6cb70627bb85ee673
1//===-- MachineLICM.cpp - Machine Loop Invariant Code Motion Pass ---------===// 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 performs loop invariant code motion on machine instructions. We 11// attempt to remove as much code from the body of a loop as possible. 12// 13//===----------------------------------------------------------------------===// 14 15#define DEBUG_TYPE "machine-licm" 16#include "llvm/CodeGen/Passes.h" 17#include "llvm/CodeGen/MachineDominators.h" 18#include "llvm/CodeGen/MachineLoopInfo.h" 19#include "llvm/CodeGen/MachineRegisterInfo.h" 20#include "llvm/Target/MRegisterInfo.h" 21#include "llvm/Target/TargetInstrInfo.h" 22#include "llvm/Target/TargetMachine.h" 23#include "llvm/ADT/SmallVector.h" 24#include "llvm/ADT/Statistic.h" 25#include "llvm/Support/CommandLine.h" 26#include "llvm/Support/Compiler.h" 27#include "llvm/Support/Debug.h" 28 29using namespace llvm; 30 31namespace { 32 // Hidden options to help debugging 33 cl::opt<bool> 34 PerformLICM("machine-licm", 35 cl::init(false), cl::Hidden, 36 cl::desc("Perform loop-invariant code motion on machine code")); 37} 38 39STATISTIC(NumHoisted, "Number of machine instructions hoisted out of loops"); 40 41namespace { 42 class VISIBILITY_HIDDEN MachineLICM : public MachineFunctionPass { 43 const TargetMachine *TM; 44 const TargetInstrInfo *TII; 45 MachineFunction *CurMF; // Current MachineFunction 46 47 // Various analyses that we use... 48 MachineLoopInfo *LI; // Current MachineLoopInfo 49 MachineDominatorTree *DT; // Machine dominator tree for the current Loop 50 MachineRegisterInfo *RegInfo; // Machine register information 51 52 // State that is updated as we process loops 53 bool Changed; // True if a loop is changed. 54 MachineLoop *CurLoop; // The current loop we are working on. 55 public: 56 static char ID; // Pass identification, replacement for typeid 57 MachineLICM() : MachineFunctionPass((intptr_t)&ID) {} 58 59 virtual bool runOnMachineFunction(MachineFunction &MF); 60 61 /// FIXME: Loop preheaders? 62 /// 63 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 64 AU.setPreservesCFG(); 65 AU.addRequired<MachineLoopInfo>(); 66 AU.addRequired<MachineDominatorTree>(); 67 } 68 private: 69 /// VisitAllLoops - Visit all of the loops in depth first order and try to 70 /// hoist invariant instructions from them. 71 /// 72 void VisitAllLoops(MachineLoop *L) { 73 const std::vector<MachineLoop*> &SubLoops = L->getSubLoops(); 74 75 for (MachineLoop::iterator 76 I = SubLoops.begin(), E = SubLoops.end(); I != E; ++I) { 77 MachineLoop *ML = *I; 78 79 // Traverse the body of the loop in depth first order on the dominator 80 // tree so that we are guaranteed to see definitions before we see uses. 81 VisitAllLoops(ML); 82 HoistRegion(DT->getNode(ML->getHeader())); 83 } 84 85 HoistRegion(DT->getNode(L->getHeader())); 86 } 87 88 /// IsInSubLoop - A little predicate that returns true if the specified 89 /// basic block is in a subloop of the current one, not the current one 90 /// itself. 91 /// 92 bool IsInSubLoop(MachineBasicBlock *BB) { 93 assert(CurLoop->contains(BB) && "Only valid if BB is IN the loop"); 94 return LI->getLoopFor(BB) != CurLoop; 95 } 96 97 /// IsLoopInvariantInst - Returns true if the instruction is loop 98 /// invariant. I.e., all virtual register operands are defined outside of 99 /// the loop, physical registers aren't accessed (explicitly or implicitly), 100 /// and the instruction is hoistable. 101 /// 102 bool IsLoopInvariantInst(MachineInstr &I); 103 104 /// FindPredecessors - Get all of the predecessors of the loop that are not 105 /// back-edges. 106 /// 107 void FindPredecessors(std::vector<MachineBasicBlock*> &Preds) { 108 const MachineBasicBlock *Header = CurLoop->getHeader(); 109 110 for (MachineBasicBlock::const_pred_iterator 111 I = Header->pred_begin(), E = Header->pred_end(); I != E; ++I) 112 if (!CurLoop->contains(*I)) 113 Preds.push_back(*I); 114 } 115 116 /// MoveInstToEndOfBlock - Moves the machine instruction to the bottom of 117 /// the predecessor basic block (but before the terminator instructions). 118 /// 119 void MoveInstToEndOfBlock(MachineBasicBlock *ToMBB, 120 MachineBasicBlock *FromMBB, 121 MachineInstr *MI) { 122 DEBUG({ 123 DOUT << "Hoisting " << *MI; 124 if (ToMBB->getBasicBlock()) 125 DOUT << " to MachineBasicBlock " 126 << ToMBB->getBasicBlock()->getName(); 127 DOUT << "\n"; 128 }); 129 130 MachineBasicBlock::iterator WhereIter = ToMBB->getFirstTerminator(); 131 MachineBasicBlock::iterator To, From = FromMBB->begin(); 132 133 while (&*From != MI) 134 ++From; 135 136 assert(From != FromMBB->end() && "Didn't find instr in BB!"); 137 138 To = From; 139 ToMBB->splice(WhereIter, FromMBB, From, ++To); 140 ++NumHoisted; 141 } 142 143 /// HoistRegion - Walk the specified region of the CFG (defined by all 144 /// blocks dominated by the specified block, and that are in the current 145 /// loop) in depth first order w.r.t the DominatorTree. This allows us to 146 /// visit definitions before uses, allowing us to hoist a loop body in one 147 /// pass without iteration. 148 /// 149 void HoistRegion(MachineDomTreeNode *N); 150 151 /// Hoist - When an instruction is found to only use loop invariant operands 152 /// that is safe to hoist, this instruction is called to do the dirty work. 153 /// 154 void Hoist(MachineInstr &MI); 155 }; 156 157 char MachineLICM::ID = 0; 158 RegisterPass<MachineLICM> X("machine-licm", 159 "Machine Loop Invariant Code Motion"); 160} // end anonymous namespace 161 162FunctionPass *llvm::createMachineLICMPass() { return new MachineLICM(); } 163 164/// Hoist expressions out of the specified loop. Note, alias info for inner loop 165/// is not preserved so it is not a good idea to run LICM multiple times on one 166/// loop. 167/// 168bool MachineLICM::runOnMachineFunction(MachineFunction &MF) { 169 if (!PerformLICM) return false; // For debugging. 170 171 DOUT << "******** Machine LICM ********\n"; 172 173 Changed = false; 174 CurMF = &MF; 175 TM = &CurMF->getTarget(); 176 TII = TM->getInstrInfo(); 177 RegInfo = &CurMF->getRegInfo(); 178 179 // Get our Loop information... 180 LI = &getAnalysis<MachineLoopInfo>(); 181 DT = &getAnalysis<MachineDominatorTree>(); 182 183 for (MachineLoopInfo::iterator 184 I = LI->begin(), E = LI->end(); I != E; ++I) { 185 CurLoop = *I; 186 187 // Visit all of the instructions of the loop. We want to visit the subloops 188 // first, though, so that we can hoist their invariants first into their 189 // containing loop before we process that loop. 190 VisitAllLoops(CurLoop); 191 } 192 193 return Changed; 194} 195 196/// HoistRegion - Walk the specified region of the CFG (defined by all blocks 197/// dominated by the specified block, and that are in the current loop) in depth 198/// first order w.r.t the DominatorTree. This allows us to visit definitions 199/// before uses, allowing us to hoist a loop body in one pass without iteration. 200/// 201void MachineLICM::HoistRegion(MachineDomTreeNode *N) { 202 assert(N != 0 && "Null dominator tree node?"); 203 MachineBasicBlock *BB = N->getBlock(); 204 205 // If this subregion is not in the top level loop at all, exit. 206 if (!CurLoop->contains(BB)) return; 207 208 // Only need to process the contents of this block if it is not part of a 209 // subloop (which would already have been processed). 210 if (!IsInSubLoop(BB)) 211 for (MachineBasicBlock::iterator 212 I = BB->begin(), E = BB->end(); I != E; ) { 213 MachineInstr &MI = *I++; 214 215 // Try hoisting the instruction out of the loop. We can only do this if 216 // all of the operands of the instruction are loop invariant and if it is 217 // safe to hoist the instruction. 218 Hoist(MI); 219 } 220 221 const std::vector<MachineDomTreeNode*> &Children = N->getChildren(); 222 223 for (unsigned I = 0, E = Children.size(); I != E; ++I) 224 HoistRegion(Children[I]); 225} 226 227/// IsLoopInvariantInst - Returns true if the instruction is loop 228/// invariant. I.e., all virtual register operands are defined outside of the 229/// loop, physical registers aren't accessed explicitly, and there are no side 230/// effects that aren't captured by the operands or other flags. 231/// 232bool MachineLICM::IsLoopInvariantInst(MachineInstr &I) { 233 DEBUG({ 234 DOUT << "--- Checking if we can hoist " << I; 235 if (I.getInstrDescriptor()->ImplicitUses) { 236 DOUT << " * Instruction has implicit uses:\n"; 237 238 const MRegisterInfo *MRI = TM->getRegisterInfo(); 239 const unsigned *ImpUses = I.getInstrDescriptor()->ImplicitUses; 240 241 for (; *ImpUses; ++ImpUses) 242 DOUT << " -> " << MRI->getName(*ImpUses) << "\n"; 243 } 244 245 if (I.getInstrDescriptor()->ImplicitDefs) { 246 DOUT << " * Instruction has implicit defines:\n"; 247 248 const MRegisterInfo *MRI = TM->getRegisterInfo(); 249 const unsigned *ImpDefs = I.getInstrDescriptor()->ImplicitDefs; 250 251 for (; *ImpDefs; ++ImpDefs) 252 DOUT << " -> " << MRI->getName(*ImpDefs) << "\n"; 253 } 254 255 if (TII->hasUnmodelledSideEffects(&I)) 256 DOUT << " * Instruction has side effects.\n"; 257 }); 258 259 // The instruction is loop invariant if all of its operands are loop-invariant 260 for (unsigned i = 0, e = I.getNumOperands(); i != e; ++i) { 261 const MachineOperand &MO = I.getOperand(i); 262 263 if (!(MO.isRegister() && MO.getReg() && MO.isUse())) 264 continue; 265 266 unsigned Reg = MO.getReg(); 267 268 // Don't hoist instructions that access physical registers. 269 if (!MRegisterInfo::isVirtualRegister(Reg)) 270 return false; 271 272 assert(RegInfo->getVRegDef(Reg)&&"Machine instr not mapped for this vreg?"); 273 274 // If the loop contains the definition of an operand, then the instruction 275 // isn't loop invariant. 276 if (CurLoop->contains(RegInfo->getVRegDef(Reg)->getParent())) 277 return false; 278 } 279 280 // Don't hoist something that has unmodelled side effects. 281 if (TII->hasUnmodelledSideEffects(&I)) return false; 282 283 // If we got this far, the instruction is loop invariant! 284 return true; 285} 286 287/// Hoist - When an instruction is found to only use loop invariant operands 288/// that is safe to hoist, this instruction is called to do the dirty work. 289/// 290void MachineLICM::Hoist(MachineInstr &MI) { 291 if (!IsLoopInvariantInst(MI)) return; 292 293 std::vector<MachineBasicBlock*> Preds; 294 295 // Non-back-edge predecessors. 296 FindPredecessors(Preds); 297 298 // Either we don't have any predecessors(?!) or we have more than one, which 299 // is forbidden. 300 if (Preds.empty() || Preds.size() != 1) return; 301 302 // Check that the predecessor is qualified to take the hoisted 303 // instruction. I.e., there is only one edge from the predecessor, and it's to 304 // the loop header. 305 MachineBasicBlock *MBB = Preds.front(); 306 307 // FIXME: We are assuming at first that the basic block coming into this loop 308 // has only one successor. This isn't the case in general because we haven't 309 // broken critical edges or added preheaders. 310 if (MBB->succ_size() != 1) return; 311 assert(*MBB->succ_begin() == CurLoop->getHeader() && 312 "The predecessor doesn't feed directly into the loop header!"); 313 314 // Now move the instructions to the predecessor. 315 MoveInstToEndOfBlock(MBB, MI.getParent(), &MI); 316 Changed = true; 317} 318