MachineLICM.cpp revision b43ca19cacd0bee7aa506ac6db7bde56abc67c9f
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 MachineFunctionPass::getAnalysisUsage(AU); 65 AU.setPreservesCFG(); 66 AU.addRequired<MachineLoopInfo>(); 67 AU.addRequired<MachineDominatorTree>(); 68 } 69 private: 70 /// VisitAllLoops - Visit all of the loops in depth first order and try to 71 /// hoist invariant instructions from them. 72 /// 73 void VisitAllLoops(MachineLoop *L) { 74 const std::vector<MachineLoop*> &SubLoops = L->getSubLoops(); 75 76 for (MachineLoop::iterator 77 I = SubLoops.begin(), E = SubLoops.end(); I != E; ++I) { 78 MachineLoop *ML = *I; 79 80 // Traverse the body of the loop in depth first order on the dominator 81 // tree so that we are guaranteed to see definitions before we see uses. 82 VisitAllLoops(ML); 83 HoistRegion(DT->getNode(ML->getHeader())); 84 } 85 86 HoistRegion(DT->getNode(L->getHeader())); 87 } 88 89 /// IsInSubLoop - A little predicate that returns true if the specified 90 /// basic block is in a subloop of the current one, not the current one 91 /// itself. 92 /// 93 bool IsInSubLoop(MachineBasicBlock *BB) { 94 assert(CurLoop->contains(BB) && "Only valid if BB is IN the loop"); 95 return LI->getLoopFor(BB) != CurLoop; 96 } 97 98 /// IsLoopInvariantInst - Returns true if the instruction is loop 99 /// invariant. I.e., all virtual register operands are defined outside of 100 /// the loop, physical registers aren't accessed (explicitly or implicitly), 101 /// and the instruction is hoistable. 102 /// 103 bool IsLoopInvariantInst(MachineInstr &I); 104 105 /// FindPredecessors - Get all of the predecessors of the loop that are not 106 /// back-edges. 107 /// 108 void FindPredecessors(std::vector<MachineBasicBlock*> &Preds) { 109 const MachineBasicBlock *Header = CurLoop->getHeader(); 110 111 for (MachineBasicBlock::const_pred_iterator 112 I = Header->pred_begin(), E = Header->pred_end(); I != E; ++I) 113 if (!CurLoop->contains(*I)) 114 Preds.push_back(*I); 115 } 116 117 /// MoveInstToEndOfBlock - Moves the machine instruction to the bottom of 118 /// the predecessor basic block (but before the terminator instructions). 119 /// 120 void MoveInstToEndOfBlock(MachineBasicBlock *ToMBB, 121 MachineBasicBlock *FromMBB, 122 MachineInstr *MI) { 123 DEBUG({ 124 DOUT << "Hoisting " << *MI; 125 if (ToMBB->getBasicBlock()) 126 DOUT << " to MachineBasicBlock " 127 << ToMBB->getBasicBlock()->getName(); 128 DOUT << "\n"; 129 }); 130 131 MachineBasicBlock::iterator WhereIter = ToMBB->getFirstTerminator(); 132 MachineBasicBlock::iterator To, From = FromMBB->begin(); 133 134 while (&*From != MI) 135 ++From; 136 137 assert(From != FromMBB->end() && "Didn't find instr in BB!"); 138 139 To = From; 140 ToMBB->splice(WhereIter, FromMBB, From, ++To); 141 ++NumHoisted; 142 } 143 144 /// HoistRegion - Walk the specified region of the CFG (defined by all 145 /// blocks dominated by the specified block, and that are in the current 146 /// loop) in depth first order w.r.t the DominatorTree. This allows us to 147 /// visit definitions before uses, allowing us to hoist a loop body in one 148 /// pass without iteration. 149 /// 150 void HoistRegion(MachineDomTreeNode *N); 151 152 /// Hoist - When an instruction is found to only use loop invariant operands 153 /// that is safe to hoist, this instruction is called to do the dirty work. 154 /// 155 void Hoist(MachineInstr &MI); 156 }; 157 158 char MachineLICM::ID = 0; 159 RegisterPass<MachineLICM> X("machine-licm", 160 "Machine Loop Invariant Code Motion"); 161} // end anonymous namespace 162 163FunctionPass *llvm::createMachineLICMPass() { return new MachineLICM(); } 164 165/// Hoist expressions out of the specified loop. Note, alias info for inner loop 166/// is not preserved so it is not a good idea to run LICM multiple times on one 167/// loop. 168/// 169bool MachineLICM::runOnMachineFunction(MachineFunction &MF) { 170 if (!PerformLICM) return false; // For debugging. 171 172 DOUT << "******** Machine LICM ********\n"; 173 174 Changed = false; 175 CurMF = &MF; 176 TM = &CurMF->getTarget(); 177 TII = TM->getInstrInfo(); 178 RegInfo = &CurMF->getRegInfo(); 179 180 // Get our Loop information... 181 LI = &getAnalysis<MachineLoopInfo>(); 182 DT = &getAnalysis<MachineDominatorTree>(); 183 184 for (MachineLoopInfo::iterator 185 I = LI->begin(), E = LI->end(); I != E; ++I) { 186 CurLoop = *I; 187 188 // Visit all of the instructions of the loop. We want to visit the subloops 189 // first, though, so that we can hoist their invariants first into their 190 // containing loop before we process that loop. 191 VisitAllLoops(CurLoop); 192 } 193 194 return Changed; 195} 196 197/// HoistRegion - Walk the specified region of the CFG (defined by all blocks 198/// dominated by the specified block, and that are in the current loop) in depth 199/// first order w.r.t the DominatorTree. This allows us to visit definitions 200/// before uses, allowing us to hoist a loop body in one pass without iteration. 201/// 202void MachineLICM::HoistRegion(MachineDomTreeNode *N) { 203 assert(N != 0 && "Null dominator tree node?"); 204 MachineBasicBlock *BB = N->getBlock(); 205 206 // If this subregion is not in the top level loop at all, exit. 207 if (!CurLoop->contains(BB)) return; 208 209 // Only need to process the contents of this block if it is not part of a 210 // subloop (which would already have been processed). 211 if (!IsInSubLoop(BB)) 212 for (MachineBasicBlock::iterator 213 I = BB->begin(), E = BB->end(); I != E; ) { 214 MachineInstr &MI = *I++; 215 216 // Try hoisting the instruction out of the loop. We can only do this if 217 // all of the operands of the instruction are loop invariant and if it is 218 // safe to hoist the instruction. 219 Hoist(MI); 220 } 221 222 const std::vector<MachineDomTreeNode*> &Children = N->getChildren(); 223 224 for (unsigned I = 0, E = Children.size(); I != E; ++I) 225 HoistRegion(Children[I]); 226} 227 228/// IsLoopInvariantInst - Returns true if the instruction is loop 229/// invariant. I.e., all virtual register operands are defined outside of the 230/// loop, physical registers aren't accessed explicitly, and there are no side 231/// effects that aren't captured by the operands or other flags. 232/// 233bool MachineLICM::IsLoopInvariantInst(MachineInstr &I) { 234 DEBUG({ 235 DOUT << "--- Checking if we can hoist " << I; 236 if (I.getInstrDescriptor()->ImplicitUses) { 237 DOUT << " * Instruction has implicit uses:\n"; 238 239 const MRegisterInfo *MRI = TM->getRegisterInfo(); 240 const unsigned *ImpUses = I.getInstrDescriptor()->ImplicitUses; 241 242 for (; *ImpUses; ++ImpUses) 243 DOUT << " -> " << MRI->getName(*ImpUses) << "\n"; 244 } 245 246 if (I.getInstrDescriptor()->ImplicitDefs) { 247 DOUT << " * Instruction has implicit defines:\n"; 248 249 const MRegisterInfo *MRI = TM->getRegisterInfo(); 250 const unsigned *ImpDefs = I.getInstrDescriptor()->ImplicitDefs; 251 252 for (; *ImpDefs; ++ImpDefs) 253 DOUT << " -> " << MRI->getName(*ImpDefs) << "\n"; 254 } 255 256 if (TII->hasUnmodelledSideEffects(&I)) 257 DOUT << " * Instruction has side effects.\n"; 258 }); 259 260 // The instruction is loop invariant if all of its operands are loop-invariant 261 for (unsigned i = 0, e = I.getNumOperands(); i != e; ++i) { 262 const MachineOperand &MO = I.getOperand(i); 263 264 if (!(MO.isRegister() && MO.getReg() && MO.isUse())) 265 continue; 266 267 unsigned Reg = MO.getReg(); 268 269 // Don't hoist instructions that access physical registers. 270 if (!MRegisterInfo::isVirtualRegister(Reg)) 271 return false; 272 273 assert(RegInfo->getVRegDef(Reg)&&"Machine instr not mapped for this vreg?"); 274 275 // If the loop contains the definition of an operand, then the instruction 276 // isn't loop invariant. 277 if (CurLoop->contains(RegInfo->getVRegDef(Reg)->getParent())) 278 return false; 279 } 280 281 // Don't hoist something that has unmodelled side effects. 282 if (TII->hasUnmodelledSideEffects(&I)) return false; 283 284 // If we got this far, the instruction is loop invariant! 285 return true; 286} 287 288/// Hoist - When an instruction is found to only use loop invariant operands 289/// that is safe to hoist, this instruction is called to do the dirty work. 290/// 291void MachineLICM::Hoist(MachineInstr &MI) { 292 if (!IsLoopInvariantInst(MI)) return; 293 294 std::vector<MachineBasicBlock*> Preds; 295 296 // Non-back-edge predecessors. 297 FindPredecessors(Preds); 298 299 // Either we don't have any predecessors(?!) or we have more than one, which 300 // is forbidden. 301 if (Preds.empty() || Preds.size() != 1) return; 302 303 // Check that the predecessor is qualified to take the hoisted 304 // instruction. I.e., there is only one edge from the predecessor, and it's to 305 // the loop header. 306 MachineBasicBlock *MBB = Preds.front(); 307 308 // FIXME: We are assuming at first that the basic block coming into this loop 309 // has only one successor. This isn't the case in general because we haven't 310 // broken critical edges or added preheaders. 311 if (MBB->succ_size() != 1) return; 312 assert(*MBB->succ_begin() == CurLoop->getHeader() && 313 "The predecessor doesn't feed directly into the loop header!"); 314 315 // Now move the instructions to the predecessor. 316 MoveInstToEndOfBlock(MBB, MI.getParent(), &MI); 317 Changed = true; 318} 319