MachineSink.cpp revision 5dedd5c716f4c2741afd8ac264d5e4e09adbcf1b
1//===-- MachineSink.cpp - Sinking for machine instructions ----------------===// 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 moves instructions into successor blocks, when possible, so that 11// they aren't executed on paths where their results aren't needed. 12// 13// This pass is not intended to be a replacement or a complete alternative 14// for an LLVM-IR-level sinking pass. It is only designed to sink simple 15// constructs that are not exposed before lowering and instruction selection. 16// 17//===----------------------------------------------------------------------===// 18 19#define DEBUG_TYPE "machine-sink" 20#include "llvm/CodeGen/Passes.h" 21#include "llvm/CodeGen/MachineRegisterInfo.h" 22#include "llvm/CodeGen/MachineDominators.h" 23#include "llvm/Target/TargetRegisterInfo.h" 24#include "llvm/Target/TargetInstrInfo.h" 25#include "llvm/Target/TargetMachine.h" 26#include "llvm/ADT/Statistic.h" 27#include "llvm/Support/Compiler.h" 28#include "llvm/Support/Debug.h" 29#include "llvm/Support/raw_ostream.h" 30using namespace llvm; 31 32STATISTIC(NumSunk, "Number of machine instructions sunk"); 33 34namespace { 35 class VISIBILITY_HIDDEN MachineSinking : public MachineFunctionPass { 36 const TargetMachine *TM; 37 const TargetInstrInfo *TII; 38 const TargetRegisterInfo *TRI; 39 MachineFunction *CurMF; // Current MachineFunction 40 MachineRegisterInfo *RegInfo; // Machine register information 41 MachineDominatorTree *DT; // Machine dominator tree 42 43 public: 44 static char ID; // Pass identification 45 MachineSinking() : MachineFunctionPass(&ID) {} 46 47 virtual bool runOnMachineFunction(MachineFunction &MF); 48 49 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 50 AU.setPreservesCFG(); 51 MachineFunctionPass::getAnalysisUsage(AU); 52 AU.addRequired<MachineDominatorTree>(); 53 AU.addPreserved<MachineDominatorTree>(); 54 } 55 private: 56 bool ProcessBlock(MachineBasicBlock &MBB); 57 bool SinkInstruction(MachineInstr *MI, bool &SawStore); 58 bool AllUsesDominatedByBlock(unsigned Reg, MachineBasicBlock *MBB) const; 59 }; 60} // end anonymous namespace 61 62char MachineSinking::ID = 0; 63static RegisterPass<MachineSinking> 64X("machine-sink", "Machine code sinking"); 65 66FunctionPass *llvm::createMachineSinkingPass() { return new MachineSinking(); } 67 68/// AllUsesDominatedByBlock - Return true if all uses of the specified register 69/// occur in blocks dominated by the specified block. 70bool MachineSinking::AllUsesDominatedByBlock(unsigned Reg, 71 MachineBasicBlock *MBB) const { 72 assert(TargetRegisterInfo::isVirtualRegister(Reg) && 73 "Only makes sense for vregs"); 74 for (MachineRegisterInfo::use_iterator I = RegInfo->use_begin(Reg), 75 E = RegInfo->use_end(); I != E; ++I) { 76 // Determine the block of the use. 77 MachineInstr *UseInst = &*I; 78 MachineBasicBlock *UseBlock = UseInst->getParent(); 79 if (UseInst->getOpcode() == TargetInstrInfo::PHI) { 80 // PHI nodes use the operand in the predecessor block, not the block with 81 // the PHI. 82 UseBlock = UseInst->getOperand(I.getOperandNo()+1).getMBB(); 83 } 84 // Check that it dominates. 85 if (!DT->dominates(MBB, UseBlock)) 86 return false; 87 } 88 return true; 89} 90 91 92 93bool MachineSinking::runOnMachineFunction(MachineFunction &MF) { 94 DEBUG(errs() << "******** Machine Sinking ********\n"); 95 96 CurMF = &MF; 97 TM = &CurMF->getTarget(); 98 TII = TM->getInstrInfo(); 99 TRI = TM->getRegisterInfo(); 100 RegInfo = &CurMF->getRegInfo(); 101 DT = &getAnalysis<MachineDominatorTree>(); 102 103 bool EverMadeChange = false; 104 105 while (1) { 106 bool MadeChange = false; 107 108 // Process all basic blocks. 109 for (MachineFunction::iterator I = CurMF->begin(), E = CurMF->end(); 110 I != E; ++I) 111 MadeChange |= ProcessBlock(*I); 112 113 // If this iteration over the code changed anything, keep iterating. 114 if (!MadeChange) break; 115 EverMadeChange = true; 116 } 117 return EverMadeChange; 118} 119 120bool MachineSinking::ProcessBlock(MachineBasicBlock &MBB) { 121 // Can't sink anything out of a block that has less than two successors. 122 if (MBB.succ_size() <= 1 || MBB.empty()) return false; 123 124 bool MadeChange = false; 125 126 // Walk the basic block bottom-up. Remember if we saw a store. 127 MachineBasicBlock::iterator I = MBB.end(); 128 --I; 129 bool ProcessedBegin, SawStore = false; 130 do { 131 MachineInstr *MI = I; // The instruction to sink. 132 133 // Predecrement I (if it's not begin) so that it isn't invalidated by 134 // sinking. 135 ProcessedBegin = I == MBB.begin(); 136 if (!ProcessedBegin) 137 --I; 138 139 if (SinkInstruction(MI, SawStore)) 140 ++NumSunk, MadeChange = true; 141 142 // If we just processed the first instruction in the block, we're done. 143 } while (!ProcessedBegin); 144 145 return MadeChange; 146} 147 148/// SinkInstruction - Determine whether it is safe to sink the specified machine 149/// instruction out of its current block into a successor. 150bool MachineSinking::SinkInstruction(MachineInstr *MI, bool &SawStore) { 151 // Check if it's safe to move the instruction. 152 if (!MI->isSafeToMove(TII, SawStore)) 153 return false; 154 155 // FIXME: This should include support for sinking instructions within the 156 // block they are currently in to shorten the live ranges. We often get 157 // instructions sunk into the top of a large block, but it would be better to 158 // also sink them down before their first use in the block. This xform has to 159 // be careful not to *increase* register pressure though, e.g. sinking 160 // "x = y + z" down if it kills y and z would increase the live ranges of y 161 // and z and only shrink the live range of x. 162 163 // Loop over all the operands of the specified instruction. If there is 164 // anything we can't handle, bail out. 165 MachineBasicBlock *ParentBlock = MI->getParent(); 166 167 // SuccToSinkTo - This is the successor to sink this instruction to, once we 168 // decide. 169 MachineBasicBlock *SuccToSinkTo = 0; 170 171 for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) { 172 const MachineOperand &MO = MI->getOperand(i); 173 if (!MO.isReg()) continue; // Ignore non-register operands. 174 175 unsigned Reg = MO.getReg(); 176 if (Reg == 0) continue; 177 178 if (TargetRegisterInfo::isPhysicalRegister(Reg)) { 179 // If this is a physical register use, we can't move it. If it is a def, 180 // we can move it, but only if the def is dead. 181 if (MO.isUse()) { 182 // If the physreg has no defs anywhere, it's just an ambient register 183 // and we can freely move its uses. 184 if (!RegInfo->def_empty(Reg)) 185 return false; 186 // Check for a def among the register's aliases too. 187 for (const unsigned *Alias = TRI->getAliasSet(Reg); *Alias; ++Alias) 188 if (!RegInfo->def_empty(*Alias)) 189 return false; 190 } else if (!MO.isDead()) { 191 // A def that isn't dead. We can't move it. 192 return false; 193 } 194 } else { 195 // Virtual register uses are always safe to sink. 196 if (MO.isUse()) continue; 197 198 // If it's not safe to move defs of the register class, then abort. 199 if (!TII->isSafeToMoveRegClassDefs(RegInfo->getRegClass(Reg))) 200 return false; 201 202 // FIXME: This picks a successor to sink into based on having one 203 // successor that dominates all the uses. However, there are cases where 204 // sinking can happen but where the sink point isn't a successor. For 205 // example: 206 // x = computation 207 // if () {} else {} 208 // use x 209 // the instruction could be sunk over the whole diamond for the 210 // if/then/else (or loop, etc), allowing it to be sunk into other blocks 211 // after that. 212 213 // Virtual register defs can only be sunk if all their uses are in blocks 214 // dominated by one of the successors. 215 if (SuccToSinkTo) { 216 // If a previous operand picked a block to sink to, then this operand 217 // must be sinkable to the same block. 218 if (!AllUsesDominatedByBlock(Reg, SuccToSinkTo)) 219 return false; 220 continue; 221 } 222 223 // Otherwise, we should look at all the successors and decide which one 224 // we should sink to. 225 for (MachineBasicBlock::succ_iterator SI = ParentBlock->succ_begin(), 226 E = ParentBlock->succ_end(); SI != E; ++SI) { 227 if (AllUsesDominatedByBlock(Reg, *SI)) { 228 SuccToSinkTo = *SI; 229 break; 230 } 231 } 232 233 // If we couldn't find a block to sink to, ignore this instruction. 234 if (SuccToSinkTo == 0) 235 return false; 236 } 237 } 238 239 // If there are no outputs, it must have side-effects. 240 if (SuccToSinkTo == 0) 241 return false; 242 243 // It's not safe to sink instructions to EH landing pad. Control flow into 244 // landing pad is implicitly defined. 245 if (SuccToSinkTo->isLandingPad()) 246 return false; 247 248 // If is not possible to sink an instruction into its own block. This can 249 // happen with loops. 250 if (MI->getParent() == SuccToSinkTo) 251 return false; 252 253 DEBUG(errs() << "Sink instr " << *MI); 254 DEBUG(errs() << "to block " << *SuccToSinkTo); 255 256 // If the block has multiple predecessors, this would introduce computation on 257 // a path that it doesn't already exist. We could split the critical edge, 258 // but for now we just punt. 259 // FIXME: Split critical edges if not backedges. 260 if (SuccToSinkTo->pred_size() > 1) { 261 DEBUG(errs() << " *** PUNTING: Critical edge found\n"); 262 return false; 263 } 264 265 // Determine where to insert into. Skip phi nodes. 266 MachineBasicBlock::iterator InsertPos = SuccToSinkTo->begin(); 267 while (InsertPos != SuccToSinkTo->end() && 268 InsertPos->getOpcode() == TargetInstrInfo::PHI) 269 ++InsertPos; 270 271 // Move the instruction. 272 SuccToSinkTo->splice(InsertPos, ParentBlock, MI, 273 ++MachineBasicBlock::iterator(MI)); 274 return true; 275} 276