lib/CodeGen/MachineSink.cpp

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