C++ LiveVariables::isLiveOut方法代码示例

bool PHIElimination::SplitPHIEdges(MachineFunction &MF,
                                   MachineBasicBlock &MBB,
                                   LiveVariables &LV,
                                   MachineLoopInfo *MLI) {
  if (MBB.empty() || !MBB.front().isPHI() || MBB.isLandingPad())
    return false;   // Quick exit for basic blocks without PHIs.

  bool Changed = false;
  for (MachineBasicBlock::const_iterator BBI = MBB.begin(), BBE = MBB.end();
       BBI != BBE && BBI->isPHI(); ++BBI) {
    for (unsigned i = 1, e = BBI->getNumOperands(); i != e; i += 2) {
      unsigned Reg = BBI->getOperand(i).getReg();
      MachineBasicBlock *PreMBB = BBI->getOperand(i+1).getMBB();
      // We break edges when registers are live out from the predecessor block
      // (not considering PHI nodes). If the register is live in to this block
      // anyway, we would gain nothing from splitting.
      // Avoid splitting backedges of loops. It would introduce small
      // out-of-line blocks into the loop which is very bad for code placement.
      if (PreMBB != &MBB &&
          !LV.isLiveIn(Reg, MBB) && LV.isLiveOut(Reg, *PreMBB)) {
        if (!MLI ||
            !(MLI->getLoopFor(PreMBB) == MLI->getLoopFor(&MBB) &&
              MLI->isLoopHeader(&MBB))) {
          if (PreMBB->SplitCriticalEdge(&MBB, this)) {
            Changed = true;
            ++NumCriticalEdgesSplit;
          }
        }
      }
    }
  }
  return Changed;
}

bool llvm::PHIElimination::SplitPHIEdges(MachineFunction &MF,
                                         MachineBasicBlock &MBB,
                                         LiveVariables &LV) {
  if (MBB.empty() || !MBB.front().isPHI() || MBB.isLandingPad())
    return false;   // Quick exit for basic blocks without PHIs.

  for (MachineBasicBlock::const_iterator BBI = MBB.begin(), BBE = MBB.end();
       BBI != BBE && BBI->isPHI(); ++BBI) {
    for (unsigned i = 1, e = BBI->getNumOperands(); i != e; i += 2) {
      unsigned Reg = BBI->getOperand(i).getReg();
      MachineBasicBlock *PreMBB = BBI->getOperand(i+1).getMBB();
      // We break edges when registers are live out from the predecessor block
      // (not considering PHI nodes). If the register is live in to this block
      // anyway, we would gain nothing from splitting.
      if (!LV.isLiveIn(Reg, MBB) && LV.isLiveOut(Reg, *PreMBB))
        SplitCriticalEdge(PreMBB, &MBB);
    }
  }
  return true;
}

//.........这里部分代码省略.........
           "Machine PHI Operands must all be virtual registers!");

    // Get the MachineBasicBlock equivalent of the BasicBlock that is the source
    // path the PHI.
    MachineBasicBlock &opBlock = *MPhi->getOperand(i*2+2).getMBB();

    // If source is defined by an implicit def, there is no need to insert a
    // copy.
    MachineInstr *DefMI = MRI->getVRegDef(SrcReg);
    if (DefMI->isImplicitDef()) {
      ImpDefs.insert(DefMI);
      continue;
    }

    // Check to make sure we haven't already emitted the copy for this block.
    // This can happen because PHI nodes may have multiple entries for the same
    // basic block.
    if (!MBBsInsertedInto.insert(&opBlock))
      continue;  // If the copy has already been emitted, we're done.

    // Find a safe location to insert the copy, this may be the first terminator
    // in the block (or end()).
    MachineBasicBlock::iterator InsertPos =
      findPHICopyInsertPoint(&opBlock, &MBB, SrcReg);

    // Insert the copy.
    if (!reusedIncoming && IncomingReg)
      BuildMI(opBlock, InsertPos, MPhi->getDebugLoc(),
              TII->get(TargetOpcode::COPY), IncomingReg).addReg(SrcReg, 0, SrcSubReg);

    // Now update live variable information if we have it.  Otherwise we're done
    if (!LV) continue;

    // We want to be able to insert a kill of the register if this PHI (aka, the
    // copy we just inserted) is the last use of the source value.  Live
    // variable analysis conservatively handles this by saying that the value is
    // live until the end of the block the PHI entry lives in.  If the value
    // really is dead at the PHI copy, there will be no successor blocks which
    // have the value live-in.

    // Also check to see if this register is in use by another PHI node which
    // has not yet been eliminated.  If so, it will be killed at an appropriate
    // point later.

    // Is it used by any PHI instructions in this block?
    bool ValueIsUsed = VRegPHIUseCount[BBVRegPair(opBlock.getNumber(), SrcReg)];

    // Okay, if we now know that the value is not live out of the block, we can
    // add a kill marker in this block saying that it kills the incoming value!
    if (!ValueIsUsed && !LV->isLiveOut(SrcReg, opBlock)) {
      // In our final twist, we have to decide which instruction kills the
      // register.  In most cases this is the copy, however, the first
      // terminator instruction at the end of the block may also use the value.
      // In this case, we should mark *it* as being the killing block, not the
      // copy.
      MachineBasicBlock::iterator KillInst;
      MachineBasicBlock::iterator Term = opBlock.getFirstTerminator();
      if (Term != opBlock.end() && Term->readsRegister(SrcReg)) {
        KillInst = Term;

        // Check that no other terminators use values.
#ifndef NDEBUG
        for (MachineBasicBlock::iterator TI = llvm::next(Term);
             TI != opBlock.end(); ++TI) {
          if (TI->isDebugValue())
            continue;
          assert(!TI->readsRegister(SrcReg) &&
                 "Terminator instructions cannot use virtual registers unless"
                 "they are the first terminator in a block!");
        }
#endif
      } else if (reusedIncoming || !IncomingReg) {
        // We may have to rewind a bit if we didn't insert a copy this time.
        KillInst = Term;
        while (KillInst != opBlock.begin()) {
          --KillInst;
          if (KillInst->isDebugValue())
            continue;
          if (KillInst->readsRegister(SrcReg))
            break;
        }
      } else {
        // We just inserted this copy.
        KillInst = prior(InsertPos);
      }
      assert(KillInst->readsRegister(SrcReg) && "Cannot find kill instruction");

      // Finally, mark it killed.
      LV->addVirtualRegisterKilled(SrcReg, KillInst);

      // This vreg no longer lives all of the way through opBlock.
      unsigned opBlockNum = opBlock.getNumber();
      LV->getVarInfo(SrcReg).AliveBlocks.reset(opBlockNum);
    }
  }

  // Really delete the PHI instruction now, if it is not in the LoweredPHIs map.
  if (reusedIncoming || !IncomingReg)
    MF.DeleteMachineInstr(MPhi);
}

bool PHIElimination::SplitPHIEdges(MachineFunction &MF,
                                   MachineBasicBlock &MBB,
                                   LiveVariables &LV,
                                   MachineLoopInfo *MLI) {
  if (MBB.empty() || !MBB.front().isPHI() || MBB.isLandingPad())
    return false;   // Quick exit for basic blocks without PHIs.

  const MachineLoop *CurLoop = MLI ? MLI->getLoopFor(&MBB) : 0;
  bool IsLoopHeader = CurLoop && &MBB == CurLoop->getHeader();

  bool Changed = false;
  for (MachineBasicBlock::iterator BBI = MBB.begin(), BBE = MBB.end();
       BBI != BBE && BBI->isPHI(); ++BBI) {
    for (unsigned i = 1, e = BBI->getNumOperands(); i != e; i += 2) {
      unsigned Reg = BBI->getOperand(i).getReg();
      MachineBasicBlock *PreMBB = BBI->getOperand(i+1).getMBB();
      // Is there a critical edge from PreMBB to MBB?
      if (PreMBB->succ_size() == 1)
        continue;

      // Avoid splitting backedges of loops. It would introduce small
      // out-of-line blocks into the loop which is very bad for code placement.
      if (PreMBB == &MBB)
        continue;
      const MachineLoop *PreLoop = MLI ? MLI->getLoopFor(PreMBB) : 0;
      if (IsLoopHeader && PreLoop == CurLoop)
        continue;

      // LV doesn't consider a phi use live-out, so isLiveOut only returns true
      // when the source register is live-out for some other reason than a phi
      // use. That means the copy we will insert in PreMBB won't be a kill, and
      // there is a risk it may not be coalesced away.
      //
      // If the copy would be a kill, there is no need to split the edge.
      if (!LV.isLiveOut(Reg, *PreMBB))
        continue;

      DEBUG(dbgs() << PrintReg(Reg) << " live-out before critical edge BB#"
                   << PreMBB->getNumber() << " -> BB#" << MBB.getNumber()
                   << ": " << *BBI);

      // If Reg is not live-in to MBB, it means it must be live-in to some
      // other PreMBB successor, and we can avoid the interference by splitting
      // the edge.
      //
      // If Reg *is* live-in to MBB, the interference is inevitable and a copy
      // is likely to be left after coalescing. If we are looking at a loop
      // exiting edge, split it so we won't insert code in the loop, otherwise
      // don't bother.
      bool ShouldSplit = !LV.isLiveIn(Reg, MBB);

      // Check for a loop exiting edge.
      if (!ShouldSplit && CurLoop != PreLoop) {
        DEBUG({
          dbgs() << "Split wouldn't help, maybe avoid loop copies?\n";
          if (PreLoop) dbgs() << "PreLoop: " << *PreLoop;
          if (CurLoop) dbgs() << "CurLoop: " << *CurLoop;
        });
        // This edge could be entering a loop, exiting a loop, or it could be
        // both: Jumping directly form one loop to the header of a sibling
        // loop.
        // Split unless this edge is entering CurLoop from an outer loop.
        ShouldSplit = PreLoop && !PreLoop->contains(CurLoop);
      }
      if (!ShouldSplit)
        continue;
      if (!PreMBB->SplitCriticalEdge(&MBB, this)) {
        DEBUG(dbgs() << "Failed to split ciritcal edge.\n");
        continue;
      }
      Changed = true;
      ++NumCriticalEdgesSplit;
    }