C++ SelectionDAGBuilder::getFrameIndexTy方法代码示例

/// Lower a single value incoming to a statepoint node.  This value can be
/// either a deopt value or a gc value, the handling is the same.  We special
/// case constants and allocas, then fall back to spilling if required.
static void lowerIncomingStatepointValue(SDValue Incoming, bool LiveInOnly,
                                         SmallVectorImpl<SDValue> &Ops,
                                         SelectionDAGBuilder &Builder) {
  SDValue Chain = Builder.getRoot();

  if (ConstantSDNode *C = dyn_cast<ConstantSDNode>(Incoming)) {
    // If the original value was a constant, make sure it gets recorded as
    // such in the stackmap.  This is required so that the consumer can
    // parse any internal format to the deopt state.  It also handles null
    // pointers and other constant pointers in GC states.  Note the constant
    // vectors do not appear to actually hit this path and that anything larger
    // than an i64 value (not type!) will fail asserts here.
    pushStackMapConstant(Ops, Builder, C->getSExtValue());
  } else if (FrameIndexSDNode *FI = dyn_cast<FrameIndexSDNode>(Incoming)) {
    // This handles allocas as arguments to the statepoint (this is only
    // really meaningful for a deopt value.  For GC, we'd be trying to
    // relocate the address of the alloca itself?)
    assert(Incoming.getValueType() == Builder.getFrameIndexTy() &&
           "Incoming value is a frame index!");
    Ops.push_back(Builder.DAG.getTargetFrameIndex(FI->getIndex(),
                                                  Builder.getFrameIndexTy()));
  } else if (LiveInOnly) {
    // If this value is live in (not live-on-return, or live-through), we can
    // treat it the same way patchpoint treats it's "live in" values.  We'll 
    // end up folding some of these into stack references, but they'll be 
    // handled by the register allocator.  Note that we do not have the notion
    // of a late use so these values might be placed in registers which are 
    // clobbered by the call.  This is fine for live-in.
    Ops.push_back(Incoming);
  } else {
    // Otherwise, locate a spill slot and explicitly spill it so it
    // can be found by the runtime later.  We currently do not support
    // tracking values through callee saved registers to their eventual
    // spill location.  This would be a useful optimization, but would
    // need to be optional since it requires a lot of complexity on the
    // runtime side which not all would support.
    auto Res = spillIncomingStatepointValue(Incoming, Chain, Builder);
    Ops.push_back(Res.first);
    Chain = Res.second;
  }

  Builder.DAG.setRoot(Chain);
}

/// Try to find existing copies of the incoming values in stack slots used for
/// statepoint spilling.  If we can find a spill slot for the incoming value,
/// mark that slot as allocated, and reuse the same slot for this safepoint.
/// This helps to avoid series of loads and stores that only serve to reshuffle
/// values on the stack between calls.
static void reservePreviousStackSlotForValue(const Value *IncomingValue,
                                             SelectionDAGBuilder &Builder) {

  SDValue Incoming = Builder.getValue(IncomingValue);

  if (isa<ConstantSDNode>(Incoming) || isa<FrameIndexSDNode>(Incoming)) {
    // We won't need to spill this, so no need to check for previously
    // allocated stack slots
    return;
  }

  SDValue OldLocation = Builder.StatepointLowering.getLocation(Incoming);
  if (OldLocation.getNode())
    // Duplicates in input
    return;

  const int LookUpDepth = 6;
  Optional<int> Index =
      findPreviousSpillSlot(IncomingValue, Builder, LookUpDepth);
  if (!Index.hasValue())
    return;

  const auto &StatepointSlots = Builder.FuncInfo.StatepointStackSlots;

  auto SlotIt = find(StatepointSlots, *Index);
  assert(SlotIt != StatepointSlots.end() &&
         "Value spilled to the unknown stack slot");

  // This is one of our dedicated lowering slots
  const int Offset = std::distance(StatepointSlots.begin(), SlotIt);
  if (Builder.StatepointLowering.isStackSlotAllocated(Offset)) {
    // stack slot already assigned to someone else, can't use it!
    // TODO: currently we reserve space for gc arguments after doing
    // normal allocation for deopt arguments.  We should reserve for
    // _all_ deopt and gc arguments, then start allocating.  This
    // will prevent some moves being inserted when vm state changes,
    // but gc state doesn't between two calls.
    return;
  }
  // Reserve this stack slot
  Builder.StatepointLowering.reserveStackSlot(Offset);

  // Cache this slot so we find it when going through the normal
  // assignment loop.
  SDValue Loc =
      Builder.DAG.getTargetFrameIndex(*Index, Builder.getFrameIndexTy());
  Builder.StatepointLowering.setLocation(Incoming, Loc);
}

/// Spill a value incoming to the statepoint. It might be either part of
/// vmstate
/// or gcstate. In both cases unconditionally spill it on the stack unless it
/// is a null constant. Return pair with first element being frame index
/// containing saved value and second element with outgoing chain from the
/// emitted store
static std::pair<SDValue, SDValue>
spillIncomingStatepointValue(SDValue Incoming, SDValue Chain,
                             SelectionDAGBuilder &Builder) {
  SDValue Loc = Builder.StatepointLowering.getLocation(Incoming);

  // Emit new store if we didn't do it for this ptr before
  if (!Loc.getNode()) {
    Loc = Builder.StatepointLowering.allocateStackSlot(Incoming.getValueType(),
                                                       Builder);
    int Index = cast<FrameIndexSDNode>(Loc)->getIndex();
    // We use TargetFrameIndex so that isel will not select it into LEA
    Loc = Builder.DAG.getTargetFrameIndex(Index, Builder.getFrameIndexTy());

    // TODO: We can create TokenFactor node instead of
    //       chaining stores one after another, this may allow
    //       a bit more optimal scheduling for them

#ifndef NDEBUG
    // Right now we always allocate spill slots that are of the same
    // size as the value we're about to spill (the size of spillee can
    // vary since we spill vectors of pointers too).  At some point we
    // can consider allowing spills of smaller values to larger slots
    // (i.e. change the '==' in the assert below to a '>=').
    MachineFrameInfo &MFI = Builder.DAG.getMachineFunction().getFrameInfo();
    assert((MFI.getObjectSize(Index) * 8) == Incoming.getValueSizeInBits() &&
           "Bad spill:  stack slot does not match!");
#endif

    Chain = Builder.DAG.getStore(Chain, Builder.getCurSDLoc(), Incoming, Loc,
                                 MachinePointerInfo::getFixedStack(
                                     Builder.DAG.getMachineFunction(), Index));

    Builder.StatepointLowering.setLocation(Incoming, Loc);
  }

  assert(Loc.getNode());
  return std::make_pair(Loc, Chain);
}

//.........这里部分代码省略.........
  // potentially allow live-through values in callee saved registers.
  const bool LiveInDeopt =
    SI.StatepointFlags & (uint64_t)StatepointFlags::DeoptLiveIn;

  auto isGCValue =[&](const Value *V) {
    return is_contained(SI.Ptrs, V) || is_contained(SI.Bases, V);
  };
  
  // Before we actually start lowering (and allocating spill slots for values),
  // reserve any stack slots which we judge to be profitable to reuse for a
  // particular value.  This is purely an optimization over the code below and
  // doesn't change semantics at all.  It is important for performance that we
  // reserve slots for both deopt and gc values before lowering either.
  for (const Value *V : SI.DeoptState) {
    if (!LiveInDeopt || isGCValue(V))
      reservePreviousStackSlotForValue(V, Builder);
  }
  for (unsigned i = 0; i < SI.Bases.size(); ++i) {
    reservePreviousStackSlotForValue(SI.Bases[i], Builder);
    reservePreviousStackSlotForValue(SI.Ptrs[i], Builder);
  }

  // First, prefix the list with the number of unique values to be
  // lowered.  Note that this is the number of *Values* not the
  // number of SDValues required to lower them.
  const int NumVMSArgs = SI.DeoptState.size();
  pushStackMapConstant(Ops, Builder, NumVMSArgs);

  // The vm state arguments are lowered in an opaque manner.  We do not know
  // what type of values are contained within.
  for (const Value *V : SI.DeoptState) {
    SDValue Incoming = Builder.getValue(V);
    const bool LiveInValue = LiveInDeopt && !isGCValue(V);
    lowerIncomingStatepointValue(Incoming, LiveInValue, Ops, Builder);
  }

  // Finally, go ahead and lower all the gc arguments.  There's no prefixed
  // length for this one.  After lowering, we'll have the base and pointer
  // arrays interwoven with each (lowered) base pointer immediately followed by
  // it's (lowered) derived pointer.  i.e
  // (base[0], ptr[0], base[1], ptr[1], ...)
  for (unsigned i = 0; i < SI.Bases.size(); ++i) {
    const Value *Base = SI.Bases[i];
    lowerIncomingStatepointValue(Builder.getValue(Base), /*LiveInOnly*/ false,
                                 Ops, Builder);

    const Value *Ptr = SI.Ptrs[i];
    lowerIncomingStatepointValue(Builder.getValue(Ptr), /*LiveInOnly*/ false,
                                 Ops, Builder);
  }

  // If there are any explicit spill slots passed to the statepoint, record
  // them, but otherwise do not do anything special.  These are user provided
  // allocas and give control over placement to the consumer.  In this case,
  // it is the contents of the slot which may get updated, not the pointer to
  // the alloca
  for (Value *V : SI.GCArgs) {
    SDValue Incoming = Builder.getValue(V);
    if (FrameIndexSDNode *FI = dyn_cast<FrameIndexSDNode>(Incoming)) {
      // This handles allocas as arguments to the statepoint
      assert(Incoming.getValueType() == Builder.getFrameIndexTy() &&
             "Incoming value is a frame index!");
      Ops.push_back(Builder.DAG.getTargetFrameIndex(FI->getIndex(),
                                                    Builder.getFrameIndexTy()));
    }
  }

  // Record computed locations for all lowered values.
  // This can not be embedded in lowering loops as we need to record *all*
  // values, while previous loops account only values with unique SDValues.
  const Instruction *StatepointInstr = SI.StatepointInstr;
  auto &SpillMap = Builder.FuncInfo.StatepointSpillMaps[StatepointInstr];

  for (const GCRelocateInst *Relocate : SI.GCRelocates) {
    const Value *V = Relocate->getDerivedPtr();
    SDValue SDV = Builder.getValue(V);
    SDValue Loc = Builder.StatepointLowering.getLocation(SDV);

    if (Loc.getNode()) {
      SpillMap.SlotMap[V] = cast<FrameIndexSDNode>(Loc)->getIndex();
    } else {
      // Record value as visited, but not spilled. This is case for allocas
      // and constants. For this values we can avoid emitting spill load while
      // visiting corresponding gc_relocate.
      // Actually we do not need to record them in this map at all.
      // We do this only to check that we are not relocating any unvisited
      // value.
      SpillMap.SlotMap[V] = None;

      // Default llvm mechanisms for exporting values which are used in
      // different basic blocks does not work for gc relocates.
      // Note that it would be incorrect to teach llvm that all relocates are
      // uses of the corresponding values so that it would automatically
      // export them. Relocates of the spilled values does not use original
      // value.
      if (Relocate->getParent() != StatepointInstr->getParent())
        Builder.ExportFromCurrentBlock(V);
    }
  }
}