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//===- StatepointLowering.cpp - SDAGBuilder's statepoint code -------------===//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
// This file includes support code use by SelectionDAGBuilder when lowering a
// statepoint sequence in SelectionDAG IR.
#include "StatepointLowering.h"
#include "SelectionDAGBuilder.h"
#include "llvm/ADT/ArrayRef.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SetVector.h"
#include "llvm/ADT/SmallBitVector.h"
#include "llvm/ADT/SmallSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/CodeGen/FunctionLoweringInfo.h"
#include "llvm/CodeGen/GCMetadata.h"
#include "llvm/CodeGen/ISDOpcodes.h"
#include "llvm/CodeGen/MachineFrameInfo.h"
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/MachineMemOperand.h"
#include "llvm/CodeGen/RuntimeLibcalls.h"
#include "llvm/CodeGen/SelectionDAG.h"
#include "llvm/CodeGen/SelectionDAGNodes.h"
#include "llvm/CodeGen/StackMaps.h"
#include "llvm/CodeGen/TargetLowering.h"
#include "llvm/CodeGen/TargetOpcodes.h"
#include "llvm/CodeGenTypes/MachineValueType.h"
#include "llvm/IR/CallingConv.h"
#include "llvm/IR/DerivedTypes.h"
#include "llvm/IR/GCStrategy.h"
#include "llvm/IR/Instruction.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/LLVMContext.h"
#include "llvm/IR/Statepoint.h"
#include "llvm/IR/Type.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Target/TargetMachine.h"
#include "llvm/Target/TargetOptions.h"
#include <cassert>
#include <cstddef>
#include <cstdint>
#include <iterator>
#include <tuple>
#include <utility>
using namespace llvm;
#define DEBUG_TYPE "statepoint-lowering"
"Number of stack slots allocated for statepoints");
STATISTIC(NumOfStatepoints, "Number of statepoint nodes encountered");
"Maximum number of stack slots required for a singe statepoint");
static cl::opt<bool> UseRegistersForDeoptValues(
"use-registers-for-deopt-values", cl::Hidden, cl::init(false),
cl::desc("Allow using registers for non pointer deopt args"));
static cl::opt<bool> UseRegistersForGCPointersInLandingPad(
"use-registers-for-gc-values-in-landing-pad", cl::Hidden, cl::init(false),
cl::desc("Allow using registers for gc pointer in landing pad"));
static cl::opt<unsigned> MaxRegistersForGCPointers(
"max-registers-for-gc-values", cl::Hidden, cl::init(0),
cl::desc("Max number of VRegs allowed to pass GC pointer meta args in"));
typedef FunctionLoweringInfo::StatepointRelocationRecord RecordType;
static void pushStackMapConstant(SmallVectorImpl<SDValue>& Ops,
SelectionDAGBuilder &Builder, uint64_t Value) {
SDLoc L = Builder.getCurSDLoc();
Ops.push_back(Builder.DAG.getTargetConstant(StackMaps::ConstantOp, L,
Ops.push_back(Builder.DAG.getTargetConstant(Value, L, MVT::i64));
void StatepointLoweringState::startNewStatepoint(SelectionDAGBuilder &Builder) {
// Consistency check
assert(PendingGCRelocateCalls.empty() &&
"Trying to visit statepoint before finished processing previous one");
NextSlotToAllocate = 0;
// Need to resize this on each safepoint - we need the two to stay in sync and
// the clear patterns of a SelectionDAGBuilder have no relation to
// FunctionLoweringInfo. Also need to ensure used bits get cleared.
void StatepointLoweringState::clear() {
assert(PendingGCRelocateCalls.empty() &&
"cleared before statepoint sequence completed");
StatepointLoweringState::allocateStackSlot(EVT ValueType,
SelectionDAGBuilder &Builder) {
MachineFrameInfo &MFI = Builder.DAG.getMachineFunction().getFrameInfo();
unsigned SpillSize = ValueType.getStoreSize();
assert((SpillSize * 8) ==
(-8u & (7 + ValueType.getSizeInBits())) && // Round up modulo 8.
"Size not in bytes?");
// First look for a previously created stack slot which is not in
// use (accounting for the fact arbitrary slots may already be
// reserved), or to create a new stack slot and use it.
const size_t NumSlots = AllocatedStackSlots.size();
assert(NextSlotToAllocate <= NumSlots && "Broken invariant");
assert(AllocatedStackSlots.size() ==
Builder.FuncInfo.StatepointStackSlots.size() &&
"Broken invariant");
for (; NextSlotToAllocate < NumSlots; NextSlotToAllocate++) {
if (!AllocatedStackSlots.test(NextSlotToAllocate)) {
const int FI = Builder.FuncInfo.StatepointStackSlots[NextSlotToAllocate];
if (MFI.getObjectSize(FI) == SpillSize) {
// TODO: Is ValueType the right thing to use here?
return Builder.DAG.getFrameIndex(FI, ValueType);
// Couldn't find a free slot, so create a new one:
SDValue SpillSlot = Builder.DAG.CreateStackTemporary(ValueType);
const unsigned FI = cast<FrameIndexSDNode>(SpillSlot)->getIndex();
AllocatedStackSlots.resize(AllocatedStackSlots.size()+1, true);
assert(AllocatedStackSlots.size() ==
Builder.FuncInfo.StatepointStackSlots.size() &&
"Broken invariant");
return SpillSlot;
/// Utility function for reservePreviousStackSlotForValue. Tries to find
/// stack slot index to which we have spilled value for previous statepoints.
/// LookUpDepth specifies maximum DFS depth this function is allowed to look.
static std::optional<int> findPreviousSpillSlot(const Value *Val,
SelectionDAGBuilder &Builder,
int LookUpDepth) {
// Can not look any further - give up now
if (LookUpDepth <= 0)
return std::nullopt;
// Spill location is known for gc relocates
if (const auto *Relocate = dyn_cast<GCRelocateInst>(Val)) {
const Value *Statepoint = Relocate->getStatepoint();
assert((isa<GCStatepointInst>(Statepoint) || isa<UndefValue>(Statepoint)) &&
"GetStatepoint must return one of two types");
if (isa<UndefValue>(Statepoint))
return std::nullopt;
const auto &RelocationMap = Builder.FuncInfo.StatepointRelocationMaps
auto It = RelocationMap.find(Relocate);
if (It == RelocationMap.end())
return std::nullopt;
auto &Record = It->second;
if (Record.type != RecordType::Spill)
return std::nullopt;
return Record.payload.FI;
// Look through bitcast instructions.
if (const BitCastInst *Cast = dyn_cast<BitCastInst>(Val))
return findPreviousSpillSlot(Cast->getOperand(0), Builder, LookUpDepth - 1);
// Look through phi nodes
// All incoming values should have same known stack slot, otherwise result
// is unknown.
if (const PHINode *Phi = dyn_cast<PHINode>(Val)) {
std::optional<int> MergedResult;
for (const auto &IncomingValue : Phi->incoming_values()) {
std::optional<int> SpillSlot =
findPreviousSpillSlot(IncomingValue, Builder, LookUpDepth - 1);
if (!SpillSlot)
return std::nullopt;
if (MergedResult && *MergedResult != *SpillSlot)
return std::nullopt;
MergedResult = SpillSlot;
return MergedResult;
// TODO: We can do better for PHI nodes. In cases like this:
// ptr = phi(relocated_pointer, not_relocated_pointer)
// statepoint(ptr)
// We will return that stack slot for ptr is unknown. And later we might
// assign different stack slots for ptr and relocated_pointer. This limits
// llvm's ability to remove redundant stores.
// Unfortunately it's hard to accomplish in current infrastructure.
// We use this function to eliminate spill store completely, while
// in example we still need to emit store, but instead of any location
// we need to use special "preferred" location.
// TODO: handle simple updates. If a value is modified and the original
// value is no longer live, it would be nice to put the modified value in the
// same slot. This allows folding of the memory accesses for some
// instructions types (like an increment).
// statepoint (i)
// i1 = i+1
// statepoint (i1)
// However we need to be careful for cases like this:
// statepoint(i)
// i1 = i+1
// statepoint(i, i1)
// Here we want to reserve spill slot for 'i', but not for 'i+1'. If we just
// put handling of simple modifications in this function like it's done
// for bitcasts we might end up reserving i's slot for 'i+1' because order in
// which we visit values is unspecified.
// Don't know any information about this instruction
return std::nullopt;
/// Return true if-and-only-if the given SDValue can be lowered as either a
/// constant argument or a stack reference. The key point is that the value
/// doesn't need to be spilled or tracked as a vreg use.
static bool willLowerDirectly(SDValue Incoming) {
// We are making an unchecked assumption that the frame size <= 2^16 as that
// is the largest offset which can be encoded in the stackmap format.
if (isa<FrameIndexSDNode>(Incoming))
return true;
// The largest constant describeable in the StackMap format is 64 bits.
// Potential Optimization: Constants values are sign extended by consumer,
// and thus there are many constants of static type > 64 bits whose value
// happens to be sext(Con64) and could thus be lowered directly.
if (Incoming.getValueType().getSizeInBits() > 64)
return false;
return isIntOrFPConstant(Incoming) || Incoming.isUndef();
/// 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 we won't spill this, we don't need to check for previously allocated
// stack slots.
if (willLowerDirectly(Incoming))
SDValue OldLocation = Builder.StatepointLowering.getLocation(Incoming);
if (OldLocation.getNode())
// Duplicates in input
const int LookUpDepth = 6;
std::optional<int> Index =
findPreviousSpillSlot(IncomingValue, Builder, LookUpDepth);
if (!Index)
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.
// Reserve this stack slot
// 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);
/// Extract call from statepoint, lower it and return pointer to the
/// call node. Also update NodeMap so that getValue(statepoint) will
/// reference lowered call result
static std::pair<SDValue, SDNode *> lowerCallFromStatepointLoweringInfo(
SelectionDAGBuilder::StatepointLoweringInfo &SI,
SelectionDAGBuilder &Builder) {
SDValue ReturnValue, CallEndVal;
std::tie(ReturnValue, CallEndVal) =
Builder.lowerInvokable(SI.CLI, SI.EHPadBB);
SDNode *CallEnd = CallEndVal.getNode();
// Get a call instruction from the call sequence chain. Tail calls are not
// allowed. The following code is essentially reverse engineering X86's
// LowerCallTo.
// We are expecting DAG to have the following form:
// ch = eh_label (only in case of invoke statepoint)
// ch, glue = callseq_start ch
// ch, glue = X86::Call ch, glue
// ch, glue = callseq_end ch, glue
// get_return_value ch, glue
// get_return_value can either be a sequence of CopyFromReg instructions
// to grab the return value from the return register(s), or it can be a LOAD
// to load a value returned by reference via a stack slot.
bool HasDef = !SI.CLI.RetTy->isVoidTy();
if (HasDef) {
if (CallEnd->getOpcode() == ISD::LOAD)
CallEnd = CallEnd->getOperand(0).getNode();
while (CallEnd->getOpcode() == ISD::CopyFromReg)
CallEnd = CallEnd->getOperand(0).getNode();
assert(CallEnd->getOpcode() == ISD::CALLSEQ_END && "expected!");
return std::make_pair(ReturnValue, CallEnd->getOperand(0).getNode());
static MachineMemOperand* getMachineMemOperand(MachineFunction &MF,
FrameIndexSDNode &FI) {
auto PtrInfo = MachinePointerInfo::getFixedStack(MF, FI.getIndex());
auto MMOFlags = MachineMemOperand::MOStore |
MachineMemOperand::MOLoad | MachineMemOperand::MOVolatile;
auto &MFI = MF.getFrameInfo();
return MF.getMachineMemOperand(PtrInfo, MMOFlags,
/// 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::tuple<SDValue, SDValue, MachineMemOperand*>
spillIncomingStatepointValue(SDValue Incoming, SDValue Chain,
SelectionDAGBuilder &Builder) {
SDValue Loc = Builder.StatepointLowering.getLocation(Incoming);
MachineMemOperand* MMO = nullptr;
// Emit new store if we didn't do it for this ptr before
if (!Loc.getNode()) {
Loc = Builder.StatepointLowering.allocateStackSlot(Incoming.getValueType(),
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());
// 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) ==
(-8 & (7 + // Round up modulo 8.
(int64_t)Incoming.getValueSizeInBits())) &&
"Bad spill: stack slot does not match!");
// Note: Using the alignment of the spill slot (rather than the abi or
// preferred alignment) is required for correctness when dealing with spill
// slots with preferred alignments larger than frame alignment..
auto &MF = Builder.DAG.getMachineFunction();
auto PtrInfo = MachinePointerInfo::getFixedStack(MF, Index);
auto *StoreMMO = MF.getMachineMemOperand(
PtrInfo, MachineMemOperand::MOStore, MFI.getObjectSize(Index),
Chain = Builder.DAG.getStore(Chain, Builder.getCurSDLoc(), Incoming, Loc,
MMO = getMachineMemOperand(MF, *cast<FrameIndexSDNode>(Loc));
Builder.StatepointLowering.setLocation(Incoming, Loc);
return std::make_tuple(Loc, Chain, MMO);
/// 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 RequireSpillSlot,
SmallVectorImpl<SDValue> &Ops,
SmallVectorImpl<MachineMemOperand *> &MemRefs,
SelectionDAGBuilder &Builder) {
if (willLowerDirectly(Incoming)) {
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!");
auto &MF = Builder.DAG.getMachineFunction();
auto *MMO = getMachineMemOperand(MF, *FI);
assert(Incoming.getValueType().getSizeInBits() <= 64);
if (Incoming.isUndef()) {
// Put an easily recognized constant that's unlikely to be a valid
// value so that uses of undef by the consumer of the stackmap is
// easily recognized. This is legal since the compiler is always
// allowed to chose an arbitrary value for undef.
pushStackMapConstant(Ops, Builder, 0xFEFEFEFE);
// 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.
if (ConstantSDNode *C = dyn_cast<ConstantSDNode>(Incoming)) {
pushStackMapConstant(Ops, Builder, C->getSExtValue());
} else if (ConstantFPSDNode *C = dyn_cast<ConstantFPSDNode>(Incoming)) {
pushStackMapConstant(Ops, Builder,
llvm_unreachable("unhandled direct lowering case");
if (!RequireSpillSlot) {
// 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. For live-through
// fix-up pass should be executed to force spilling of such registers.
} else {
// Otherwise, locate a spill slot and explicitly spill it so it can be
// found by the runtime later. Note: We know all of these spills are
// independent, but don't bother to exploit that chain wise. DAGCombine
// will happily do so as needed, so doing it here would be a small compile
// time win at most.
SDValue Chain = Builder.getRoot();
auto Res = spillIncomingStatepointValue(Incoming, Chain, Builder);
if (auto *MMO = std::get<2>(Res))
Chain = std::get<1>(Res);
/// Return true if value V represents the GC value. The behavior is conservative
/// in case it is not sure that value is not GC the function returns true.
static bool isGCValue(const Value *V, SelectionDAGBuilder &Builder) {
auto *Ty = V->getType();
if (!Ty->isPtrOrPtrVectorTy())
return false;
if (auto *GFI = Builder.GFI)
if (auto IsManaged = GFI->getStrategy().isGCManagedPointer(Ty))
return *IsManaged;
return true; // conservative
/// Lower deopt state and gc pointer arguments of the statepoint. The actual
/// lowering is described in lowerIncomingStatepointValue. This function is
/// responsible for lowering everything in the right position and playing some
/// tricks to avoid redundant stack manipulation where possible. On
/// completion, 'Ops' will contain ready to use operands for machine code
/// statepoint. The chain nodes will have already been created and the DAG root
/// will be set to the last value spilled (if any were).
static void
lowerStatepointMetaArgs(SmallVectorImpl<SDValue> &Ops,
SmallVectorImpl<MachineMemOperand *> &MemRefs,
SmallVectorImpl<SDValue> &GCPtrs,
DenseMap<SDValue, int> &LowerAsVReg,
SelectionDAGBuilder::StatepointLoweringInfo &SI,
SelectionDAGBuilder &Builder) {
// Lower the deopt and gc arguments for this statepoint. Layout will be:
// deopt argument length, deopt arguments.., gc arguments...
// Figure out what lowering strategy we're going to use for each part
// Note: It is conservatively correct to lower both "live-in" and "live-out"
// as "live-through". A "live-through" variable is one which is "live-in",
// "live-out", and live throughout the lifetime of the call (i.e. we can find
// it from any PC within the transitive callee of the statepoint). In
// particular, if the callee spills callee preserved registers we may not
// be able to find a value placed in that register during the call. This is
// fine for live-out, but not for live-through. If we were willing to make
// assumptions about the code generator producing the callee, we could
// potentially allow live-through values in callee saved registers.
const bool LiveInDeopt =
SI.StatepointFlags & (uint64_t)StatepointFlags::DeoptLiveIn;
// Decide which deriver pointers will go on VRegs
unsigned MaxVRegPtrs = MaxRegistersForGCPointers.getValue();
// Pointers used on exceptional path of invoke statepoint.
// We cannot assing them to VRegs.
SmallSet<SDValue, 8> LPadPointers;
if (!UseRegistersForGCPointersInLandingPad)
if (const auto *StInvoke =
dyn_cast_or_null<InvokeInst>(SI.StatepointInstr)) {
LandingPadInst *LPI = StInvoke->getLandingPadInst();
for (const auto *Relocate : SI.GCRelocates)
if (Relocate->getOperand(0) == LPI) {
LLVM_DEBUG(dbgs() << "Deciding how to lower GC Pointers:\n");
// List of unique lowered GC Pointer values.
SmallSetVector<SDValue, 16> LoweredGCPtrs;
// Map lowered GC Pointer value to the index in above vector
DenseMap<SDValue, unsigned> GCPtrIndexMap;
unsigned CurNumVRegs = 0;
auto canPassGCPtrOnVReg = [&](SDValue SD) {
if (SD.getValueType().isVector())
return false;
if (LPadPointers.count(SD))
return false;
return !willLowerDirectly(SD);
auto processGCPtr = [&](const Value *V) {
SDValue PtrSD = Builder.getValue(V);
if (!LoweredGCPtrs.insert(PtrSD))
return; // skip duplicates
GCPtrIndexMap[PtrSD] = LoweredGCPtrs.size() - 1;
assert(!LowerAsVReg.count(PtrSD) && "must not have been seen");
if (LowerAsVReg.size() == MaxVRegPtrs)
assert(V->getType()->isVectorTy() == PtrSD.getValueType().isVector() &&
"IR and SD types disagree");
if (!canPassGCPtrOnVReg(PtrSD)) {
LLVM_DEBUG(dbgs() << "direct/spill "; PtrSD.dump(&Builder.DAG));
LLVM_DEBUG(dbgs() << "vreg "; PtrSD.dump(&Builder.DAG));
LowerAsVReg[PtrSD] = CurNumVRegs++;
// Process derived pointers first to give them more chance to go on VReg.
for (const Value *V : SI.Ptrs)
for (const Value *V : SI.Bases)
LLVM_DEBUG(dbgs() << LowerAsVReg.size() << " pointers will go in vregs\n");
auto requireSpillSlot = [&](const Value *V) {
if (!Builder.DAG.getTargetLoweringInfo().isTypeLegal(
return true;
if (isGCValue(V, Builder))
return !LowerAsVReg.count(Builder.getValue(V));
return !(LiveInDeopt || UseRegistersForDeoptValues);
// 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 (requireSpillSlot(V))
reservePreviousStackSlotForValue(V, Builder);
for (const Value *V : SI.Ptrs) {
SDValue SDV = Builder.getValue(V);
if (!LowerAsVReg.count(SDV))
reservePreviousStackSlotForValue(V, Builder);
for (const Value *V : SI.Bases) {
SDValue SDV = Builder.getValue(V);
if (!LowerAsVReg.count(SDV))
reservePreviousStackSlotForValue(V, 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.
LLVM_DEBUG(dbgs() << "Lowering deopt state\n");
for (const Value *V : SI.DeoptState) {
SDValue Incoming;
// If this is a function argument at a static frame index, generate it as
// the frame index.
if (const Argument *Arg = dyn_cast<Argument>(V)) {
int FI = Builder.FuncInfo.getArgumentFrameIndex(Arg);
if (FI != INT_MAX)
Incoming = Builder.DAG.getFrameIndex(FI, Builder.getFrameIndexTy());
if (!Incoming.getNode())
Incoming = Builder.getValue(V);
LLVM_DEBUG(dbgs() << "Value " << *V
<< " requireSpillSlot = " << requireSpillSlot(V) << "\n");
lowerIncomingStatepointValue(Incoming, requireSpillSlot(V), Ops, MemRefs,
// Finally, go ahead and lower all the gc arguments.
pushStackMapConstant(Ops, Builder, LoweredGCPtrs.size());
for (SDValue SDV : LoweredGCPtrs)
lowerIncomingStatepointValue(SDV, !LowerAsVReg.count(SDV), Ops, MemRefs,
// Copy to out vector. LoweredGCPtrs will be empty after this point.
GCPtrs = LoweredGCPtrs.takeVector();
// 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
SmallVector<SDValue, 4> Allocas;
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!");
FI->getIndex(), Builder.getFrameIndexTy()));
auto &MF = Builder.DAG.getMachineFunction();
auto *MMO = getMachineMemOperand(MF, *FI);
pushStackMapConstant(Ops, Builder, Allocas.size());
Ops.append(Allocas.begin(), Allocas.end());
// Now construct GC base/derived map;
pushStackMapConstant(Ops, Builder, SI.Ptrs.size());
SDLoc L = Builder.getCurSDLoc();
for (unsigned i = 0; i < SI.Ptrs.size(); ++i) {
SDValue Base = Builder.getValue(SI.Bases[i]);
assert(GCPtrIndexMap.count(Base) && "base not found in index map");
Builder.DAG.getTargetConstant(GCPtrIndexMap[Base], L, MVT::i64));
SDValue Derived = Builder.getValue(SI.Ptrs[i]);
assert(GCPtrIndexMap.count(Derived) && "derived not found in index map");
Builder.DAG.getTargetConstant(GCPtrIndexMap[Derived], L, MVT::i64));
SDValue SelectionDAGBuilder::LowerAsSTATEPOINT(
SelectionDAGBuilder::StatepointLoweringInfo &SI) {
// The basic scheme here is that information about both the original call and
// the safepoint is encoded in the CallInst. We create a temporary call and
// lower it, then reverse engineer the calling sequence.
// Clear state
assert(SI.Bases.size() == SI.Ptrs.size() && "Pointer without base!");
assert((GFI || SI.Bases.empty()) &&
"No gc specified, so cannot relocate pointers!");
LLVM_DEBUG(if (SI.StatepointInstr) dbgs()
<< "Lowering statepoint " << *SI.StatepointInstr << "\n");
#ifndef NDEBUG
for (const auto *Reloc : SI.GCRelocates)
if (Reloc->getParent() == SI.StatepointInstr->getParent())
// Lower statepoint vmstate and gcstate arguments
// All lowered meta args.
SmallVector<SDValue, 10> LoweredMetaArgs;
// Lowered GC pointers (subset of above).
SmallVector<SDValue, 16> LoweredGCArgs;
SmallVector<MachineMemOperand*, 16> MemRefs;
// Maps derived pointer SDValue to statepoint result of relocated pointer.
DenseMap<SDValue, int> LowerAsVReg;
lowerStatepointMetaArgs(LoweredMetaArgs, MemRefs, LoweredGCArgs, LowerAsVReg,
SI, *this);
// Now that we've emitted the spills, we need to update the root so that the
// call sequence is ordered correctly.
// Get call node, we will replace it later with statepoint
SDValue ReturnVal;
SDNode *CallNode;
std::tie(ReturnVal, CallNode) = lowerCallFromStatepointLoweringInfo(SI, *this);
// nodes with all the appropriate arguments and return values.
// Call Node: Chain, Target, {Args}, RegMask, [Glue]
SDValue Chain = CallNode->getOperand(0);
SDValue Glue;
bool CallHasIncomingGlue = CallNode->getGluedNode();
if (CallHasIncomingGlue) {
// Glue is always last operand
Glue = CallNode->getOperand(CallNode->getNumOperands() - 1);
// Build the GC_TRANSITION_START node if necessary.
// The operands to the GC_TRANSITION_{START,END} nodes are laid out in the
// order in which they appear in the call to the statepoint intrinsic. If
// any of the operands is a pointer-typed, that operand is immediately
// followed by a SRCVALUE for the pointer that may be used during lowering
// (e.g. to form MachinePointerInfo values for loads/stores).
const bool IsGCTransition =
(SI.StatepointFlags & (uint64_t)StatepointFlags::GCTransition) ==
if (IsGCTransition) {
SmallVector<SDValue, 8> TSOps;
// Add chain
// Add GC transition arguments
for (const Value *V : SI.GCTransitionArgs) {
if (V->getType()->isPointerTy())
// Add glue if necessary
if (CallHasIncomingGlue)
SDVTList NodeTys = DAG.getVTList(MVT::Other, MVT::Glue);
SDValue GCTransitionStart =
DAG.getNode(ISD::GC_TRANSITION_START, getCurSDLoc(), NodeTys, TSOps);
Chain = GCTransitionStart.getValue(0);
Glue = GCTransitionStart.getValue(1);
// TODO: Currently, all of these operands are being marked as read/write in
// PrologEpilougeInserter.cpp, we should special case the VMState arguments
// and flags to be read-only.
SmallVector<SDValue, 40> Ops;
// Add the <id> and <numBytes> constants.
Ops.push_back(DAG.getTargetConstant(SI.ID, getCurSDLoc(), MVT::i64));
DAG.getTargetConstant(SI.NumPatchBytes, getCurSDLoc(), MVT::i32));
// Calculate and push starting position of vmstate arguments
// Get number of arguments incoming directly into call node
unsigned NumCallRegArgs =
CallNode->getNumOperands() - (CallHasIncomingGlue ? 4 : 3);
Ops.push_back(DAG.getTargetConstant(NumCallRegArgs, getCurSDLoc(), MVT::i32));
// Add call target
SDValue CallTarget = SDValue(CallNode->getOperand(1).getNode(), 0);
// Add call arguments
// Get position of register mask in the call
SDNode::op_iterator RegMaskIt;
if (CallHasIncomingGlue)
RegMaskIt = CallNode->op_end() - 2;
RegMaskIt = CallNode->op_end() - 1;
Ops.insert(Ops.end(), CallNode->op_begin() + 2, RegMaskIt);
// Add a constant argument for the calling convention
pushStackMapConstant(Ops, *this, SI.CLI.CallConv);
// Add a constant argument for the flags
uint64_t Flags = SI.StatepointFlags;
assert(((Flags & ~(uint64_t)StatepointFlags::MaskAll) == 0) &&
"Unknown flag used");
pushStackMapConstant(Ops, *this, Flags);
// Insert all vmstate and gcstate arguments
llvm::append_range(Ops, LoweredMetaArgs);
// Add register mask from call node
// Add chain
// Same for the glue, but we add it only if original call had it
if (Glue.getNode())
// Compute return values. Provide a glue output since we consume one as
// input. This allows someone else to chain off us as needed.
SmallVector<EVT, 8> NodeTys;
for (auto SD : LoweredGCArgs) {
if (!LowerAsVReg.count(SD))
LLVM_DEBUG(dbgs() << "Statepoint has " << NodeTys.size() << " results\n");
assert(NodeTys.size() == LowerAsVReg.size() && "Inconsistent GC Ptr lowering");
unsigned NumResults = NodeTys.size();
MachineSDNode *StatepointMCNode =
DAG.getMachineNode(TargetOpcode::STATEPOINT, getCurSDLoc(), NodeTys, Ops);
DAG.setNodeMemRefs(StatepointMCNode, MemRefs);
// For values lowered to tied-defs, create the virtual registers if used
// in other blocks. For local gc.relocate record appropriate statepoint
// result in StatepointLoweringState.
DenseMap<SDValue, Register> VirtRegs;
for (const auto *Relocate : SI.GCRelocates) {
Value *Derived = Relocate->getDerivedPtr();
SDValue SD = getValue(Derived);
if (!LowerAsVReg.count(SD))
SDValue Relocated = SDValue(StatepointMCNode, LowerAsVReg[SD]);
// Handle local relocate. Note that different relocates might
// map to the same SDValue.
if (SI.StatepointInstr->getParent() == Relocate->getParent()) {
SDValue Res = StatepointLowering.getLocation(SD);
if (Res)
assert(Res == Relocated);
StatepointLowering.setLocation(SD, Relocated);
// Handle multiple gc.relocates of the same input efficiently.
if (VirtRegs.count(SD))
auto *RetTy = Relocate->getType();
Register Reg = FuncInfo.CreateRegs(RetTy);
RegsForValue RFV(*DAG.getContext(), DAG.getTargetLoweringInfo(),
DAG.getDataLayout(), Reg, RetTy, std::nullopt);
SDValue Chain = DAG.getRoot();
RFV.getCopyToRegs(Relocated, DAG, getCurSDLoc(), Chain, nullptr);
VirtRegs[SD] = Reg;
// Record for later use how each relocation was lowered. This is needed to
// allow later gc.relocates to mirror the lowering chosen.
const Instruction *StatepointInstr = SI.StatepointInstr;
auto &RelocationMap = FuncInfo.StatepointRelocationMaps[StatepointInstr];
for (const GCRelocateInst *Relocate : SI.GCRelocates) {
const Value *V = Relocate->getDerivedPtr();
SDValue SDV = getValue(V);
SDValue Loc = StatepointLowering.getLocation(SDV);
bool IsLocal = (Relocate->getParent() == StatepointInstr->getParent());
RecordType Record;
if (IsLocal && LowerAsVReg.count(SDV)) {
// Result is already stored in StatepointLowering
Record.type = RecordType::SDValueNode;
} else if (LowerAsVReg.count(SDV)) {
Record.type = RecordType::VReg;
Record.payload.Reg = VirtRegs[SDV];
} else if (Loc.getNode()) {
Record.type = RecordType::Spill;
Record.payload.FI = cast<FrameIndexSDNode>(Loc)->getIndex();
} else {
Record.type = RecordType::NoRelocate;
// If we didn't relocate a value, we'll essentialy end up inserting an
// additional use of the original value when lowering the gc.relocate.
// We need to make sure the value is available at the new use, which
// might be in another block.
if (Relocate->getParent() != StatepointInstr->getParent())
RelocationMap[Relocate] = Record;
SDNode *SinkNode = StatepointMCNode;
// Build the GC_TRANSITION_END node if necessary.
// See the comment above regarding GC_TRANSITION_START for the layout of
// the operands to the GC_TRANSITION_END node.
if (IsGCTransition) {
SmallVector<SDValue, 8> TEOps;
// Add chain
TEOps.push_back(SDValue(StatepointMCNode, NumResults - 2));
// Add GC transition arguments
for (const Value *V : SI.GCTransitionArgs) {
if (V->getType()->isPointerTy())
// Add glue
TEOps.push_back(SDValue(StatepointMCNode, NumResults - 1));
SDVTList NodeTys = DAG.getVTList(MVT::Other, MVT::Glue);
SDValue GCTransitionStart =
DAG.getNode(ISD::GC_TRANSITION_END, getCurSDLoc(), NodeTys, TEOps);
SinkNode = GCTransitionStart.getNode();
// Replace original call
// Call: ch,glue = CALL ...
// Statepoint: [gc relocates],ch,glue = STATEPOINT ...
unsigned NumSinkValues = SinkNode->getNumValues();
SDValue StatepointValues[2] = {SDValue(SinkNode, NumSinkValues - 2),
SDValue(SinkNode, NumSinkValues - 1)};
DAG.ReplaceAllUsesWith(CallNode, StatepointValues);
// Remove original call node
// Since we always emit CopyToRegs (even for local relocates), we must
// update root, so that they are emitted before any local uses.
// TODO: A better future implementation would be to emit a single variable
// argument, variable return value STATEPOINT node here and then hookup the
// return value of each gc.relocate to the respective output of the
// previously emitted STATEPOINT value. Unfortunately, this doesn't appear
// to actually be possible today.
return ReturnVal;
/// Return two gc.results if present. First result is a block local
/// gc.result, second result is a non-block local gc.result. Corresponding
/// entry will be nullptr if not present.
static std::pair<const GCResultInst*, const GCResultInst*>
getGCResultLocality(const GCStatepointInst &S) {
std::pair<const GCResultInst *, const GCResultInst*> Res(nullptr, nullptr);
for (const auto *U : S.users()) {
auto *GRI = dyn_cast<GCResultInst>(U);
if (!GRI)
if (GRI->getParent() == S.getParent())
Res.first = GRI;
Res.second = GRI;
return Res;
SelectionDAGBuilder::LowerStatepoint(const GCStatepointInst &I,
const BasicBlock *EHPadBB /*= nullptr*/) {
assert(I.getCallingConv() != CallingConv::AnyReg &&
"anyregcc is not supported on statepoints!");
#ifndef NDEBUG
// Check that the associated GCStrategy expects to encounter statepoints.
assert(GFI->getStrategy().useStatepoints() &&
"GCStrategy does not expect to encounter statepoints");
SDValue ActualCallee;
SDValue Callee = getValue(I.getActualCalledOperand());
if (I.getNumPatchBytes() > 0) {
// If we've been asked to emit a nop sequence instead of a call instruction
// for this statepoint then don't lower the call target, but use a constant
// `undef` instead. Not lowering the call target lets statepoint clients
// get away without providing a physical address for the symbolic call
// target at link time.
ActualCallee = DAG.getUNDEF(Callee.getValueType());
} else {
ActualCallee = Callee;
const auto GCResultLocality = getGCResultLocality(I);
AttributeSet retAttrs;
if (GCResultLocality.first)
retAttrs = GCResultLocality.first->getAttributes().getRetAttrs();
StatepointLoweringInfo SI(DAG);
populateCallLoweringInfo(SI.CLI, &I, GCStatepointInst::CallArgsBeginPos,
I.getNumCallArgs(), ActualCallee,
I.getActualReturnType(), retAttrs,
// There may be duplication in the gc.relocate list; such as two copies of
// each relocation on normal and exceptional path for an invoke. We only
// need to spill once and record one copy in the stackmap, but we need to
// reload once per gc.relocate. (Dedupping gc.relocates is trickier and best
// handled as a CSE problem elsewhere.)
// TODO: There a couple of major stackmap size optimizations we could do
// here if we wished.
// 1) If we've encountered a derived pair {B, D}, we don't need to actually
// record {B,B} if it's seen later.
// 2) Due to rematerialization, actual derived pointers are somewhat rare;
// given that, we could change the format to record base pointer relocations
// separately with half the space. This would require a format rev and a
// fairly major rework of the STATEPOINT node though.
SmallSet<SDValue, 8> Seen;
for (const GCRelocateInst *Relocate : I.getGCRelocates()) {
SDValue DerivedSD = getValue(Relocate->getDerivedPtr());
if (Seen.insert(DerivedSD).second) {
// If we find a deopt value which isn't explicitly added, we need to
// ensure it gets lowered such that gc cycles occurring before the
// deoptimization event during the lifetime of the call don't invalidate
// the pointer we're deopting with. Note that we assume that all
// pointers passed to deopt are base pointers; relaxing that assumption
// would require relatively large changes to how we represent relocations.
for (Value *V : I.deopt_operands()) {
if (!isGCValue(V, *this))
if (Seen.insert(getValue(V)).second) {
SI.GCArgs = ArrayRef<const Use>(I.gc_args_begin(), I.gc_args_end());
SI.StatepointInstr = &I;
SI.ID = I.getID();
SI.DeoptState = ArrayRef<const Use>(I.deopt_begin(), I.deopt_end());
SI.GCTransitionArgs = ArrayRef<const Use>(I.gc_transition_args_begin(),
SI.StatepointFlags = I.getFlags();
SI.NumPatchBytes = I.getNumPatchBytes();
SDValue ReturnValue = LowerAsSTATEPOINT(SI);
// Export the result value if needed
if (!GCResultLocality.first && !GCResultLocality.second) {
// The return value is not needed, just generate a poison value.
// Note: This covers the void return case.
setValue(&I, DAG.getIntPtrConstant(-1, getCurSDLoc()));
if (GCResultLocality.first) {
// Result value will be used in a same basic block. Don't export it or
// perform any explicit register copies. The gc_result will simply grab
// this value.
setValue(&I, ReturnValue);
if (!GCResultLocality.second)
// Result value will be used in a different basic block so we need to export
// it now. Default exporting mechanism will not work here because statepoint
// call has a different type than the actual call. It means that by default
// llvm will create export register of the wrong type (always i32 in our
// case). So instead we need to create export register with correct type
// manually.
// TODO: To eliminate this problem we can remove gc.result intrinsics
// completely and make statepoint call to return a tuple.
Type *RetTy = GCResultLocality.second->getType();
Register Reg = FuncInfo.CreateRegs(RetTy);
RegsForValue RFV(*DAG.getContext(), DAG.getTargetLoweringInfo(),
DAG.getDataLayout(), Reg, RetTy,
SDValue Chain = DAG.getEntryNode();
RFV.getCopyToRegs(ReturnValue, DAG, getCurSDLoc(), Chain, nullptr);
FuncInfo.ValueMap[&I] = Reg;
void SelectionDAGBuilder::LowerCallSiteWithDeoptBundleImpl(
const CallBase *Call, SDValue Callee, const BasicBlock *EHPadBB,
bool VarArgDisallowed, bool ForceVoidReturnTy) {
StatepointLoweringInfo SI(DAG);
unsigned ArgBeginIndex = Call->arg_begin() - Call->op_begin();
SI.CLI, Call, ArgBeginIndex, Call->arg_size(), Callee,
ForceVoidReturnTy ? Type::getVoidTy(*DAG.getContext()) : Call->getType(),
Call->getAttributes().getRetAttrs(), /*IsPatchPoint=*/false);
if (!VarArgDisallowed)
SI.CLI.IsVarArg = Call->getFunctionType()->isVarArg();
auto DeoptBundle = *Call->getOperandBundle(LLVMContext::OB_deopt);
unsigned DefaultID = StatepointDirectives::DeoptBundleStatepointID;
auto SD = parseStatepointDirectivesFromAttrs(Call->getAttributes());
SI.ID = SD.StatepointID.value_or(DefaultID);
SI.NumPatchBytes = SD.NumPatchBytes.value_or(0);
SI.DeoptState =
ArrayRef<const Use>(DeoptBundle.Inputs.begin(), DeoptBundle.Inputs.end());
SI.StatepointFlags = static_cast<uint64_t>(StatepointFlags::None);
// NB! The GC arguments are deliberately left empty.
LLVM_DEBUG(dbgs() << "Lowering call with deopt bundle " << *Call << "\n");
if (SDValue ReturnVal = LowerAsSTATEPOINT(SI)) {
ReturnVal = lowerRangeToAssertZExt(DAG, *Call, ReturnVal);
setValue(Call, ReturnVal);
void SelectionDAGBuilder::LowerCallSiteWithDeoptBundle(
const CallBase *Call, SDValue Callee, const BasicBlock *EHPadBB) {
LowerCallSiteWithDeoptBundleImpl(Call, Callee, EHPadBB,
/* VarArgDisallowed = */ false,
/* ForceVoidReturnTy = */ false);
void SelectionDAGBuilder::visitGCResult(const GCResultInst &CI) {
// The result value of the gc_result is simply the result of the actual
// call. We've already emitted this, so just grab the value.
const Value *SI = CI.getStatepoint();
assert((isa<GCStatepointInst>(SI) || isa<UndefValue>(SI)) &&
"GetStatepoint must return one of two types");
if (isa<UndefValue>(SI))
if (cast<GCStatepointInst>(SI)->getParent() == CI.getParent()) {
setValue(&CI, getValue(SI));
// Statepoint is in different basic block so we should have stored call
// result in a virtual register.
// We can not use default getValue() functionality to copy value from this
// register because statepoint and actual call return types can be
// different, and getValue() will use CopyFromReg of the wrong type,
// which is always i32 in our case.
Type *RetTy = CI.getType();
SDValue CopyFromReg = getCopyFromRegs(SI, RetTy);
setValue(&CI, CopyFromReg);
void SelectionDAGBuilder::visitGCRelocate(const GCRelocateInst &Relocate) {
const Value *Statepoint = Relocate.getStatepoint();
#ifndef NDEBUG
// Consistency check
// We skip this check for relocates not in the same basic block as their
// statepoint. It would be too expensive to preserve validation info through
// different basic blocks.
assert((isa<GCStatepointInst>(Statepoint) || isa<UndefValue>(Statepoint)) &&
"GetStatepoint must return one of two types");
if (isa<UndefValue>(Statepoint))
if (cast<GCStatepointInst>(Statepoint)->getParent() == Relocate.getParent())
const Value *DerivedPtr = Relocate.getDerivedPtr();
auto &RelocationMap =
auto SlotIt = RelocationMap.find(&Relocate);
assert(SlotIt != RelocationMap.end() && "Relocating not lowered gc value");
const RecordType &Record = SlotIt->second;
// If relocation was done via virtual register..
if (Record.type == RecordType::SDValueNode) {
assert(cast<GCStatepointInst>(Statepoint)->getParent() ==
Relocate.getParent() &&
"Nonlocal gc.relocate mapped via SDValue");
SDValue SDV = StatepointLowering.getLocation(getValue(DerivedPtr));
assert(SDV.getNode() && "empty SDValue");
setValue(&Relocate, SDV);
if (Record.type == RecordType::VReg) {
Register InReg = Record.payload.Reg;
RegsForValue RFV(*DAG.getContext(), DAG.getTargetLoweringInfo(),
DAG.getDataLayout(), InReg, Relocate.getType(),
std::nullopt); // This is not an ABI copy.
// We generate copy to/from regs even for local uses, hence we must
// chain with current root to ensure proper ordering of copies w.r.t.
// statepoint.
SDValue Chain = DAG.getRoot();
SDValue Relocation = RFV.getCopyFromRegs(DAG, FuncInfo, getCurSDLoc(),
Chain, nullptr, nullptr);
setValue(&Relocate, Relocation);
if (Record.type == RecordType::Spill) {
unsigned Index = Record.payload.FI;
SDValue SpillSlot = DAG.getTargetFrameIndex(Index, getFrameIndexTy());
// All the reloads are independent and are reading memory only modified by
// statepoints (i.e. no other aliasing stores); informing SelectionDAG of
// this lets CSE kick in for free and allows reordering of
// instructions if possible. The lowering for statepoint sets the root,
// so this is ordering all reloads with the either
// a) the statepoint node itself, or
// b) the entry of the current block for an invoke statepoint.
const SDValue Chain = DAG.getRoot(); // != Builder.getRoot()
auto &MF = DAG.getMachineFunction();
auto &MFI = MF.getFrameInfo();
auto PtrInfo = MachinePointerInfo::getFixedStack(MF, Index);
auto *LoadMMO = MF.getMachineMemOperand(PtrInfo, MachineMemOperand::MOLoad,
auto LoadVT = DAG.getTargetLoweringInfo().getValueType(DAG.getDataLayout(),
SDValue SpillLoad =
DAG.getLoad(LoadVT, getCurSDLoc(), Chain, SpillSlot, LoadMMO);
setValue(&Relocate, SpillLoad);
assert(Record.type == RecordType::NoRelocate);
SDValue SD = getValue(DerivedPtr);
if (SD.isUndef() && SD.getValueType().getSizeInBits() <= 64) {
// Lowering relocate(undef) as arbitrary constant. Current constant value
// is chosen such that it's unlikely to be a valid pointer.
setValue(&Relocate, DAG.getConstant(0xFEFEFEFE, SDLoc(SD), MVT::i64));
// We didn't need to spill these special cases (constants and allocas).
// See the handling in spillIncomingValueForStatepoint for detail.
setValue(&Relocate, SD);
void SelectionDAGBuilder::LowerDeoptimizeCall(const CallInst *CI) {
const auto &TLI = DAG.getTargetLoweringInfo();
SDValue Callee = DAG.getExternalSymbol(TLI.getLibcallName(RTLIB::DEOPTIMIZE),
// We don't lower calls to __llvm_deoptimize as varargs, but as a regular
// call. We also do not lower the return value to any virtual register, and
// change the immediately following return to a trap instruction.
LowerCallSiteWithDeoptBundleImpl(CI, Callee, /* EHPadBB = */ nullptr,
/* VarArgDisallowed = */ true,
/* ForceVoidReturnTy = */ true);
void SelectionDAGBuilder::LowerDeoptimizingReturn() {
// We do not lower the return value from llvm.deoptimize to any virtual
// register, and change the immediately following return to a trap
// instruction.
if (DAG.getTarget().Options.TrapUnreachable)
DAG.getNode(ISD::TRAP, getCurSDLoc(), MVT::Other, DAG.getRoot()));