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diff --git a/llvm/lib/CodeGen/LocalStackSlotAllocation.cpp b/llvm/lib/CodeGen/LocalStackSlotAllocation.cpp
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index 000000000000..b14d76a585f7
--- /dev/null
+++ b/llvm/lib/CodeGen/LocalStackSlotAllocation.cpp
@@ -0,0 +1,437 @@
+//===- LocalStackSlotAllocation.cpp - Pre-allocate locals to stack slots --===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// This pass assigns local frame indices to stack slots relative to one another
+// and allocates additional base registers to access them when the target
+// estimates they are likely to be out of range of stack pointer and frame
+// pointer relative addressing.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/ADT/SetVector.h"
+#include "llvm/ADT/SmallSet.h"
+#include "llvm/ADT/SmallVector.h"
+#include "llvm/ADT/Statistic.h"
+#include "llvm/CodeGen/MachineBasicBlock.h"
+#include "llvm/CodeGen/MachineFrameInfo.h"
+#include "llvm/CodeGen/MachineFunction.h"
+#include "llvm/CodeGen/MachineFunctionPass.h"
+#include "llvm/CodeGen/MachineInstr.h"
+#include "llvm/CodeGen/MachineOperand.h"
+#include "llvm/CodeGen/MachineRegisterInfo.h"
+#include "llvm/CodeGen/TargetFrameLowering.h"
+#include "llvm/CodeGen/TargetOpcodes.h"
+#include "llvm/CodeGen/TargetRegisterInfo.h"
+#include "llvm/CodeGen/TargetSubtargetInfo.h"
+#include "llvm/Pass.h"
+#include "llvm/Support/Debug.h"
+#include "llvm/Support/ErrorHandling.h"
+#include "llvm/Support/raw_ostream.h"
+#include <algorithm>
+#include <cassert>
+#include <cstdint>
+#include <tuple>
+
+using namespace llvm;
+
+#define DEBUG_TYPE "localstackalloc"
+
+STATISTIC(NumAllocations, "Number of frame indices allocated into local block");
+STATISTIC(NumBaseRegisters, "Number of virtual frame base registers allocated");
+STATISTIC(NumReplacements, "Number of frame indices references replaced");
+
+namespace {
+
+  class FrameRef {
+    MachineBasicBlock::iterator MI; // Instr referencing the frame
+    int64_t LocalOffset;            // Local offset of the frame idx referenced
+    int FrameIdx;                   // The frame index
+
+    // Order reference instruction appears in program. Used to ensure
+    // deterministic order when multiple instructions may reference the same
+    // location.
+    unsigned Order;
+
+  public:
+    FrameRef(MachineInstr *I, int64_t Offset, int Idx, unsigned Ord) :
+      MI(I), LocalOffset(Offset), FrameIdx(Idx), Order(Ord) {}
+
+    bool operator<(const FrameRef &RHS) const {
+      return std::tie(LocalOffset, FrameIdx, Order) <
+             std::tie(RHS.LocalOffset, RHS.FrameIdx, RHS.Order);
+    }
+
+    MachineBasicBlock::iterator getMachineInstr() const { return MI; }
+    int64_t getLocalOffset() const { return LocalOffset; }
+    int getFrameIndex() const { return FrameIdx; }
+  };
+
+  class LocalStackSlotPass: public MachineFunctionPass {
+    SmallVector<int64_t, 16> LocalOffsets;
+
+    /// StackObjSet - A set of stack object indexes
+    using StackObjSet = SmallSetVector<int, 8>;
+
+    void AdjustStackOffset(MachineFrameInfo &MFI, int FrameIdx, int64_t &Offset,
+                           bool StackGrowsDown, unsigned &MaxAlign);
+    void AssignProtectedObjSet(const StackObjSet &UnassignedObjs,
+                               SmallSet<int, 16> &ProtectedObjs,
+                               MachineFrameInfo &MFI, bool StackGrowsDown,
+                               int64_t &Offset, unsigned &MaxAlign);
+    void calculateFrameObjectOffsets(MachineFunction &Fn);
+    bool insertFrameReferenceRegisters(MachineFunction &Fn);
+
+  public:
+    static char ID; // Pass identification, replacement for typeid
+
+    explicit LocalStackSlotPass() : MachineFunctionPass(ID) {
+      initializeLocalStackSlotPassPass(*PassRegistry::getPassRegistry());
+    }
+
+    bool runOnMachineFunction(MachineFunction &MF) override;
+
+    void getAnalysisUsage(AnalysisUsage &AU) const override {
+      AU.setPreservesCFG();
+      MachineFunctionPass::getAnalysisUsage(AU);
+    }
+  };
+
+} // end anonymous namespace
+
+char LocalStackSlotPass::ID = 0;
+
+char &llvm::LocalStackSlotAllocationID = LocalStackSlotPass::ID;
+INITIALIZE_PASS(LocalStackSlotPass, DEBUG_TYPE,
+                "Local Stack Slot Allocation", false, false)
+
+bool LocalStackSlotPass::runOnMachineFunction(MachineFunction &MF) {
+  MachineFrameInfo &MFI = MF.getFrameInfo();
+  const TargetRegisterInfo *TRI = MF.getSubtarget().getRegisterInfo();
+  unsigned LocalObjectCount = MFI.getObjectIndexEnd();
+
+  // If the target doesn't want/need this pass, or if there are no locals
+  // to consider, early exit.
+  if (!TRI->requiresVirtualBaseRegisters(MF) || LocalObjectCount == 0)
+    return true;
+
+  // Make sure we have enough space to store the local offsets.
+  LocalOffsets.resize(MFI.getObjectIndexEnd());
+
+  // Lay out the local blob.
+  calculateFrameObjectOffsets(MF);
+
+  // Insert virtual base registers to resolve frame index references.
+  bool UsedBaseRegs = insertFrameReferenceRegisters(MF);
+
+  // Tell MFI whether any base registers were allocated. PEI will only
+  // want to use the local block allocations from this pass if there were any.
+  // Otherwise, PEI can do a bit better job of getting the alignment right
+  // without a hole at the start since it knows the alignment of the stack
+  // at the start of local allocation, and this pass doesn't.
+  MFI.setUseLocalStackAllocationBlock(UsedBaseRegs);
+
+  return true;
+}
+
+/// AdjustStackOffset - Helper function used to adjust the stack frame offset.
+void LocalStackSlotPass::AdjustStackOffset(MachineFrameInfo &MFI,
+                                           int FrameIdx, int64_t &Offset,
+                                           bool StackGrowsDown,
+                                           unsigned &MaxAlign) {
+  // If the stack grows down, add the object size to find the lowest address.
+  if (StackGrowsDown)
+    Offset += MFI.getObjectSize(FrameIdx);
+
+  unsigned Align = MFI.getObjectAlignment(FrameIdx);
+
+  // If the alignment of this object is greater than that of the stack, then
+  // increase the stack alignment to match.
+  MaxAlign = std::max(MaxAlign, Align);
+
+  // Adjust to alignment boundary.
+  Offset = (Offset + Align - 1) / Align * Align;
+
+  int64_t LocalOffset = StackGrowsDown ? -Offset : Offset;
+  LLVM_DEBUG(dbgs() << "Allocate FI(" << FrameIdx << ") to local offset "
+                    << LocalOffset << "\n");
+  // Keep the offset available for base register allocation
+  LocalOffsets[FrameIdx] = LocalOffset;
+  // And tell MFI about it for PEI to use later
+  MFI.mapLocalFrameObject(FrameIdx, LocalOffset);
+
+  if (!StackGrowsDown)
+    Offset += MFI.getObjectSize(FrameIdx);
+
+  ++NumAllocations;
+}
+
+/// AssignProtectedObjSet - Helper function to assign large stack objects (i.e.,
+/// those required to be close to the Stack Protector) to stack offsets.
+void LocalStackSlotPass::AssignProtectedObjSet(const StackObjSet &UnassignedObjs,
+                                           SmallSet<int, 16> &ProtectedObjs,
+                                           MachineFrameInfo &MFI,
+                                           bool StackGrowsDown, int64_t &Offset,
+                                           unsigned &MaxAlign) {
+  for (StackObjSet::const_iterator I = UnassignedObjs.begin(),
+        E = UnassignedObjs.end(); I != E; ++I) {
+    int i = *I;
+    AdjustStackOffset(MFI, i, Offset, StackGrowsDown, MaxAlign);
+    ProtectedObjs.insert(i);
+  }
+}
+
+/// calculateFrameObjectOffsets - Calculate actual frame offsets for all of the
+/// abstract stack objects.
+void LocalStackSlotPass::calculateFrameObjectOffsets(MachineFunction &Fn) {
+  // Loop over all of the stack objects, assigning sequential addresses...
+  MachineFrameInfo &MFI = Fn.getFrameInfo();
+  const TargetFrameLowering &TFI = *Fn.getSubtarget().getFrameLowering();
+  bool StackGrowsDown =
+    TFI.getStackGrowthDirection() == TargetFrameLowering::StackGrowsDown;
+  int64_t Offset = 0;
+  unsigned MaxAlign = 0;
+
+  // Make sure that the stack protector comes before the local variables on the
+  // stack.
+  SmallSet<int, 16> ProtectedObjs;
+  if (MFI.hasStackProtectorIndex()) {
+    int StackProtectorFI = MFI.getStackProtectorIndex();
+
+    // We need to make sure we didn't pre-allocate the stack protector when
+    // doing this.
+    // If we already have a stack protector, this will re-assign it to a slot
+    // that is **not** covering the protected objects.
+    assert(!MFI.isObjectPreAllocated(StackProtectorFI) &&
+           "Stack protector pre-allocated in LocalStackSlotAllocation");
+
+    StackObjSet LargeArrayObjs;
+    StackObjSet SmallArrayObjs;
+    StackObjSet AddrOfObjs;
+
+    AdjustStackOffset(MFI, StackProtectorFI, Offset, StackGrowsDown, MaxAlign);
+
+    // Assign large stack objects first.
+    for (unsigned i = 0, e = MFI.getObjectIndexEnd(); i != e; ++i) {
+      if (MFI.isDeadObjectIndex(i))
+        continue;
+      if (StackProtectorFI == (int)i)
+        continue;
+
+      switch (MFI.getObjectSSPLayout(i)) {
+      case MachineFrameInfo::SSPLK_None:
+        continue;
+      case MachineFrameInfo::SSPLK_SmallArray:
+        SmallArrayObjs.insert(i);
+        continue;
+      case MachineFrameInfo::SSPLK_AddrOf:
+        AddrOfObjs.insert(i);
+        continue;
+      case MachineFrameInfo::SSPLK_LargeArray:
+        LargeArrayObjs.insert(i);
+        continue;
+      }
+      llvm_unreachable("Unexpected SSPLayoutKind.");
+    }
+
+    AssignProtectedObjSet(LargeArrayObjs, ProtectedObjs, MFI, StackGrowsDown,
+                          Offset, MaxAlign);
+    AssignProtectedObjSet(SmallArrayObjs, ProtectedObjs, MFI, StackGrowsDown,
+                          Offset, MaxAlign);
+    AssignProtectedObjSet(AddrOfObjs, ProtectedObjs, MFI, StackGrowsDown,
+                          Offset, MaxAlign);
+  }
+
+  // Then assign frame offsets to stack objects that are not used to spill
+  // callee saved registers.
+  for (unsigned i = 0, e = MFI.getObjectIndexEnd(); i != e; ++i) {
+    if (MFI.isDeadObjectIndex(i))
+      continue;
+    if (MFI.getStackProtectorIndex() == (int)i)
+      continue;
+    if (ProtectedObjs.count(i))
+      continue;
+
+    AdjustStackOffset(MFI, i, Offset, StackGrowsDown, MaxAlign);
+  }
+
+  // Remember how big this blob of stack space is
+  MFI.setLocalFrameSize(Offset);
+  MFI.setLocalFrameMaxAlign(MaxAlign);
+}
+
+static inline bool
+lookupCandidateBaseReg(unsigned BaseReg,
+                       int64_t BaseOffset,
+                       int64_t FrameSizeAdjust,
+                       int64_t LocalFrameOffset,
+                       const MachineInstr &MI,
+                       const TargetRegisterInfo *TRI) {
+  // Check if the relative offset from the where the base register references
+  // to the target address is in range for the instruction.
+  int64_t Offset = FrameSizeAdjust + LocalFrameOffset - BaseOffset;
+  return TRI->isFrameOffsetLegal(&MI, BaseReg, Offset);
+}
+
+bool LocalStackSlotPass::insertFrameReferenceRegisters(MachineFunction &Fn) {
+  // Scan the function's instructions looking for frame index references.
+  // For each, ask the target if it wants a virtual base register for it
+  // based on what we can tell it about where the local will end up in the
+  // stack frame. If it wants one, re-use a suitable one we've previously
+  // allocated, or if there isn't one that fits the bill, allocate a new one
+  // and ask the target to create a defining instruction for it.
+  bool UsedBaseReg = false;
+
+  MachineFrameInfo &MFI = Fn.getFrameInfo();
+  const TargetRegisterInfo *TRI = Fn.getSubtarget().getRegisterInfo();
+  const TargetFrameLowering &TFI = *Fn.getSubtarget().getFrameLowering();
+  bool StackGrowsDown =
+    TFI.getStackGrowthDirection() == TargetFrameLowering::StackGrowsDown;
+
+  // Collect all of the instructions in the block that reference
+  // a frame index. Also store the frame index referenced to ease later
+  // lookup. (For any insn that has more than one FI reference, we arbitrarily
+  // choose the first one).
+  SmallVector<FrameRef, 64> FrameReferenceInsns;
+
+  unsigned Order = 0;
+
+  for (MachineBasicBlock &BB : Fn) {
+    for (MachineInstr &MI : BB) {
+      // Debug value, stackmap and patchpoint instructions can't be out of
+      // range, so they don't need any updates.
+      if (MI.isDebugInstr() || MI.getOpcode() == TargetOpcode::STATEPOINT ||
+          MI.getOpcode() == TargetOpcode::STACKMAP ||
+          MI.getOpcode() == TargetOpcode::PATCHPOINT)
+        continue;
+
+      // For now, allocate the base register(s) within the basic block
+      // where they're used, and don't try to keep them around outside
+      // of that. It may be beneficial to try sharing them more broadly
+      // than that, but the increased register pressure makes that a
+      // tricky thing to balance. Investigate if re-materializing these
+      // becomes an issue.
+      for (unsigned i = 0, e = MI.getNumOperands(); i != e; ++i) {
+        // Consider replacing all frame index operands that reference
+        // an object allocated in the local block.
+        if (MI.getOperand(i).isFI()) {
+          // Don't try this with values not in the local block.
+          if (!MFI.isObjectPreAllocated(MI.getOperand(i).getIndex()))
+            break;
+          int Idx = MI.getOperand(i).getIndex();
+          int64_t LocalOffset = LocalOffsets[Idx];
+          if (!TRI->needsFrameBaseReg(&MI, LocalOffset))
+            break;
+          FrameReferenceInsns.push_back(FrameRef(&MI, LocalOffset, Idx, Order++));
+          break;
+        }
+      }
+    }
+  }
+
+  // Sort the frame references by local offset.
+  // Use frame index as a tie-breaker in case MI's have the same offset.
+  llvm::sort(FrameReferenceInsns);
+
+  MachineBasicBlock *Entry = &Fn.front();
+
+  unsigned BaseReg = 0;
+  int64_t BaseOffset = 0;
+
+  // Loop through the frame references and allocate for them as necessary.
+  for (int ref = 0, e = FrameReferenceInsns.size(); ref < e ; ++ref) {
+    FrameRef &FR = FrameReferenceInsns[ref];
+    MachineInstr &MI = *FR.getMachineInstr();
+    int64_t LocalOffset = FR.getLocalOffset();
+    int FrameIdx = FR.getFrameIndex();
+    assert(MFI.isObjectPreAllocated(FrameIdx) &&
+           "Only pre-allocated locals expected!");
+
+    LLVM_DEBUG(dbgs() << "Considering: " << MI);
+
+    unsigned idx = 0;
+    for (unsigned f = MI.getNumOperands(); idx != f; ++idx) {
+      if (!MI.getOperand(idx).isFI())
+        continue;
+
+      if (FrameIdx == MI.getOperand(idx).getIndex())
+        break;
+    }
+
+    assert(idx < MI.getNumOperands() && "Cannot find FI operand");
+
+    int64_t Offset = 0;
+    int64_t FrameSizeAdjust = StackGrowsDown ? MFI.getLocalFrameSize() : 0;
+
+    LLVM_DEBUG(dbgs() << "  Replacing FI in: " << MI);
+
+    // If we have a suitable base register available, use it; otherwise
+    // create a new one. Note that any offset encoded in the
+    // instruction itself will be taken into account by the target,
+    // so we don't have to adjust for it here when reusing a base
+    // register.
+    if (UsedBaseReg &&
+        lookupCandidateBaseReg(BaseReg, BaseOffset, FrameSizeAdjust,
+                               LocalOffset, MI, TRI)) {
+      LLVM_DEBUG(dbgs() << "  Reusing base register " << BaseReg << "\n");
+      // We found a register to reuse.
+      Offset = FrameSizeAdjust + LocalOffset - BaseOffset;
+    } else {
+      // No previously defined register was in range, so create a new one.
+      int64_t InstrOffset = TRI->getFrameIndexInstrOffset(&MI, idx);
+
+      int64_t PrevBaseOffset = BaseOffset;
+      BaseOffset = FrameSizeAdjust + LocalOffset + InstrOffset;
+
+      // We'd like to avoid creating single-use virtual base registers.
+      // Because the FrameRefs are in sorted order, and we've already
+      // processed all FrameRefs before this one, just check whether or not
+      // the next FrameRef will be able to reuse this new register. If not,
+      // then don't bother creating it.
+      if (ref + 1 >= e ||
+          !lookupCandidateBaseReg(
+              BaseReg, BaseOffset, FrameSizeAdjust,
+              FrameReferenceInsns[ref + 1].getLocalOffset(),
+              *FrameReferenceInsns[ref + 1].getMachineInstr(), TRI)) {
+        BaseOffset = PrevBaseOffset;
+        continue;
+      }
+
+      const MachineFunction *MF = MI.getMF();
+      const TargetRegisterClass *RC = TRI->getPointerRegClass(*MF);
+      BaseReg = Fn.getRegInfo().createVirtualRegister(RC);
+
+      LLVM_DEBUG(dbgs() << "  Materializing base register " << BaseReg
+                        << " at frame local offset "
+                        << LocalOffset + InstrOffset << "\n");
+
+      // Tell the target to insert the instruction to initialize
+      // the base register.
+      //            MachineBasicBlock::iterator InsertionPt = Entry->begin();
+      TRI->materializeFrameBaseRegister(Entry, BaseReg, FrameIdx,
+                                        InstrOffset);
+
+      // The base register already includes any offset specified
+      // by the instruction, so account for that so it doesn't get
+      // applied twice.
+      Offset = -InstrOffset;
+
+      ++NumBaseRegisters;
+      UsedBaseReg = true;
+    }
+    assert(BaseReg != 0 && "Unable to allocate virtual base register!");
+
+    // Modify the instruction to use the new base register rather
+    // than the frame index operand.
+    TRI->resolveFrameIndex(MI, BaseReg, Offset);
+    LLVM_DEBUG(dbgs() << "Resolved: " << MI);
+
+    ++NumReplacements;
+  }
+
+  return UsedBaseReg;
+}