text: //===----- X86WinAllocaExpander.cpp - Expand WinAlloca pseudo instruction -===//
  //
  //                     The LLVM Compiler Infrastructure
  //
  // This file is distributed under the University of Illinois Open Source
  // License. See LICENSE.TXT for details.
  //
  //===----------------------------------------------------------------------===//
  //
  // This file defines a pass that expands WinAlloca pseudo-instructions.
  //
  // It performs a conservative analysis to determine whether each allocation
  // falls within a region of the stack that is safe to use, or whether stack
  // probes must be emitted.
  //
  //===----------------------------------------------------------------------===//
  
  #include "X86.h"
  #include "X86InstrBuilder.h"
  #include "X86InstrInfo.h"
  #include "X86MachineFunctionInfo.h"
  #include "X86Subtarget.h"
  #include "llvm/ADT/PostOrderIterator.h"
  #include "llvm/CodeGen/MachineFunctionPass.h"
  #include "llvm/CodeGen/MachineInstrBuilder.h"
  #include "llvm/CodeGen/MachineRegisterInfo.h"
  #include "llvm/CodeGen/Passes.h"
  #include "llvm/CodeGen/TargetInstrInfo.h"
  #include "llvm/IR/Function.h"
  #include "llvm/Support/raw_ostream.h"
  
  using namespace llvm;
  
  namespace {
  
  class X86WinAllocaExpander : public MachineFunctionPass {
  public:
    X86WinAllocaExpander() : MachineFunctionPass(ID) {}
  
    bool runOnMachineFunction(MachineFunction &MF) override;
  
  private:
    /// Strategies for lowering a WinAlloca.
    enum Lowering { TouchAndSub, Sub, Probe };
  
    /// Deterministic-order map from WinAlloca instruction to desired lowering.
    typedef MapVector<MachineInstr*, Lowering> LoweringMap;
  
    /// Compute which lowering to use for each WinAlloca instruction.
    void computeLowerings(MachineFunction &MF, LoweringMap& Lowerings);
  
    /// Get the appropriate lowering based on current offset and amount.
    Lowering getLowering(int64_t CurrentOffset, int64_t AllocaAmount);
  
    /// Lower a WinAlloca instruction.
    void lower(MachineInstr* MI, Lowering L);
  
    MachineRegisterInfo *MRI;
    const X86Subtarget *STI;
    const TargetInstrInfo *TII;
    const X86RegisterInfo *TRI;
    unsigned StackPtr;
    unsigned SlotSize;
    int64_t StackProbeSize;
    bool NoStackArgProbe;
  
    StringRef getPassName() const override { return "X86 WinAlloca Expander"; }
    static char ID;
  };
  
  char X86WinAllocaExpander::ID = 0;
  
  } // end anonymous namespace
  
  FunctionPass *llvm::createX86WinAllocaExpander() {
    return new X86WinAllocaExpander();
  }
  
  /// Return the allocation amount for a WinAlloca instruction, or -1 if unknown.
  static int64_t getWinAllocaAmount(MachineInstr *MI, MachineRegisterInfo *MRI) {
    assert(MI->getOpcode() == X86::WIN_ALLOCA_32 ||
           MI->getOpcode() == X86::WIN_ALLOCA_64);
    assert(MI->getOperand(0).isReg());
  
    unsigned AmountReg = MI->getOperand(0).getReg();
    MachineInstr *Def = MRI->getUniqueVRegDef(AmountReg);
  
    // Look through copies.
    while (Def && Def->isCopy() && Def->getOperand(1).isReg())
      Def = MRI->getUniqueVRegDef(Def->getOperand(1).getReg());
  
    if (!Def ||
        (Def->getOpcode() != X86::MOV32ri && Def->getOpcode() != X86::MOV64ri) ||
        !Def->getOperand(1).isImm())
      return -1;
  
    return Def->getOperand(1).getImm();
  }
  
  X86WinAllocaExpander::Lowering
  X86WinAllocaExpander::getLowering(int64_t CurrentOffset,
                                    int64_t AllocaAmount) {
    // For a non-constant amount or a large amount, we have to probe.
    if (AllocaAmount < 0 || AllocaAmount > StackProbeSize)
      return Probe;
  
    // If it fits within the safe region of the stack, just subtract.
    if (CurrentOffset + AllocaAmount <= StackProbeSize)
      return Sub;
  
    // Otherwise, touch the current tip of the stack, then subtract.
    return TouchAndSub;
  }
  
  static bool isPushPop(const MachineInstr &MI) {
    switch (MI.getOpcode()) {
    case X86::PUSH32i8:
    case X86::PUSH32r:
    case X86::PUSH32rmm:
    case X86::PUSH32rmr:
    case X86::PUSHi32:
    case X86::PUSH64i8:
    case X86::PUSH64r:
    case X86::PUSH64rmm:
    case X86::PUSH64rmr:
    case X86::PUSH64i32:
    case X86::POP32r:
    case X86::POP64r:
      return true;
    default:
      return false;
    }
  }
  
  void X86WinAllocaExpander::computeLowerings(MachineFunction &MF,
                                              LoweringMap &Lowerings) {
    // Do a one-pass reverse post-order walk of the CFG to conservatively estimate
    // the offset between the stack pointer and the lowest touched part of the
    // stack, and use that to decide how to lower each WinAlloca instruction.
  
    // Initialize OutOffset[B], the stack offset at exit from B, to something big.
    DenseMap<MachineBasicBlock *, int64_t> OutOffset;
    for (MachineBasicBlock &MBB : MF)
      OutOffset[&MBB] = INT32_MAX;
  
    // Note: we don't know the offset at the start of the entry block since the
    // prologue hasn't been inserted yet, and how much that will adjust the stack
    // pointer depends on register spills, which have not been computed yet.
  
    // Compute the reverse post-order.
    ReversePostOrderTraversal<MachineFunction*> RPO(&MF);
  
    for (MachineBasicBlock *MBB : RPO) {
      int64_t Offset = -1;
      for (MachineBasicBlock *Pred : MBB->predecessors())
        Offset = std::max(Offset, OutOffset[Pred]);
      if (Offset == -1) Offset = INT32_MAX;
  
      for (MachineInstr &MI : *MBB) {
        if (MI.getOpcode() == X86::WIN_ALLOCA_32 ||
            MI.getOpcode() == X86::WIN_ALLOCA_64) {
          // A WinAlloca moves StackPtr, and potentially touches it.
          int64_t Amount = getWinAllocaAmount(&MI, MRI);
          Lowering L = getLowering(Offset, Amount);
          Lowerings[&MI] = L;
          switch (L) {
          case Sub:
            Offset += Amount;
            break;
          case TouchAndSub:
            Offset = Amount;
            break;
          case Probe:
            Offset = 0;
            break;
          }
        } else if (MI.isCall() || isPushPop(MI)) {
          // Calls, pushes and pops touch the tip of the stack.
          Offset = 0;
        } else if (MI.getOpcode() == X86::ADJCALLSTACKUP32 ||
                   MI.getOpcode() == X86::ADJCALLSTACKUP64) {
          Offset -= MI.getOperand(0).getImm();
        } else if (MI.getOpcode() == X86::ADJCALLSTACKDOWN32 ||
                   MI.getOpcode() == X86::ADJCALLSTACKDOWN64) {
          Offset += MI.getOperand(0).getImm();
        } else if (MI.modifiesRegister(StackPtr, TRI)) {
          // Any other modification of SP means we've lost track of it.
          Offset = INT32_MAX;
        }
      }
  
      OutOffset[MBB] = Offset;
    }
  }
  
  static unsigned getSubOpcode(bool Is64Bit, int64_t Amount) {
    if (Is64Bit)
      return isInt<8>(Amount) ? X86::SUB64ri8 : X86::SUB64ri32;
    return isInt<8>(Amount) ? X86::SUB32ri8 : X86::SUB32ri;
  }
  
  void X86WinAllocaExpander::lower(MachineInstr* MI, Lowering L) {
    DebugLoc DL = MI->getDebugLoc();
    MachineBasicBlock *MBB = MI->getParent();
    MachineBasicBlock::iterator I = *MI;
  
    int64_t Amount = getWinAllocaAmount(MI, MRI);
    if (Amount == 0) {
      MI->eraseFromParent();
      return;
    }
  
    bool Is64Bit = STI->is64Bit();
    assert(SlotSize == 4 || SlotSize == 8);
    unsigned RegA = (SlotSize == 8) ? X86::RAX : X86::EAX;
  
    switch (L) {
    case TouchAndSub:
      assert(Amount >= SlotSize);
  
      // Use a push to touch the top of the stack.
      BuildMI(*MBB, I, DL, TII->get(Is64Bit ? X86::PUSH64r : X86::PUSH32r))
          .addReg(RegA, RegState::Undef);
      Amount -= SlotSize;
      if (!Amount)
        break;
  
      // Fall through to make any remaining adjustment.
      LLVM_FALLTHROUGH;
    case Sub:
      assert(Amount > 0);
      if (Amount == SlotSize) {
        // Use push to save size.
        BuildMI(*MBB, I, DL, TII->get(Is64Bit ? X86::PUSH64r : X86::PUSH32r))
            .addReg(RegA, RegState::Undef);
      } else {
        // Sub.
        BuildMI(*MBB, I, DL, TII->get(getSubOpcode(Is64Bit, Amount)), StackPtr)
            .addReg(StackPtr)
            .addImm(Amount);
      }
      break;
    case Probe:
      if (!NoStackArgProbe) {
        // The probe lowering expects the amount in RAX/EAX.
        BuildMI(*MBB, MI, DL, TII->get(TargetOpcode::COPY), RegA)
            .addReg(MI->getOperand(0).getReg());
  
        // Do the probe.
        STI->getFrameLowering()->emitStackProbe(*MBB->getParent(), *MBB, MI, DL,
                                                /*InPrologue=*/false);
      } else {
        // Sub
        BuildMI(*MBB, I, DL, TII->get(Is64Bit ? X86::SUB64rr : X86::SUB32rr),
                StackPtr)
            .addReg(StackPtr)
            .addReg(MI->getOperand(0).getReg());
      }
      break;
    }
  
    unsigned AmountReg = MI->getOperand(0).getReg();
    MI->eraseFromParent();
  
    // Delete the definition of AmountReg, possibly walking a chain of copies.
    for (;;) {
      if (!MRI->use_empty(AmountReg))
        break;
      MachineInstr *AmountDef = MRI->getUniqueVRegDef(AmountReg);
      if (!AmountDef)
        break;
      if (AmountDef->isCopy() && AmountDef->getOperand(1).isReg())
        AmountReg = AmountDef->getOperand(1).isReg();
      AmountDef->eraseFromParent();
      break;
    }
  }
  
  bool X86WinAllocaExpander::runOnMachineFunction(MachineFunction &MF) {
    if (!MF.getInfo<X86MachineFunctionInfo>()->hasWinAlloca())
      return false;
  
    MRI = &MF.getRegInfo();
    STI = &MF.getSubtarget<X86Subtarget>();
    TII = STI->getInstrInfo();
    TRI = STI->getRegisterInfo();
    StackPtr = TRI->getStackRegister();
    SlotSize = TRI->getSlotSize();
  
    StackProbeSize = 4096;
    if (MF.getFunction().hasFnAttribute("stack-probe-size")) {
      MF.getFunction()
          .getFnAttribute("stack-probe-size")
          .getValueAsString()
          .getAsInteger(0, StackProbeSize);
    }
    NoStackArgProbe = MF.getFunction().hasFnAttribute("no-stack-arg-probe");
    if (NoStackArgProbe)
      StackProbeSize = INT64_MAX;
  
    LoweringMap Lowerings;
    computeLowerings(MF, Lowerings);
    for (auto &P : Lowerings)
      lower(P.first, P.second);
  
    return true;
  }
  