CFIInstrInserter.cpp

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//===------ CFIInstrInserter.cpp - Insert additional CFI instructions -----===//
//
// 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
//
//===----------------------------------------------------------------------===//
//
/// \file This pass verifies incoming and outgoing CFA information of basic
/// blocks. CFA information is information about offset and register set by CFI
/// directives, valid at the start and end of a basic block. This pass checks
/// that outgoing information of predecessors matches incoming information of
/// their successors. Then it checks if blocks have correct CFA calculation rule
/// set and inserts additional CFI instruction at their beginnings if they
/// don't. CFI instructions are inserted if basic blocks have incorrect offset
/// or register set by previous blocks, as a result of a non-linear layout of
/// blocks in a function.
//===----------------------------------------------------------------------===//
 
#include "llvm/ADT/DepthFirstIterator.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstrBuilder.h"
#include "llvm/CodeGen/Passes.h"
#include "llvm/CodeGen/TargetFrameLowering.h"
#include "llvm/CodeGen/TargetInstrInfo.h"
#include "llvm/CodeGen/TargetSubtargetInfo.h"
#include "llvm/InitializePasses.h"
#include "llvm/MC/MCContext.h"
#include "llvm/MC/MCDwarf.h"
using namespace llvm;
 
static cl::opt<bool> VerifyCFI("verify-cfiinstrs",
    cl::desc("Verify Call Frame Information instructions"),
    cl::init(false),
    cl::Hidden);
 
namespace {
class CFIInstrInserter : public MachineFunctionPass {
 public:
  static char ID;
 
  CFIInstrInserter() : MachineFunctionPass(ID) {}
 
  void getAnalysisUsage(AnalysisUsage &AU) const override {
    AU.setPreservesAll();
    MachineFunctionPass::getAnalysisUsage(AU);
  }
 
  bool runOnMachineFunction(MachineFunction &MF) override {
    if (!MF.needsFrameMoves())
      return false;
 
    MBBVector.resize(MF.getNumBlockIDs());
    calculateCFAInfo(MF);
 
    if (VerifyCFI) {
      if (unsigned ErrorNum = verify(MF))
        report_fatal_error("Found " + Twine(ErrorNum) +
                           " in/out CFI information errors.");
    }
    bool insertedCFI = insertCFIInstrs(MF);
    MBBVector.clear();
    return insertedCFI;
  }
 
private:
  /// contains the location where CSR register is saved.
  class CSRSavedLocation {
  public:
    enum Kind { Invalid, Register, CFAOffset };
    Kind K = Invalid;
 
  private:
    union {
      // Dwarf register number
      unsigned Reg;
      // CFA offset
      int64_t Offset;
    };
 
  public:
    CSRSavedLocation() {}
 
    static CSRSavedLocation createCFAOffset(int64_t Offset) {
      CSRSavedLocation Loc;
      Loc.K = Kind::CFAOffset;
      Loc.Offset = Offset;
      return Loc;
    }
 
    static CSRSavedLocation createRegister(unsigned Reg) {
      CSRSavedLocation Loc;
      Loc.K = Kind::Register;
      Loc.Reg = Reg;
      return Loc;
    }
 
    bool isValid() const { return K != Kind::Invalid; }
 
    unsigned getRegister() const {
      assert(K == Kind::Register);
      return Reg;
    }
 
    int64_t getOffset() const {
      assert(K == Kind::CFAOffset);
      return Offset;
    }
 
    bool operator==(const CSRSavedLocation &RHS) const {
      if (K != RHS.K)
        return false;
      switch (K) {
      case Kind::Invalid:
        return true;
      case Kind::Register:
        return getRegister() == RHS.getRegister();
      case Kind::CFAOffset:
        return getOffset() == RHS.getOffset();
      }
      llvm_unreachable("Unknown CSRSavedLocation Kind!");
    }
    bool operator!=(const CSRSavedLocation &RHS) const {
      return !(*this == RHS);
    }
    void dump(raw_ostream &OS) const {
      switch (K) {
      case Kind::Invalid:
        OS << "Invalid";
        break;
      case Kind::Register:
        OS << "In Dwarf register: " << Reg;
        break;
      case Kind::CFAOffset:
        OS << "At CFA offset: " << Offset;
        break;
      }
    }
  };
 
  struct MBBCFAInfo {
    MachineBasicBlock *MBB;
    /// Value of cfa offset valid at basic block entry.
    int64_t IncomingCFAOffset = -1;
    /// Value of cfa offset valid at basic block exit.
    int64_t OutgoingCFAOffset = -1;
    /// Value of cfa register valid at basic block entry.
    unsigned IncomingCFARegister = 0;
    /// Value of cfa register valid at basic block exit.
    unsigned OutgoingCFARegister = 0;
    /// Set of callee saved registers saved at basic block entry.
    BitVector IncomingCSRSaved;
    /// Set of callee saved registers saved at basic block exit.
    BitVector OutgoingCSRSaved;
    /// If in/out cfa offset and register values for this block have already
    /// been set or not.
    bool Processed = false;
  };
 
  /// Contains cfa offset and register values valid at entry and exit of basic
  /// blocks.
  std::vector<MBBCFAInfo> MBBVector;
 
  /// Map the callee save registers to the locations where they are saved.
  SmallDenseMap<unsigned, CSRSavedLocation, 16> CSRLocMap;
 
  /// Calculate cfa offset and register values valid at entry and exit for all
  /// basic blocks in a function.
  void calculateCFAInfo(MachineFunction &MF);
  /// Calculate cfa offset and register values valid at basic block exit by
  /// checking the block for CFI instructions. Block's incoming CFA info remains
  /// the same.
  void calculateOutgoingCFAInfo(MBBCFAInfo &MBBInfo);
  /// Update in/out cfa offset and register values for successors of the basic
  /// block.
  void updateSuccCFAInfo(MBBCFAInfo &MBBInfo);
 
  /// Check if incoming CFA information of a basic block matches outgoing CFA
  /// information of the previous block. If it doesn't, insert CFI instruction
  /// at the beginning of the block that corrects the CFA calculation rule for
  /// that block.
  bool insertCFIInstrs(MachineFunction &MF);
  /// Return the cfa offset value that should be set at the beginning of a MBB
  /// if needed. The negated value is needed when creating CFI instructions that
  /// set absolute offset.
  int64_t getCorrectCFAOffset(MachineBasicBlock *MBB) {
    return MBBVector[MBB->getNumber()].IncomingCFAOffset;
  }
 
  void reportCFAError(const MBBCFAInfo &Pred, const MBBCFAInfo &Succ);
  void reportCSRError(const MBBCFAInfo &Pred, const MBBCFAInfo &Succ);
  /// Go through each MBB in a function and check that outgoing offset and
  /// register of its predecessors match incoming offset and register of that
  /// MBB, as well as that incoming offset and register of its successors match
  /// outgoing offset and register of the MBB.
  unsigned verify(MachineFunction &MF);
};
}  // namespace
 
char CFIInstrInserter::ID = 0;
INITIALIZE_PASS(CFIInstrInserter, "cfi-instr-inserter",
                "Check CFA info and insert CFI instructions if needed", false,
                false)
FunctionPass *llvm::createCFIInstrInserter() { return new CFIInstrInserter(); }
 
void CFIInstrInserter::calculateCFAInfo(MachineFunction &MF) {
  const TargetRegisterInfo &TRI = *MF.getSubtarget().getRegisterInfo();
  // Initial CFA offset value i.e. the one valid at the beginning of the
  // function.
  int InitialOffset =
      MF.getSubtarget().getFrameLowering()->getInitialCFAOffset(MF);
  // Initial CFA register value i.e. the one valid at the beginning of the
  // function.
  Register InitialRegister =
      MF.getSubtarget().getFrameLowering()->getInitialCFARegister(MF);
  unsigned DwarfInitialRegister = TRI.getDwarfRegNum(InitialRegister, true);
  unsigned NumRegs = TRI.getNumSupportedRegs(MF);
 
  // Initialize MBBMap.
  for (MachineBasicBlock &MBB : MF) {
    MBBCFAInfo &MBBInfo = MBBVector[MBB.getNumber()];
    MBBInfo.MBB = &MBB;
    MBBInfo.IncomingCFAOffset = InitialOffset;
    MBBInfo.OutgoingCFAOffset = InitialOffset;
    MBBInfo.IncomingCFARegister = DwarfInitialRegister;
    MBBInfo.OutgoingCFARegister = DwarfInitialRegister;
    MBBInfo.IncomingCSRSaved.resize(NumRegs);
    MBBInfo.OutgoingCSRSaved.resize(NumRegs);
  }
  CSRLocMap.clear();
 
  // Set in/out cfa info for all blocks in the function. This traversal is based
  // on the assumption that the first block in the function is the entry block
  // i.e. that it has initial cfa offset and register values as incoming CFA
  // information.
  updateSuccCFAInfo(MBBVector[MF.front().getNumber()]);
}
 
void CFIInstrInserter::calculateOutgoingCFAInfo(MBBCFAInfo &MBBInfo) {
  // Outgoing cfa offset set by the block.
  int64_t SetOffset = MBBInfo.IncomingCFAOffset;
  // Outgoing cfa register set by the block.
  unsigned SetRegister = MBBInfo.IncomingCFARegister;
  MachineFunction *MF = MBBInfo.MBB->getParent();
  const std::vector<MCCFIInstruction> &Instrs = MF->getFrameInstructions();
  const TargetRegisterInfo &TRI = *MF->getSubtarget().getRegisterInfo();
  unsigned NumRegs = TRI.getNumSupportedRegs(*MF);
  BitVector CSRSaved(NumRegs), CSRRestored(NumRegs);
 
#ifndef NDEBUG
  int RememberState = 0;
#endif
 
  // Determine cfa offset and register set by the block.
  for (MachineInstr &MI : *MBBInfo.MBB) {
    if (MI.isCFIInstruction()) {
      std::optional<unsigned> CSRReg;
      std::optional<int64_t> CSROffset;
      unsigned CFIIndex = MI.getOperand(0).getCFIIndex();
      const MCCFIInstruction &CFI = Instrs[CFIIndex];
      switch (CFI.getOperation()) {
      case MCCFIInstruction::OpDefCfaRegister:
        SetRegister = CFI.getRegister();
        break;
      case MCCFIInstruction::OpDefCfaOffset:
        SetOffset = CFI.getOffset();
        break;
      case MCCFIInstruction::OpAdjustCfaOffset:
        SetOffset += CFI.getOffset();
        break;
      case MCCFIInstruction::OpDefCfa:
        SetRegister = CFI.getRegister();
        SetOffset = CFI.getOffset();
        break;
      case MCCFIInstruction::OpOffset:
        CSROffset = CFI.getOffset();
        break;
      case MCCFIInstruction::OpRegister:
        CSRReg = CFI.getRegister2();
        break;
      case MCCFIInstruction::OpRelOffset:
        CSROffset = CFI.getOffset() - SetOffset;
        break;
      case MCCFIInstruction::OpRestore:
        CSRRestored.set(CFI.getRegister());
        break;
      case MCCFIInstruction::OpLLVMDefAspaceCfa:
        // TODO: Add support for handling cfi_def_aspace_cfa.
#ifndef NDEBUG
        report_fatal_error(
            "Support for cfi_llvm_def_aspace_cfa not implemented! Value of CFA "
            "may be incorrect!\n");
#endif
        break;
      case MCCFIInstruction::OpRememberState:
        // TODO: Add support for handling cfi_remember_state.
#ifndef NDEBUG
        // Currently we need cfi_remember_state and cfi_restore_state to be in
        // the same BB, so it will not impact outgoing CFA.
        ++RememberState;
        if (RememberState != 1)
          MF->getContext().reportError(
              SMLoc(),
              "Support for cfi_remember_state not implemented! Value of CFA "
              "may be incorrect!\n");
#endif
        break;
      case MCCFIInstruction::OpRestoreState:
        // TODO: Add support for handling cfi_restore_state.
#ifndef NDEBUG
        --RememberState;
        if (RememberState != 0)
          MF->getContext().reportError(
              SMLoc(),
              "Support for cfi_restore_state not implemented! Value of CFA may "
              "be incorrect!\n");
#endif
        break;
      // Other CFI directives do not affect CFA value.
      case MCCFIInstruction::OpUndefined:
      case MCCFIInstruction::OpSameValue:
      case MCCFIInstruction::OpEscape:
      case MCCFIInstruction::OpWindowSave:
      case MCCFIInstruction::OpNegateRAState:
      case MCCFIInstruction::OpNegateRAStateWithPC:
      case MCCFIInstruction::OpGnuArgsSize:
      case MCCFIInstruction::OpLabel:
      case MCCFIInstruction::OpValOffset:
        break;
      }
      assert((!CSRReg.has_value() || !CSROffset.has_value()) &&
             "A register can only be at an offset from CFA or in another "
             "register, but not both!");
      CSRSavedLocation CSRLoc;
      if (CSRReg)
        CSRLoc = CSRSavedLocation::createRegister(*CSRReg);
      else if (CSROffset)
        CSRLoc = CSRSavedLocation::createCFAOffset(*CSROffset);
      if (CSRLoc.isValid()) {
        auto [It, Inserted] = CSRLocMap.insert({CFI.getRegister(), CSRLoc});
        if (!Inserted && It->second != CSRLoc)
          reportFatalInternalError(
              "Different saved locations for the same CSR");
        CSRSaved.set(CFI.getRegister());
      }
    }
  }
 
#ifndef NDEBUG
  if (RememberState != 0)
    MF->getContext().reportError(
        SMLoc(),
        "Support for cfi_remember_state not implemented! Value of CFA may be "
        "incorrect!\n");
#endif
 
  MBBInfo.Processed = true;
 
  // Update outgoing CFA info.
  MBBInfo.OutgoingCFAOffset = SetOffset;
  MBBInfo.OutgoingCFARegister = SetRegister;
 
  // Update outgoing CSR info.
  BitVector::apply([](auto x, auto y, auto z) { return (x | y) & ~z; },
                   MBBInfo.OutgoingCSRSaved, MBBInfo.IncomingCSRSaved, CSRSaved,
                   CSRRestored);
}
 
void CFIInstrInserter::updateSuccCFAInfo(MBBCFAInfo &MBBInfo) {
  SmallVector<MachineBasicBlock *, 4> Stack;
  Stack.push_back(MBBInfo.MBB);
 
  do {
    MachineBasicBlock *Current = Stack.pop_back_val();
    MBBCFAInfo &CurrentInfo = MBBVector[Current->getNumber()];
    calculateOutgoingCFAInfo(CurrentInfo);
    for (auto *Succ : CurrentInfo.MBB->successors()) {
      MBBCFAInfo &SuccInfo = MBBVector[Succ->getNumber()];
      if (!SuccInfo.Processed) {
        SuccInfo.IncomingCFAOffset = CurrentInfo.OutgoingCFAOffset;
        SuccInfo.IncomingCFARegister = CurrentInfo.OutgoingCFARegister;
        SuccInfo.IncomingCSRSaved = CurrentInfo.OutgoingCSRSaved;
        Stack.push_back(Succ);
      }
    }
  } while (!Stack.empty());
}
 
bool CFIInstrInserter::insertCFIInstrs(MachineFunction &MF) {
  const MBBCFAInfo *PrevMBBInfo = &MBBVector[MF.front().getNumber()];
  const TargetInstrInfo *TII = MF.getSubtarget().getInstrInfo();
  bool InsertedCFIInstr = false;
 
  BitVector SetDifference;
  for (MachineBasicBlock &MBB : MF) {
    // Skip the first MBB in a function
    if (MBB.getNumber() == MF.front().getNumber()) continue;
 
    const MBBCFAInfo &MBBInfo = MBBVector[MBB.getNumber()];
    auto MBBI = MBBInfo.MBB->begin();
    DebugLoc DL = MBBInfo.MBB->findDebugLoc(MBBI);
 
    // If the current MBB will be placed in a unique section, a full DefCfa
    // must be emitted.
    const bool ForceFullCFA = MBB.isBeginSection();
 
    if ((PrevMBBInfo->OutgoingCFAOffset != MBBInfo.IncomingCFAOffset &&
         PrevMBBInfo->OutgoingCFARegister != MBBInfo.IncomingCFARegister) ||
        ForceFullCFA) {
      // If both outgoing offset and register of a previous block don't match
      // incoming offset and register of this block, or if this block begins a
      // section, add a def_cfa instruction with the correct offset and
      // register for this block.
      unsigned CFIIndex = MF.addFrameInst(MCCFIInstruction::cfiDefCfa(
          nullptr, MBBInfo.IncomingCFARegister, getCorrectCFAOffset(&MBB)));
      BuildMI(*MBBInfo.MBB, MBBI, DL, TII->get(TargetOpcode::CFI_INSTRUCTION))
          .addCFIIndex(CFIIndex);
      InsertedCFIInstr = true;
    } else if (PrevMBBInfo->OutgoingCFAOffset != MBBInfo.IncomingCFAOffset) {
      // If outgoing offset of a previous block doesn't match incoming offset
      // of this block, add a def_cfa_offset instruction with the correct
      // offset for this block.
      unsigned CFIIndex = MF.addFrameInst(MCCFIInstruction::cfiDefCfaOffset(
          nullptr, getCorrectCFAOffset(&MBB)));
      BuildMI(*MBBInfo.MBB, MBBI, DL, TII->get(TargetOpcode::CFI_INSTRUCTION))
          .addCFIIndex(CFIIndex);
      InsertedCFIInstr = true;
    } else if (PrevMBBInfo->OutgoingCFARegister !=
               MBBInfo.IncomingCFARegister) {
      unsigned CFIIndex =
          MF.addFrameInst(MCCFIInstruction::createDefCfaRegister(
              nullptr, MBBInfo.IncomingCFARegister));
      BuildMI(*MBBInfo.MBB, MBBI, DL, TII->get(TargetOpcode::CFI_INSTRUCTION))
          .addCFIIndex(CFIIndex);
      InsertedCFIInstr = true;
    }
 
    if (ForceFullCFA) {
      MF.getSubtarget().getFrameLowering()->emitCalleeSavedFrameMovesFullCFA(
          *MBBInfo.MBB, MBBI);
      InsertedCFIInstr = true;
      PrevMBBInfo = &MBBInfo;
      continue;
    }
 
    BitVector::apply([](auto x, auto y) { return x & ~y; }, SetDifference,
                     PrevMBBInfo->OutgoingCSRSaved, MBBInfo.IncomingCSRSaved);
    for (int Reg : SetDifference.set_bits()) {
      unsigned CFIIndex =
          MF.addFrameInst(MCCFIInstruction::createRestore(nullptr, Reg));
      BuildMI(*MBBInfo.MBB, MBBI, DL, TII->get(TargetOpcode::CFI_INSTRUCTION))
          .addCFIIndex(CFIIndex);
      InsertedCFIInstr = true;
    }
 
    BitVector::apply([](auto x, auto y) { return x & ~y; }, SetDifference,
                     MBBInfo.IncomingCSRSaved, PrevMBBInfo->OutgoingCSRSaved);
    for (int Reg : SetDifference.set_bits()) {
      auto it = CSRLocMap.find(Reg);
      assert(it != CSRLocMap.end() && "Reg should have an entry in CSRLocMap");
      unsigned CFIIndex;
      CSRSavedLocation RO = it->second;
      switch (RO.K) {
      case CSRSavedLocation::CFAOffset: {
        CFIIndex = MF.addFrameInst(
            MCCFIInstruction::createOffset(nullptr, Reg, RO.getOffset()));
        break;
      }
      case CSRSavedLocation::Register: {
        CFIIndex = MF.addFrameInst(
            MCCFIInstruction::createRegister(nullptr, Reg, RO.getRegister()));
        break;
      }
      default:
        llvm_unreachable("Invalid CSRSavedLocation!");
      }
      BuildMI(*MBBInfo.MBB, MBBI, DL, TII->get(TargetOpcode::CFI_INSTRUCTION))
          .addCFIIndex(CFIIndex);
      InsertedCFIInstr = true;
    }
 
    PrevMBBInfo = &MBBInfo;
  }
  return InsertedCFIInstr;
}
 
void CFIInstrInserter::reportCFAError(const MBBCFAInfo &Pred,
                                      const MBBCFAInfo &Succ) {
  errs() << "*** Inconsistent CFA register and/or offset between pred and succ "
            "***\n";
  errs() << "Pred: " << Pred.MBB->getName() << " #" << Pred.MBB->getNumber()
         << " in " << Pred.MBB->getParent()->getName()
         << " outgoing CFA Reg:" << Pred.OutgoingCFARegister << "\n";
  errs() << "Pred: " << Pred.MBB->getName() << " #" << Pred.MBB->getNumber()
         << " in " << Pred.MBB->getParent()->getName()
         << " outgoing CFA Offset:" << Pred.OutgoingCFAOffset << "\n";
  errs() << "Succ: " << Succ.MBB->getName() << " #" << Succ.MBB->getNumber()
         << " incoming CFA Reg:" << Succ.IncomingCFARegister << "\n";
  errs() << "Succ: " << Succ.MBB->getName() << " #" << Succ.MBB->getNumber()
         << " incoming CFA Offset:" << Succ.IncomingCFAOffset << "\n";
}
 
void CFIInstrInserter::reportCSRError(const MBBCFAInfo &Pred,
                                      const MBBCFAInfo &Succ) {
  errs() << "*** Inconsistent CSR Saved between pred and succ in function "
         << Pred.MBB->getParent()->getName() << " ***\n";
  errs() << "Pred: " << Pred.MBB->getName() << " #" << Pred.MBB->getNumber()
         << " outgoing CSR Saved: ";
  for (int Reg : Pred.OutgoingCSRSaved.set_bits())
    errs() << Reg << " ";
  errs() << "\n";
  errs() << "Succ: " << Succ.MBB->getName() << " #" << Succ.MBB->getNumber()
         << " incoming CSR Saved: ";
  for (int Reg : Succ.IncomingCSRSaved.set_bits())
    errs() << Reg << " ";
  errs() << "\n";
}
 
unsigned CFIInstrInserter::verify(MachineFunction &MF) {
  unsigned ErrorNum = 0;
  for (auto *CurrMBB : depth_first(&MF)) {
    const MBBCFAInfo &CurrMBBInfo = MBBVector[CurrMBB->getNumber()];
    for (MachineBasicBlock *Succ : CurrMBB->successors()) {
      const MBBCFAInfo &SuccMBBInfo = MBBVector[Succ->getNumber()];
      // Check that incoming offset and register values of successors match the
      // outgoing offset and register values of CurrMBB
      if (SuccMBBInfo.IncomingCFAOffset != CurrMBBInfo.OutgoingCFAOffset ||
          SuccMBBInfo.IncomingCFARegister != CurrMBBInfo.OutgoingCFARegister) {
        // Inconsistent offsets/registers are ok for 'noreturn' blocks because
        // we don't generate epilogues inside such blocks.
        if (SuccMBBInfo.MBB->succ_empty() && !SuccMBBInfo.MBB->isReturnBlock())
          continue;
        reportCFAError(CurrMBBInfo, SuccMBBInfo);
        ErrorNum++;
      }
      // Check that IncomingCSRSaved of every successor matches the
      // OutgoingCSRSaved of CurrMBB
      if (SuccMBBInfo.IncomingCSRSaved != CurrMBBInfo.OutgoingCSRSaved) {
        reportCSRError(CurrMBBInfo, SuccMBBInfo);
        ErrorNum++;
      }
    }
  }
  return ErrorNum;
}