Commit c3e65556 authored by Krzysztof Pszeniczny's avatar Krzysztof Pszeniczny Committed by Sriraman Tallam
Browse files

Call Frame Information (CFI) Handling for Basic Block Sections 

This patch handles CFI with basic block sections, which unlike DebugInfo does
not support ranges. The DWARF standard explicitly requires emitting separate
CFI Frame Descriptor Entries for each contiguous fragment of a function. Thus,
the CFI information for all callee-saved registers (possibly including the
frame pointer, if necessary) have to be emitted along with redefining the
Call Frame Address (CFA), viz. where the current frame starts.

CFI directives are emitted in FDE’s in the object file with a low_pc, high_pc
specification. So, a single FDE must point to a contiguous code region unlike
debug info which has the support for ranges. This is what complicates CFI for
basic block sections.

Now, what happens when we start placing individual basic blocks in unique
sections:

* Basic block sections allow the linker to randomly reorder basic blocks in the
address space such that a given basic block can become non-contiguous with the
original function.
* The different basic block sections can no longer share the cfi_startproc and
cfi_endproc directives. So, each basic block section should emit this
independently.
* Each (cfi_startproc, cfi_endproc) directive will result in a new FDE that
caters to that basic block section.
* Now, this basic block section needs to duplicate the information from the
entry block to compute the CFA as it is an independent entity. It cannot refer
to the FDE of the original function and hence must duplicate all the stuff that
is needed to compute the CFA on its own.
* We are working on a de-duplication patch that can share common information in
FDEs in a CIE (Common Information Entry) and we will present this as a follow up
patch. This can significantly reduce the duplication overhead and is
particularly useful when several basic block sections are created.
* The CFI directives are emitted similarly for registers that are pushed onto
the stack, like callee saved registers in the prologue. There are cfi
directives that emit how to retrieve the value of the register at that point
when the push happened. This has to be duplicated too in a basic block that is
floated as a separate section.

Differential Revision: https://reviews.llvm.org/D79978
parent 16f777f4

llvm/include/llvm/CodeGen/TargetFrameLowering.h

+11 −0
Original line number Diff line number Diff line
@@ -202,6 +202,17 @@ public: 
  virtual void emitEpilogue(MachineFunction &MF,

                            MachineBasicBlock &MBB) const = 0;



  /// With basic block sections, emit callee saved frame moves for basic blocks

  /// that are in a different section.

  virtual void

  emitCalleeSavedFrameMoves(MachineBasicBlock &MBB,

                            MachineBasicBlock::iterator MBBI) const {}



  virtual void emitCalleeSavedFrameMoves(MachineBasicBlock &MBB,

                                         MachineBasicBlock::iterator MBBI,

                                         const DebugLoc &DL,

                                         bool IsPrologue) const {}



  /// Replace a StackProbe stub (if any) with the actual probe code inline

  virtual void inlineStackProbe(MachineFunction &MF,

                                MachineBasicBlock &PrologueMBB) const {}

llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp

+15 −3
Original line number Diff line number Diff line
@@ -3067,18 +3067,30 @@ void AsmPrinter::emitBasicBlockStart(const MachineBasicBlock &MBB) { 
    if (isVerbose() && MBB.hasLabelMustBeEmitted()) {

      OutStreamer->AddComment("Label of block must be emitted");

    }

    auto *BBSymbol = MBB.getSymbol();

    // Switch to a new section if this basic block must begin a section.

    if (MBB.isBeginSection()) {

      OutStreamer->SwitchSection(

          getObjFileLowering().getSectionForMachineBasicBlock(MF->getFunction(),

                                                              MBB, TM));

      CurrentSectionBeginSym = MBB.getSymbol();

      CurrentSectionBeginSym = BBSymbol;

    }

    OutStreamer->emitLabel(MBB.getSymbol());

    OutStreamer->emitLabel(BBSymbol);

    // With BB sections, each basic block must handle CFI information on its own

    // if it begins a section.

    if (MBB.isBeginSection())

      for (const HandlerInfo &HI : Handlers)

        HI.Handler->beginBasicBlock(MBB);

  }

}



void AsmPrinter::emitBasicBlockEnd(const MachineBasicBlock &MBB) {}

void AsmPrinter::emitBasicBlockEnd(const MachineBasicBlock &MBB) {

  // Check if CFI information needs to be updated for this MBB with basic block

  // sections.

  if (MBB.isEndSection())

    for (const HandlerInfo &HI : Handlers)

      HI.Handler->endBasicBlock(MBB);

}



void AsmPrinter::emitVisibility(MCSymbol *Sym, unsigned Visibility,

                                bool IsDefinition) const {

llvm/lib/CodeGen/AsmPrinter/DwarfCFIException.cpp

+10 −1
Original line number Diff line number Diff line
@@ -47,7 +47,7 @@ void DwarfCFIExceptionBase::markFunctionEnd() { 
}



void DwarfCFIExceptionBase::endFragment() {

  if (shouldEmitCFI)

  if (shouldEmitCFI && !Asm->MF->hasBBSections())

    Asm->OutStreamer->emitCFIEndProc();

}
@@ -172,3 +172,12 @@ void DwarfCFIException::endFunction(const MachineFunction *MF) { 


  emitExceptionTable();

}



void DwarfCFIException::beginBasicBlock(const MachineBasicBlock &MBB) {

  beginFragment(&MBB, getExceptionSym);

}



void DwarfCFIException::endBasicBlock(const MachineBasicBlock &MBB) {

  if (shouldEmitCFI)

    Asm->OutStreamer->emitCFIEndProc();

}

llvm/lib/CodeGen/AsmPrinter/DwarfException.h

+3 −0
Original line number Diff line number Diff line
@@ -66,6 +66,9 @@ public: 


  void beginFragment(const MachineBasicBlock *MBB,

                     ExceptionSymbolProvider ESP) override;



  void beginBasicBlock(const MachineBasicBlock &MBB) override;

  void endBasicBlock(const MachineBasicBlock &MBB) override;

};



class LLVM_LIBRARY_VISIBILITY ARMException : public DwarfCFIExceptionBase {

llvm/lib/CodeGen/CFIInstrInserter.cpp

+31 −20
Original line number Diff line number Diff line
@@ -303,28 +303,31 @@ bool CFIInstrInserter::insertCFIInstrs(MachineFunction &MF) { 
    auto MBBI = MBBInfo.MBB->begin();

    DebugLoc DL = MBBInfo.MBB->findDebugLoc(MBBI);



    if (PrevMBBInfo->OutgoingCFAOffset != MBBInfo.IncomingCFAOffset) {

    // 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, add a def_cfa instruction

      // with the correct offset and register for this block.

      if (PrevMBBInfo->OutgoingCFARegister != MBBInfo.IncomingCFARegister) {

      // 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.

      } else {

      unsigned CFIIndex = MF.addFrameInst(MCCFIInstruction::cfiDefCfaOffset(

          nullptr, getCorrectCFAOffset(&MBB)));

      BuildMI(*MBBInfo.MBB, MBBI, DL, TII->get(TargetOpcode::CFI_INSTRUCTION))

          .addCFIIndex(CFIIndex);

      }

      InsertedCFIInstr = true;

      // If outgoing register of a previous block doesn't match incoming

      // register of this block, add a def_cfa_register instruction with the

      // correct register for this block.

    } else if (PrevMBBInfo->OutgoingCFARegister !=

               MBBInfo.IncomingCFARegister) {

      unsigned CFIIndex =
@@ -335,6 +338,14 @@ bool CFIInstrInserter::insertCFIInstrs(MachineFunction &MF) { 
      InsertedCFIInstr = true;

    }



    if (ForceFullCFA) {

      MF.getSubtarget().getFrameLowering()->emitCalleeSavedFrameMoves(

          *MBBInfo.MBB, MBBI);

      InsertedCFIInstr = true;

      PrevMBBInfo = &MBBInfo;

      continue;

    }



    BitVector SetDifference = PrevMBBInfo->OutgoingCSRSaved;

    SetDifference.reset(MBBInfo.IncomingCSRSaved);

    for (int Reg : SetDifference.set_bits()) {