Cpu0TargetObjectFile.cpp

// Copyright 2022 All Rights Reserved.
// Author: lanzongwei541@gmail.com (lanzongwei)
//
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception

#include "Cpu0TargetObjectFile.h"

#include "Cpu0Subtarget.h"
#include "Cpu0TargetMachine.h"
#include "llvm/BinaryFormat/ELF.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/DerivedTypes.h"
#include "llvm/IR/GlobalVariable.h"
#include "llvm/MC/MCContext.h"
#include "llvm/MC/MCSectionELF.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Target/TargetMachine.h"
using namespace llvm;

static cl::opt<unsigned> SSThreshold(
    "cpu0-ssection-threshold", cl::Hidden,
    cl::desc("Small data and bss section threshold size (default=8)"),
    cl::init(8));

void Cpu0TargetObjectFile::Initialize(MCContext &Ctx, const TargetMachine &TM) {
  TargetLoweringObjectFileELF::Initialize(Ctx, TM);
  InitializeELF(TM.Options.UseInitArray);

  SmallDataSection = getContext().getELFSection(
      ".sdata", ELF::SHT_PROGBITS, ELF::SHF_WRITE | ELF::SHF_ALLOC);

  SmallBSSSection = getContext().getELFSection(".sbss", ELF::SHT_NOBITS,
                                               ELF::SHF_WRITE | ELF::SHF_ALLOC);
  this->TM = &static_cast<const Cpu0TargetMachine &>(TM);
}

// A address must be loaded from a small section if its size is less than the
// small section size threshold. Data in this section must be addressed using
// gp_rel operator.
static bool IsInSmallSection(uint64_t Size) {
  return Size > 0 && Size <= SSThreshold;
}

bool Cpu0TargetObjectFile::IsGlobalInSmallSection(
    const GlobalObject *GO, const TargetMachine &TM) const {
  // We first check the case where global is a declaration, because finding
  // section kind using getKindForGlobal() is only allowed for global
  // definitions.
  if (GO->isDeclaration() || GO->hasAvailableExternallyLinkage())
    return IsGlobalInSmallSectionImpl(GO, TM);

  return IsGlobalInSmallSection(GO, TM, getKindForGlobal(GO, TM));
}

/// IsGlobalInSmallSection - Return true if this global address should be
/// placed into small data/bss section.
bool Cpu0TargetObjectFile::IsGlobalInSmallSection(const GlobalObject *GO,
                                                  const TargetMachine &TM,
                                                  SectionKind Kind) const {
  return IsGlobalInSmallSectionImpl(GO, TM) &&
         (Kind.isData() || Kind.isBSS() || Kind.isCommon() ||
          Kind.isReadOnly());
}

/// Return true if this global address should be placed into small data/bss
/// section. This method does all the work, except for checking the section
/// kind.
bool Cpu0TargetObjectFile::IsGlobalInSmallSectionImpl(
    const GlobalObject *GV, const TargetMachine &TM) const {
  const Cpu0Subtarget &Subtarget =
      *static_cast<const Cpu0TargetMachine &>(TM).getSubtargetImpl();

  // Return if small section is not available.
  if (!Subtarget.useSmallSection())
    return false;

  // Only global variables, not functions.
  const GlobalVariable *GVA = dyn_cast<GlobalVariable>(GV);
  if (!GVA)
    return false;

  Type *Ty = GV->getValueType();
  return IsInSmallSection(
      GV->getParent()->getDataLayout().getTypeAllocSize(Ty));
}

MCSection *Cpu0TargetObjectFile::SelectSectionForGlobal(
    const GlobalObject *GO, SectionKind Kind, const TargetMachine &TM) const {
  // TODO: Could also support "weak" symbols as well with ".gnu.linkonce.s.*"
  // sections?

  // Handle Small Section classification here.
  if (Kind.isBSS() && IsGlobalInSmallSection(GO, TM, Kind))
    return SmallBSSSection;
  if (Kind.isData() && IsGlobalInSmallSection(GO, TM, Kind))
    return SmallDataSection;
  if (Kind.isReadOnly() && IsGlobalInSmallSection(GO, TM, Kind))
    return SmallDataSection;

  // Otherwise, we work the same as ELF.
  return TargetLoweringObjectFileELF::SelectSectionForGlobal(GO, Kind, TM);
}