LanaiTargetObjectFile.cpp

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//
// 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
//
//===----------------------------------------------------------------------===//
 
#include "LanaiTargetObjectFile.h"
 
#include "llvm/BinaryFormat/ELF.h"
#include "llvm/IR/DataLayout.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(
    "lanai-ssection-threshold", cl::Hidden,
    cl::desc("Small data and bss section threshold size (default=0)"),
    cl::init(0));
 
void LanaiTargetObjectFile::Initialize(MCContext &Ctx,
                                       const TargetMachine &TM) {
  TargetLoweringObjectFileELF::Initialize(Ctx, TM);
 
  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);
}
 
// 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) {
  // gcc has traditionally not treated zero-sized objects as small data, so this
  // is effectively part of the ABI.
  return Size > 0 && Size <= SSThreshold;
}
 
// Return true if this global address should be placed into small data/bss
// section.
bool LanaiTargetObjectFile::isGlobalInSmallSection(
    const GlobalObject *GO, const TargetMachine &TM) const {
  if (GO == nullptr) return TM.getCodeModel() == CodeModel::Small;
 
  // 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));
}
 
// Return true if this global address should be placed into small data/bss
// section.
bool LanaiTargetObjectFile::isGlobalInSmallSection(const GlobalObject *GO,
                                                   const TargetMachine &TM,
                                                   SectionKind Kind) const {
  return isGlobalInSmallSectionImpl(GO, TM);
}
 
// 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 LanaiTargetObjectFile::isGlobalInSmallSectionImpl(
    const GlobalObject *GO, const TargetMachine &TM) const {
  const auto *GVA = dyn_cast<GlobalVariable>(GO);
 
  // If not a GlobalVariable, only consider the code model.
  if (!GVA) return TM.getCodeModel() == CodeModel::Small;
 
  // Global values placed in sections starting with .ldata do not fit in
  // 21-bits, so always use large memory access for them. FIXME: This is a
  // workaround for a tool limitation.
  if (GVA->getSection().starts_with(".ldata"))
    return false;
 
  if (TM.getCodeModel() == CodeModel::Small)
    return true;
 
  if (GVA->hasLocalLinkage())
    return false;
 
  if (((GVA->hasExternalLinkage() && GVA->isDeclaration()) ||
       GVA->hasCommonLinkage()))
    return false;
 
  Type *Ty = GVA->getValueType();
  return isInSmallSection(
      GVA->getDataLayout().getTypeAllocSize(Ty));
}
 
MCSection *LanaiTargetObjectFile::SelectSectionForGlobal(
    const GlobalObject *GO, SectionKind Kind, const TargetMachine &TM) const {
  // Handle Small Section classification here.
  if (Kind.isBSS() && isGlobalInSmallSection(GO, TM, Kind))
    return SmallBSSSection;
  if (Kind.isData() && isGlobalInSmallSection(GO, TM, Kind))
    return SmallDataSection;
 
  // Otherwise, we work the same as ELF.
  return TargetLoweringObjectFileELF::SelectSectionForGlobal(GO, Kind, TM);
}
 
/// Return true if this constant should be placed into small data section.
bool LanaiTargetObjectFile::isConstantInSmallSection(const DataLayout &DL,
                                                     const Constant *CN) const {
  return isInSmallSection(DL.getTypeAllocSize(CN->getType()));
}
 
MCSection *LanaiTargetObjectFile::getSectionForConstant(
    const DataLayout &DL, SectionKind Kind, const Constant *C,
    Align &Alignment) const {
  if (isConstantInSmallSection(DL, C))
    return SmallDataSection;
 
  // Otherwise, we work the same as ELF.
  return TargetLoweringObjectFileELF::getSectionForConstant(DL, Kind, C,
                                                            Alignment);
}