llvm-project/llvm/lib/Target/Mips/MipsTargetObjectFile.cpp
Rafael Espindola 449711cb36 Stop producing .data.rel sections.
If a section is rw, it is irrelevant if the dynamic linker will write to
it or not.

It looks like llvm implemented this because gcc was doing it. It looks
like gcc implemented this in the hope that it would put all the
relocated items close together and speed up the dynamic linker.

There are two problem with this:
* It doesn't work. Both bfd and gold will map .data.rel to .data and
  concatenate the input sections in the order they are seen.
* If we want a feature like that, it can be implemented directly in the
  linker since it knowns where the dynamic relocations are.

llvm-svn: 253436
2015-11-18 06:02:15 +00:00

149 lines
5.5 KiB
C++

//===-- MipsTargetObjectFile.cpp - Mips Object Files ----------------------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
#include "MipsTargetObjectFile.h"
#include "MipsSubtarget.h"
#include "MipsTargetMachine.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/Support/ELF.h"
#include "llvm/Target/TargetMachine.h"
using namespace llvm;
static cl::opt<unsigned>
SSThreshold("mips-ssection-threshold", cl::Hidden,
cl::desc("Small data and bss section threshold size (default=8)"),
cl::init(8));
static cl::opt<bool>
LocalSData("mlocal-sdata", cl::Hidden,
cl::desc("MIPS: Use gp_rel for object-local data."),
cl::init(true));
static cl::opt<bool>
ExternSData("mextern-sdata", cl::Hidden,
cl::desc("MIPS: Use gp_rel for data that is not defined by the "
"current object."),
cl::init(true));
void MipsTargetObjectFile::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 MipsTargetMachine &>(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) {
// 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 MipsTargetObjectFile::
IsGlobalInSmallSection(const GlobalValue *GV, 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 (GV->isDeclaration() || GV->hasAvailableExternallyLinkage())
return IsGlobalInSmallSectionImpl(GV, TM);
return IsGlobalInSmallSection(GV, TM, getKindForGlobal(GV, TM));
}
/// Return true if this global address should be placed into small data/bss
/// section.
bool MipsTargetObjectFile::
IsGlobalInSmallSection(const GlobalValue *GV, const TargetMachine &TM,
SectionKind Kind) const {
return (IsGlobalInSmallSectionImpl(GV, TM) &&
(Kind.isData() || Kind.isBSS() || Kind.isCommon()));
}
/// 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 MipsTargetObjectFile::
IsGlobalInSmallSectionImpl(const GlobalValue *GV,
const TargetMachine &TM) const {
const MipsSubtarget &Subtarget =
*static_cast<const MipsTargetMachine &>(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;
// Enforce -mlocal-sdata.
if (!LocalSData && GV->hasLocalLinkage())
return false;
// Enforce -mextern-sdata.
if (!ExternSData && ((GV->hasExternalLinkage() && GV->isDeclaration()) ||
GV->hasCommonLinkage()))
return false;
Type *Ty = GV->getType()->getElementType();
return IsInSmallSection(
GV->getParent()->getDataLayout().getTypeAllocSize(Ty));
}
MCSection *
MipsTargetObjectFile::SelectSectionForGlobal(const GlobalValue *GV,
SectionKind Kind, Mangler &Mang,
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(GV, TM, Kind))
return SmallBSSSection;
if (Kind.isData() && IsGlobalInSmallSection(GV, TM, Kind))
return SmallDataSection;
// Otherwise, we work the same as ELF.
return TargetLoweringObjectFileELF::SelectSectionForGlobal(GV, Kind, Mang,TM);
}
/// Return true if this constant should be placed into small data section.
bool MipsTargetObjectFile::IsConstantInSmallSection(
const DataLayout &DL, const Constant *CN, const TargetMachine &TM) const {
return (static_cast<const MipsTargetMachine &>(TM)
.getSubtargetImpl()
->useSmallSection() &&
LocalSData && IsInSmallSection(DL.getTypeAllocSize(CN->getType())));
}
/// Return true if this constant should be placed into small data section.
MCSection *MipsTargetObjectFile::getSectionForConstant(
const DataLayout &DL, SectionKind Kind, const Constant *C) const {
if (IsConstantInSmallSection(DL, C, *TM))
return SmallDataSection;
// Otherwise, we work the same as ELF.
return TargetLoweringObjectFileELF::getSectionForConstant(DL, Kind, C);
}