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//===-- llvm/Target/TargetLoweringObjectFile.cpp - Object File Info -------===//
// The LLVM Compiler Infrastructure
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
// This file implements classes used to handle lowerings specific to common
// object file formats.
#include "llvm/CodeGen/TargetLoweringObjectFile.h"
#include "llvm/BinaryFormat/Dwarf.h"
#include "llvm/CodeGen/TargetLowering.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/DerivedTypes.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/GlobalVariable.h"
#include "llvm/IR/Mangler.h"
#include "llvm/MC/MCContext.h"
#include "llvm/MC/MCExpr.h"
#include "llvm/MC/MCStreamer.h"
#include "llvm/MC/MCSymbol.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Target/TargetMachine.h"
#include "llvm/Target/TargetOptions.h"
using namespace llvm;
// Generic Code
/// Initialize - this method must be called before any actual lowering is
/// done. This specifies the current context for codegen, and gives the
/// lowering implementations a chance to set up their default sections.
void TargetLoweringObjectFile::Initialize(MCContext &ctx,
const TargetMachine &TM) {
Ctx = &ctx;
// `Initialize` can be called more than once.
delete Mang;
Mang = new Mangler();
InitMCObjectFileInfo(TM.getTargetTriple(), TM.isPositionIndependent(), *Ctx,
TM.getCodeModel() == CodeModel::Large);
TargetLoweringObjectFile::~TargetLoweringObjectFile() {
delete Mang;
static bool isSuitableForBSS(const GlobalVariable *GV, bool NoZerosInBSS) {
const Constant *C = GV->getInitializer();
// Must have zero initializer.
if (!C->isNullValue())
return false;
// Leave constant zeros in readonly constant sections, so they can be shared.
if (GV->isConstant())
return false;
// If the global has an explicit section specified, don't put it in BSS.
if (GV->hasSection())
return false;
// If -nozero-initialized-in-bss is specified, don't ever use BSS.
if (NoZerosInBSS)
return false;
// Otherwise, put it in BSS!
return true;
/// IsNullTerminatedString - Return true if the specified constant (which is
/// known to have a type that is an array of 1/2/4 byte elements) ends with a
/// nul value and contains no other nuls in it. Note that this is more general
/// than ConstantDataSequential::isString because we allow 2 & 4 byte strings.
static bool IsNullTerminatedString(const Constant *C) {
// First check: is we have constant array terminated with zero
if (const ConstantDataSequential *CDS = dyn_cast<ConstantDataSequential>(C)) {
unsigned NumElts = CDS->getNumElements();
assert(NumElts != 0 && "Can't have an empty CDS");
if (CDS->getElementAsInteger(NumElts-1) != 0)
return false; // Not null terminated.
// Verify that the null doesn't occur anywhere else in the string.
for (unsigned i = 0; i != NumElts-1; ++i)
if (CDS->getElementAsInteger(i) == 0)
return false;
return true;
// Another possibility: [1 x i8] zeroinitializer
if (isa<ConstantAggregateZero>(C))
return cast<ArrayType>(C->getType())->getNumElements() == 1;
return false;
MCSymbol *TargetLoweringObjectFile::getSymbolWithGlobalValueBase(
const GlobalValue *GV, StringRef Suffix, const TargetMachine &TM) const {
SmallString<60> NameStr;
NameStr += GV->getParent()->getDataLayout().getPrivateGlobalPrefix();
TM.getNameWithPrefix(NameStr, GV, *Mang);
NameStr.append(Suffix.begin(), Suffix.end());
return Ctx->getOrCreateSymbol(NameStr);
MCSymbol *TargetLoweringObjectFile::getCFIPersonalitySymbol(
const GlobalValue *GV, const TargetMachine &TM,
MachineModuleInfo *MMI) const {
return TM.getSymbol(GV);
void TargetLoweringObjectFile::emitPersonalityValue(MCStreamer &Streamer,
const DataLayout &,
const MCSymbol *Sym) const {
/// getKindForGlobal - This is a top-level target-independent classifier for
/// a global variable. Given an global variable and information from TM, it
/// classifies the global in a variety of ways that make various target
/// implementations simpler. The target implementation is free to ignore this
/// extra info of course.
SectionKind TargetLoweringObjectFile::getKindForGlobal(const GlobalObject *GO,
const TargetMachine &TM){
assert(!GO->isDeclaration() && !GO->hasAvailableExternallyLinkage() &&
"Can only be used for global definitions");
Reloc::Model ReloModel = TM.getRelocationModel();
// Early exit - functions should be always in text sections.
const auto *GVar = dyn_cast<GlobalVariable>(GO);
if (!GVar)
return SectionKind::getText();
// Handle thread-local data first.
if (GVar->isThreadLocal()) {
if (isSuitableForBSS(GVar, TM.Options.NoZerosInBSS))
return SectionKind::getThreadBSS();
return SectionKind::getThreadData();
// Variables with common linkage always get classified as common.
if (GVar->hasCommonLinkage())
return SectionKind::getCommon();
// Variable can be easily put to BSS section.
if (isSuitableForBSS(GVar, TM.Options.NoZerosInBSS)) {
if (GVar->hasLocalLinkage())
return SectionKind::getBSSLocal();
else if (GVar->hasExternalLinkage())
return SectionKind::getBSSExtern();
return SectionKind::getBSS();
const Constant *C = GVar->getInitializer();
// If the global is marked constant, we can put it into a mergable section,
// a mergable string section, or general .data if it contains relocations.
if (GVar->isConstant()) {
// If the initializer for the global contains something that requires a
// relocation, then we may have to drop this into a writable data section
// even though it is marked const.
if (!C->needsRelocation()) {
// If the global is required to have a unique address, it can't be put
// into a mergable section: just drop it into the general read-only
// section instead.
if (!GVar->hasGlobalUnnamedAddr())
return SectionKind::getReadOnly();
// If initializer is a null-terminated string, put it in a "cstring"
// section of the right width.
if (ArrayType *ATy = dyn_cast<ArrayType>(C->getType())) {
if (IntegerType *ITy =
dyn_cast<IntegerType>(ATy->getElementType())) {
if ((ITy->getBitWidth() == 8 || ITy->getBitWidth() == 16 ||
ITy->getBitWidth() == 32) &&
IsNullTerminatedString(C)) {
if (ITy->getBitWidth() == 8)
return SectionKind::getMergeable1ByteCString();
if (ITy->getBitWidth() == 16)
return SectionKind::getMergeable2ByteCString();
assert(ITy->getBitWidth() == 32 && "Unknown width");
return SectionKind::getMergeable4ByteCString();
// Otherwise, just drop it into a mergable constant section. If we have
// a section for this size, use it, otherwise use the arbitrary sized
// mergable section.
switch (
GVar->getParent()->getDataLayout().getTypeAllocSize(C->getType())) {
case 4: return SectionKind::getMergeableConst4();
case 8: return SectionKind::getMergeableConst8();
case 16: return SectionKind::getMergeableConst16();
case 32: return SectionKind::getMergeableConst32();
return SectionKind::getReadOnly();
} else {
// In static, ROPI and RWPI relocation models, the linker will resolve
// all addresses, so the relocation entries will actually be constants by
// the time the app starts up. However, we can't put this into a
// mergable section, because the linker doesn't take relocations into
// consideration when it tries to merge entries in the section.
if (ReloModel == Reloc::Static || ReloModel == Reloc::ROPI ||
ReloModel == Reloc::RWPI || ReloModel == Reloc::ROPI_RWPI)
return SectionKind::getReadOnly();
// Otherwise, the dynamic linker needs to fix it up, put it in the
// writable data.rel section.
return SectionKind::getReadOnlyWithRel();
// Okay, this isn't a constant.
return SectionKind::getData();
/// This method computes the appropriate section to emit the specified global
/// variable or function definition. This should not be passed external (or
/// available externally) globals.
MCSection *TargetLoweringObjectFile::SectionForGlobal(
const GlobalObject *GO, SectionKind Kind, const TargetMachine &TM) const {
// Select section name.
if (GO->hasSection())
return getExplicitSectionGlobal(GO, Kind, TM);
if (auto *GVar = dyn_cast<GlobalVariable>(GO)) {
auto Attrs = GVar->getAttributes();
if ((Attrs.hasAttribute("bss-section") && Kind.isBSS()) ||
(Attrs.hasAttribute("data-section") && Kind.isData()) ||
(Attrs.hasAttribute("rodata-section") && Kind.isReadOnly())) {
return getExplicitSectionGlobal(GO, Kind, TM);
if (auto *F = dyn_cast<Function>(GO)) {
if (F->hasFnAttribute("implicit-section-name"))
return getExplicitSectionGlobal(GO, Kind, TM);
// Use default section depending on the 'type' of global
return SelectSectionForGlobal(GO, Kind, TM);
MCSection *TargetLoweringObjectFile::getSectionForJumpTable(
const Function &F, const TargetMachine &TM) const {
unsigned Align = 0;
return getSectionForConstant(F.getParent()->getDataLayout(),
SectionKind::getReadOnly(), /*C=*/nullptr,
bool TargetLoweringObjectFile::shouldPutJumpTableInFunctionSection(
bool UsesLabelDifference, const Function &F) const {
// In PIC mode, we need to emit the jump table to the same section as the
// function body itself, otherwise the label differences won't make sense.
// FIXME: Need a better predicate for this: what about custom entries?
if (UsesLabelDifference)
return true;
// We should also do if the section name is NULL or function is declared
// in discardable section
// FIXME: this isn't the right predicate, should be based on the MCSection
// for the function.
return F.isWeakForLinker();
/// Given a mergable constant with the specified size and relocation
/// information, return a section that it should be placed in.
MCSection *TargetLoweringObjectFile::getSectionForConstant(
const DataLayout &DL, SectionKind Kind, const Constant *C,
unsigned &Align) const {
if (Kind.isReadOnly() && ReadOnlySection != nullptr)
return ReadOnlySection;
return DataSection;
/// getTTypeGlobalReference - Return an MCExpr to use for a
/// reference to the specified global variable from exception
/// handling information.
const MCExpr *TargetLoweringObjectFile::getTTypeGlobalReference(
const GlobalValue *GV, unsigned Encoding, const TargetMachine &TM,
MachineModuleInfo *MMI, MCStreamer &Streamer) const {
const MCSymbolRefExpr *Ref =
MCSymbolRefExpr::create(TM.getSymbol(GV), getContext());
return getTTypeReference(Ref, Encoding, Streamer);
const MCExpr *TargetLoweringObjectFile::
getTTypeReference(const MCSymbolRefExpr *Sym, unsigned Encoding,
MCStreamer &Streamer) const {
switch (Encoding & 0x70) {
report_fatal_error("We do not support this DWARF encoding yet!");
case dwarf::DW_EH_PE_absptr:
// Do nothing special
return Sym;
case dwarf::DW_EH_PE_pcrel: {
// Emit a label to the streamer for the current position. This gives us
// .-foo addressing.
MCSymbol *PCSym = getContext().createTempSymbol();
const MCExpr *PC = MCSymbolRefExpr::create(PCSym, getContext());
return MCBinaryExpr::createSub(Sym, PC, getContext());
const MCExpr *TargetLoweringObjectFile::getDebugThreadLocalSymbol(const MCSymbol *Sym) const {
// FIXME: It's not clear what, if any, default this should have - perhaps a
// null return could mean 'no location' & we should just do that here.
return MCSymbolRefExpr::create(Sym, *Ctx);
void TargetLoweringObjectFile::getNameWithPrefix(
SmallVectorImpl<char> &OutName, const GlobalValue *GV,
const TargetMachine &TM) const {
Mang->getNameWithPrefix(OutName, GV, /*CannotUsePrivateLabel=*/false);