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llvm-project/llvm/lib/MC/MCMachOStreamer.cpp
Eli Friedman 10c17c97eb [COFF] Add MC support for emitting IMAGE_WEAK_EXTERN_ANTI_DEPENDENCY symbols 
This is mostly useful for ARM64EC, which uses such symbols extensively.

One interesting quirk of ARM64EC is that we need to be able to emit weak
symbols that point at each other (so if either symbol is defined
elsewhere, both symbols point at the definition).  This required a few
changes to the way we handle weak symbols on Windows.

Differential Revision: https://reviews.llvm.org/D145208
2023-04-17 13:17:25 -07:00

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//===- MCMachOStreamer.cpp - MachO Streamer -------------------------------===//
//
// 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 "llvm/ADT/DenseMap.h"
#include "llvm/ADT/SmallString.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/StringRef.h"
#include "llvm/MC/MCAsmBackend.h"
#include "llvm/MC/MCAssembler.h"
#include "llvm/MC/MCCodeEmitter.h"
#include "llvm/MC/MCContext.h"
#include "llvm/MC/MCDirectives.h"
#include "llvm/MC/MCExpr.h"
#include "llvm/MC/MCFixup.h"
#include "llvm/MC/MCFragment.h"
#include "llvm/MC/MCLinkerOptimizationHint.h"
#include "llvm/MC/MCObjectFileInfo.h"
#include "llvm/MC/MCObjectStreamer.h"
#include "llvm/MC/MCObjectWriter.h"
#include "llvm/MC/MCSection.h"
#include "llvm/MC/MCSectionMachO.h"
#include "llvm/MC/MCSymbol.h"
#include "llvm/MC/MCSymbolMachO.h"
#include "llvm/MC/MCValue.h"
#include "llvm/MC/SectionKind.h"
#include "llvm/MC/TargetRegistry.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/ErrorHandling.h"
#include <cassert>
#include <vector>
namespace llvm {
class MCInst;
class MCStreamer;
class MCSubtargetInfo;
class Triple;
} // namespace llvm
using namespace llvm;
namespace {
class MCMachOStreamer : public MCObjectStreamer {
private:
/// LabelSections - true if each section change should emit a linker local
/// label for use in relocations for assembler local references. Obviates the
/// need for local relocations. False by default.
bool LabelSections;
bool DWARFMustBeAtTheEnd;
bool CreatedADWARFSection;
/// HasSectionLabel - map of which sections have already had a non-local
/// label emitted to them. Used so we don't emit extraneous linker local
/// labels in the middle of the section.
DenseMap<const MCSection*, bool> HasSectionLabel;
void emitInstToData(const MCInst &Inst, const MCSubtargetInfo &STI) override;
void emitDataRegion(DataRegionData::KindTy Kind);
void emitDataRegionEnd();
public:
MCMachOStreamer(MCContext &Context, std::unique_ptr<MCAsmBackend> MAB,
std::unique_ptr<MCObjectWriter> OW,
std::unique_ptr<MCCodeEmitter> Emitter,
bool DWARFMustBeAtTheEnd, bool label)
: MCObjectStreamer(Context, std::move(MAB), std::move(OW),
std::move(Emitter)),
LabelSections(label), DWARFMustBeAtTheEnd(DWARFMustBeAtTheEnd),
CreatedADWARFSection(false) {}
/// state management
void reset() override {
CreatedADWARFSection = false;
HasSectionLabel.clear();
MCObjectStreamer::reset();
}
/// @name MCStreamer Interface
/// @{
void changeSection(MCSection *Sect, const MCExpr *Subsect) override;
void emitLabel(MCSymbol *Symbol, SMLoc Loc = SMLoc()) override;
void emitAssignment(MCSymbol *Symbol, const MCExpr *Value) override;
void emitEHSymAttributes(const MCSymbol *Symbol, MCSymbol *EHSymbol) override;
void emitAssemblerFlag(MCAssemblerFlag Flag) override;
void emitLinkerOptions(ArrayRef<std::string> Options) override;
void emitDataRegion(MCDataRegionType Kind) override;
void emitVersionMin(MCVersionMinType Kind, unsigned Major, unsigned Minor,
unsigned Update, VersionTuple SDKVersion) override;
void emitBuildVersion(unsigned Platform, unsigned Major, unsigned Minor,
unsigned Update, VersionTuple SDKVersion) override;
void emitDarwinTargetVariantBuildVersion(unsigned Platform, unsigned Major,
unsigned Minor, unsigned Update,
VersionTuple SDKVersion) override;
void emitThumbFunc(MCSymbol *Func) override;
bool emitSymbolAttribute(MCSymbol *Symbol, MCSymbolAttr Attribute) override;
void emitSymbolDesc(MCSymbol *Symbol, unsigned DescValue) override;
void emitCommonSymbol(MCSymbol *Symbol, uint64_t Size,
Align ByteAlignment) override;
void emitLocalCommonSymbol(MCSymbol *Symbol, uint64_t Size,
Align ByteAlignment) override;
void emitZerofill(MCSection *Section, MCSymbol *Symbol = nullptr,
uint64_t Size = 0, Align ByteAlignment = Align(1),
SMLoc Loc = SMLoc()) override;
void emitTBSSSymbol(MCSection *Section, MCSymbol *Symbol, uint64_t Size,
Align ByteAlignment = Align(1)) override;
void emitIdent(StringRef IdentString) override {
llvm_unreachable("macho doesn't support this directive");
}
void emitLOHDirective(MCLOHType Kind, const MCLOHArgs &Args) override {
getAssembler().getLOHContainer().addDirective(Kind, Args);
}
void emitCGProfileEntry(const MCSymbolRefExpr *From,
const MCSymbolRefExpr *To, uint64_t Count) override {
if (!From->getSymbol().isTemporary() && !To->getSymbol().isTemporary())
getAssembler().CGProfile.push_back({From, To, Count});
}
void finishImpl() override;
void finalizeCGProfileEntry(const MCSymbolRefExpr *&SRE);
void finalizeCGProfile();
void createAddrSigSection();
};
} // end anonymous namespace.
static bool canGoAfterDWARF(const MCSectionMachO &MSec) {
// These sections are created by the assembler itself after the end of
// the .s file.
StringRef SegName = MSec.getSegmentName();
StringRef SecName = MSec.getName();
if (SegName == "__LD" && SecName == "__compact_unwind")
return true;
if (SegName == "__IMPORT") {
if (SecName == "__jump_table")
return true;
if (SecName == "__pointers")
return true;
}
if (SegName == "__TEXT" && SecName == "__eh_frame")
return true;
if (SegName == "__DATA" && (SecName == "__nl_symbol_ptr" ||
SecName == "__thread_ptr"))
return true;
if (SegName == "__LLVM" && SecName == "__cg_profile")
return true;
return false;
}
void MCMachOStreamer::changeSection(MCSection *Section,
const MCExpr *Subsection) {
// Change the section normally.
bool Created = changeSectionImpl(Section, Subsection);
const MCSectionMachO &MSec = *cast<MCSectionMachO>(Section);
StringRef SegName = MSec.getSegmentName();
if (SegName == "__DWARF")
CreatedADWARFSection = true;
else if (Created && DWARFMustBeAtTheEnd && !canGoAfterDWARF(MSec))
assert(!CreatedADWARFSection && "Creating regular section after DWARF");
// Output a linker-local symbol so we don't need section-relative local
// relocations. The linker hates us when we do that.
if (LabelSections && !HasSectionLabel[Section] &&
!Section->getBeginSymbol()) {
MCSymbol *Label = getContext().createLinkerPrivateTempSymbol();
Section->setBeginSymbol(Label);
HasSectionLabel[Section] = true;
}
}
void MCMachOStreamer::emitEHSymAttributes(const MCSymbol *Symbol,
MCSymbol *EHSymbol) {
getAssembler().registerSymbol(*Symbol);
if (Symbol->isExternal())
emitSymbolAttribute(EHSymbol, MCSA_Global);
if (cast<MCSymbolMachO>(Symbol)->isWeakDefinition())
emitSymbolAttribute(EHSymbol, MCSA_WeakDefinition);
if (Symbol->isPrivateExtern())
emitSymbolAttribute(EHSymbol, MCSA_PrivateExtern);
}
void MCMachOStreamer::emitLabel(MCSymbol *Symbol, SMLoc Loc) {
// We have to create a new fragment if this is an atom defining symbol,
// fragments cannot span atoms.
if (getAssembler().isSymbolLinkerVisible(*Symbol))
insert(new MCDataFragment());
MCObjectStreamer::emitLabel(Symbol, Loc);
// This causes the reference type flag to be cleared. Darwin 'as' was "trying"
// to clear the weak reference and weak definition bits too, but the
// implementation was buggy. For now we just try to match 'as', for
// diffability.
//
// FIXME: Cleanup this code, these bits should be emitted based on semantic
// properties, not on the order of definition, etc.
cast<MCSymbolMachO>(Symbol)->clearReferenceType();
}
void MCMachOStreamer::emitAssignment(MCSymbol *Symbol, const MCExpr *Value) {
MCValue Res;
if (Value->evaluateAsRelocatable(Res, nullptr, nullptr)) {
if (const MCSymbolRefExpr *SymAExpr = Res.getSymA()) {
const MCSymbol &SymA = SymAExpr->getSymbol();
if (!Res.getSymB() && (SymA.getName() == "" || Res.getConstant() != 0))
cast<MCSymbolMachO>(Symbol)->setAltEntry();
}
}
MCObjectStreamer::emitAssignment(Symbol, Value);
}
void MCMachOStreamer::emitDataRegion(DataRegionData::KindTy Kind) {
// Create a temporary label to mark the start of the data region.
MCSymbol *Start = getContext().createTempSymbol();
emitLabel(Start);
// Record the region for the object writer to use.
DataRegionData Data = { Kind, Start, nullptr };
std::vector<DataRegionData> &Regions = getAssembler().getDataRegions();
Regions.push_back(Data);
}
void MCMachOStreamer::emitDataRegionEnd() {
std::vector<DataRegionData> &Regions = getAssembler().getDataRegions();
assert(!Regions.empty() && "Mismatched .end_data_region!");
DataRegionData &Data = Regions.back();
assert(!Data.End && "Mismatched .end_data_region!");
// Create a temporary label to mark the end of the data region.
Data.End = getContext().createTempSymbol();
emitLabel(Data.End);
}
void MCMachOStreamer::emitAssemblerFlag(MCAssemblerFlag Flag) {
// Let the target do whatever target specific stuff it needs to do.
getAssembler().getBackend().handleAssemblerFlag(Flag);
// Do any generic stuff we need to do.
switch (Flag) {
case MCAF_SyntaxUnified: return; // no-op here.
case MCAF_Code16: return; // Change parsing mode; no-op here.
case MCAF_Code32: return; // Change parsing mode; no-op here.
case MCAF_Code64: return; // Change parsing mode; no-op here.
case MCAF_SubsectionsViaSymbols:
getAssembler().setSubsectionsViaSymbols(true);
return;
}
}
void MCMachOStreamer::emitLinkerOptions(ArrayRef<std::string> Options) {
getAssembler().getLinkerOptions().push_back(Options);
}
void MCMachOStreamer::emitDataRegion(MCDataRegionType Kind) {
switch (Kind) {
case MCDR_DataRegion:
emitDataRegion(DataRegionData::Data);
return;
case MCDR_DataRegionJT8:
emitDataRegion(DataRegionData::JumpTable8);
return;
case MCDR_DataRegionJT16:
emitDataRegion(DataRegionData::JumpTable16);
return;
case MCDR_DataRegionJT32:
emitDataRegion(DataRegionData::JumpTable32);
return;
case MCDR_DataRegionEnd:
emitDataRegionEnd();
return;
}
}
void MCMachOStreamer::emitVersionMin(MCVersionMinType Kind, unsigned Major,
unsigned Minor, unsigned Update,
VersionTuple SDKVersion) {
getAssembler().setVersionMin(Kind, Major, Minor, Update, SDKVersion);
}
void MCMachOStreamer::emitBuildVersion(unsigned Platform, unsigned Major,
unsigned Minor, unsigned Update,
VersionTuple SDKVersion) {
getAssembler().setBuildVersion((MachO::PlatformType)Platform, Major, Minor,
Update, SDKVersion);
}
void MCMachOStreamer::emitDarwinTargetVariantBuildVersion(
unsigned Platform, unsigned Major, unsigned Minor, unsigned Update,
VersionTuple SDKVersion) {
getAssembler().setDarwinTargetVariantBuildVersion(
(MachO::PlatformType)Platform, Major, Minor, Update, SDKVersion);
}
void MCMachOStreamer::emitThumbFunc(MCSymbol *Symbol) {
// Remember that the function is a thumb function. Fixup and relocation
// values will need adjusted.
getAssembler().setIsThumbFunc(Symbol);
cast<MCSymbolMachO>(Symbol)->setThumbFunc();
}
bool MCMachOStreamer::emitSymbolAttribute(MCSymbol *Sym,
MCSymbolAttr Attribute) {
MCSymbolMachO *Symbol = cast<MCSymbolMachO>(Sym);
// Indirect symbols are handled differently, to match how 'as' handles
// them. This makes writing matching .o files easier.
if (Attribute == MCSA_IndirectSymbol) {
// Note that we intentionally cannot use the symbol data here; this is
// important for matching the string table that 'as' generates.
IndirectSymbolData ISD;
ISD.Symbol = Symbol;
ISD.Section = getCurrentSectionOnly();
getAssembler().getIndirectSymbols().push_back(ISD);
return true;
}
// Adding a symbol attribute always introduces the symbol, note that an
// important side effect of calling registerSymbol here is to register
// the symbol with the assembler.
getAssembler().registerSymbol(*Symbol);
// The implementation of symbol attributes is designed to match 'as', but it
// leaves much to desired. It doesn't really make sense to arbitrarily add and
// remove flags, but 'as' allows this (in particular, see .desc).
//
// In the future it might be worth trying to make these operations more well
// defined.
switch (Attribute) {
case MCSA_Invalid:
case MCSA_ELF_TypeFunction:
case MCSA_ELF_TypeIndFunction:
case MCSA_ELF_TypeObject:
case MCSA_ELF_TypeTLS:
case MCSA_ELF_TypeCommon:
case MCSA_ELF_TypeNoType:
case MCSA_ELF_TypeGnuUniqueObject:
case MCSA_Extern:
case MCSA_Hidden:
case MCSA_IndirectSymbol:
case MCSA_Internal:
case MCSA_Protected:
case MCSA_Weak:
case MCSA_Local:
case MCSA_LGlobal:
case MCSA_Exported:
case MCSA_Memtag:
case MCSA_WeakAntiDep:
return false;
case MCSA_Global:
Symbol->setExternal(true);
// This effectively clears the undefined lazy bit, in Darwin 'as', although
// it isn't very consistent because it implements this as part of symbol
// lookup.
//
// FIXME: Cleanup this code, these bits should be emitted based on semantic
// properties, not on the order of definition, etc.
Symbol->setReferenceTypeUndefinedLazy(false);
break;
case MCSA_LazyReference:
// FIXME: This requires -dynamic.
Symbol->setNoDeadStrip();
if (Symbol->isUndefined())
Symbol->setReferenceTypeUndefinedLazy(true);
break;
// Since .reference sets the no dead strip bit, it is equivalent to
// .no_dead_strip in practice.
case MCSA_Reference:
case MCSA_NoDeadStrip:
Symbol->setNoDeadStrip();
break;
case MCSA_SymbolResolver:
Symbol->setSymbolResolver();
break;
case MCSA_AltEntry:
Symbol->setAltEntry();
break;
case MCSA_PrivateExtern:
Symbol->setExternal(true);
Symbol->setPrivateExtern(true);
break;
case MCSA_WeakReference:
// FIXME: This requires -dynamic.
if (Symbol->isUndefined())
Symbol->setWeakReference();
break;
case MCSA_WeakDefinition:
// FIXME: 'as' enforces that this is defined and global. The manual claims
// it has to be in a coalesced section, but this isn't enforced.
Symbol->setWeakDefinition();
break;
case MCSA_WeakDefAutoPrivate:
Symbol->setWeakDefinition();
Symbol->setWeakReference();
break;
case MCSA_Cold:
Symbol->setCold();
break;
}
return true;
}
void MCMachOStreamer::emitSymbolDesc(MCSymbol *Symbol, unsigned DescValue) {
// Encode the 'desc' value into the lowest implementation defined bits.
getAssembler().registerSymbol(*Symbol);
cast<MCSymbolMachO>(Symbol)->setDesc(DescValue);
}
void MCMachOStreamer::emitCommonSymbol(MCSymbol *Symbol, uint64_t Size,
Align ByteAlignment) {
// FIXME: Darwin 'as' does appear to allow redef of a .comm by itself.
assert(Symbol->isUndefined() && "Cannot define a symbol twice!");
getAssembler().registerSymbol(*Symbol);
Symbol->setExternal(true);
Symbol->setCommon(Size, ByteAlignment);
}
void MCMachOStreamer::emitLocalCommonSymbol(MCSymbol *Symbol, uint64_t Size,
Align ByteAlignment) {
// '.lcomm' is equivalent to '.zerofill'.
return emitZerofill(getContext().getObjectFileInfo()->getDataBSSSection(),
Symbol, Size, ByteAlignment);
}
void MCMachOStreamer::emitZerofill(MCSection *Section, MCSymbol *Symbol,
uint64_t Size, Align ByteAlignment,
SMLoc Loc) {
// On darwin all virtual sections have zerofill type. Disallow the usage of
// .zerofill in non-virtual functions. If something similar is needed, use
// .space or .zero.
if (!Section->isVirtualSection()) {
getContext().reportError(
Loc, "The usage of .zerofill is restricted to sections of "
"ZEROFILL type. Use .zero or .space instead.");
return; // Early returning here shouldn't harm. EmitZeros should work on any
// section.
}
pushSection();
switchSection(Section);
// The symbol may not be present, which only creates the section.
if (Symbol) {
emitValueToAlignment(ByteAlignment, 0, 1, 0);
emitLabel(Symbol);
emitZeros(Size);
}
popSection();
}
// This should always be called with the thread local bss section. Like the
// .zerofill directive this doesn't actually switch sections on us.
void MCMachOStreamer::emitTBSSSymbol(MCSection *Section, MCSymbol *Symbol,
uint64_t Size, Align ByteAlignment) {
emitZerofill(Section, Symbol, Size, ByteAlignment);
}
void MCMachOStreamer::emitInstToData(const MCInst &Inst,
const MCSubtargetInfo &STI) {
MCDataFragment *DF = getOrCreateDataFragment();
SmallVector<MCFixup, 4> Fixups;
SmallString<256> Code;
getAssembler().getEmitter().encodeInstruction(Inst, Code, Fixups, STI);
// Add the fixups and data.
for (MCFixup &Fixup : Fixups) {
Fixup.setOffset(Fixup.getOffset() + DF->getContents().size());
DF->getFixups().push_back(Fixup);
}
DF->setHasInstructions(STI);
DF->getContents().append(Code.begin(), Code.end());
}
void MCMachOStreamer::finishImpl() {
emitFrames(&getAssembler().getBackend());
// We have to set the fragment atom associations so we can relax properly for
// Mach-O.
// First, scan the symbol table to build a lookup table from fragments to
// defining symbols.
DenseMap<const MCFragment *, const MCSymbol *> DefiningSymbolMap;
for (const MCSymbol &Symbol : getAssembler().symbols()) {
if (getAssembler().isSymbolLinkerVisible(Symbol) && Symbol.isInSection() &&
!Symbol.isVariable()) {
// An atom defining symbol should never be internal to a fragment.
assert(Symbol.getOffset() == 0 &&
"Invalid offset in atom defining symbol!");
DefiningSymbolMap[Symbol.getFragment()] = &Symbol;
}
}
// Set the fragment atom associations by tracking the last seen atom defining
// symbol.
for (MCSection &Sec : getAssembler()) {
const MCSymbol *CurrentAtom = nullptr;
for (MCFragment &Frag : Sec) {
if (const MCSymbol *Symbol = DefiningSymbolMap.lookup(&Frag))
CurrentAtom = Symbol;
Frag.setAtom(CurrentAtom);
}
}
finalizeCGProfile();
createAddrSigSection();
this->MCObjectStreamer::finishImpl();
}
void MCMachOStreamer::finalizeCGProfileEntry(const MCSymbolRefExpr *&SRE) {
const MCSymbol *S = &SRE->getSymbol();
bool Created;
getAssembler().registerSymbol(*S, &Created);
if (Created)
S->setExternal(true);
}
void MCMachOStreamer::finalizeCGProfile() {
MCAssembler &Asm = getAssembler();
if (Asm.CGProfile.empty())
return;
for (MCAssembler::CGProfileEntry &E : Asm.CGProfile) {
finalizeCGProfileEntry(E.From);
finalizeCGProfileEntry(E.To);
}
// We can't write the section out until symbol indices are finalized which
// doesn't happen until after section layout. We need to create the section
// and set its size now so that it's accounted for in layout.
MCSection *CGProfileSection = Asm.getContext().getMachOSection(
"__LLVM", "__cg_profile", 0, SectionKind::getMetadata());
Asm.registerSection(*CGProfileSection);
auto *Frag = new MCDataFragment(CGProfileSection);
// For each entry, reserve space for 2 32-bit indices and a 64-bit count.
size_t SectionBytes =
Asm.CGProfile.size() * (2 * sizeof(uint32_t) + sizeof(uint64_t));
Frag->getContents().resize(SectionBytes);
}
MCStreamer *llvm::createMachOStreamer(MCContext &Context,
std::unique_ptr<MCAsmBackend> &&MAB,
std::unique_ptr<MCObjectWriter> &&OW,
std::unique_ptr<MCCodeEmitter> &&CE,
bool RelaxAll, bool DWARFMustBeAtTheEnd,
bool LabelSections) {
MCMachOStreamer *S =
new MCMachOStreamer(Context, std::move(MAB), std::move(OW), std::move(CE),
DWARFMustBeAtTheEnd, LabelSections);
const Triple &Target = Context.getTargetTriple();
S->emitVersionForTarget(
Target, Context.getObjectFileInfo()->getSDKVersion(),
Context.getObjectFileInfo()->getDarwinTargetVariantTriple(),
Context.getObjectFileInfo()->getDarwinTargetVariantSDKVersion());
if (RelaxAll)
S->getAssembler().setRelaxAll(true);
return S;
}
// The AddrSig section uses a series of relocations to refer to the symbols that
// should be considered address-significant. The only interesting content of
// these relocations is their symbol; the type, length etc will be ignored by
// the linker. The reason we are not referring to the symbol indices directly is
// that those indices will be invalidated by tools that update the symbol table.
// Symbol relocations OTOH will have their indices updated by e.g. llvm-strip.
void MCMachOStreamer::createAddrSigSection() {
MCAssembler &Asm = getAssembler();
MCObjectWriter &writer = Asm.getWriter();
if (!writer.getEmitAddrsigSection())
return;
// Create the AddrSig section and first data fragment here as its layout needs
// to be computed immediately after in order for it to be exported correctly.
MCSection *AddrSigSection =
Asm.getContext().getObjectFileInfo()->getAddrSigSection();
Asm.registerSection(*AddrSigSection);
auto *Frag = new MCDataFragment(AddrSigSection);
// We will generate a series of pointer-sized symbol relocations at offset
// 0x0. Set the section size to be large enough to contain a single pointer
// (instead of emitting a zero-sized section) so these relocations are
// technically valid, even though we don't expect these relocations to
// actually be applied by the linker.
Frag->getContents().resize(8);
}
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