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[lld-macho][NFC] Refactor ObjCSelRefsSection out of ObjCStubsSection #83878

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Merged
merged 3 commits into from
Mar 10, 2024
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[lld-macho][NFC] Refactor ObjCSelRefsSection out of ObjCStubsSection #83878

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80a9e0e
[lld-macho][NFC] Refactor ObjCSelRefsSection out of ObjCStubsSection
Mar 4, 2024
7954143
[lld-macho] Address Feedback #1
Mar 6, 2024
e692db0
[lld-macho] Address Feedback #2
Mar 10, 2024
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105 changes: 59 additions & 46 deletions lld/MachO/SyntheticSections.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -806,26 +806,10 @@ void StubHelperSection::setUp() {
dyldPrivate->used = true;
}

ObjCStubsSection::ObjCStubsSection()
: SyntheticSection(segment_names::text, section_names::objcStubs) {
flags = S_ATTR_SOME_INSTRUCTIONS | S_ATTR_PURE_INSTRUCTIONS;
align = config->objcStubsMode == ObjCStubsMode::fast
? target->objcStubsFastAlignment
: target->objcStubsSmallAlignment;
}

bool ObjCStubsSection::isObjCStubSymbol(Symbol *sym) {
return sym->getName().starts_with(symbolPrefix);
}
ObjCSelRefsSection::ObjCSelRefsSection()
: SyntheticSection(segment_names::data, section_names::objcSelrefs) {}

StringRef ObjCStubsSection::getMethname(Symbol *sym) {
assert(isObjCStubSymbol(sym) && "not an objc stub");
auto name = sym->getName();
StringRef methname = name.drop_front(symbolPrefix.size());
return methname;
}

void ObjCStubsSection::initialize() {
void ObjCSelRefsSection::initialize() {
// Do not fold selrefs without ICF.
if (config->icfLevel == ICFLevel::none)
return;
Expand All @@ -852,33 +836,62 @@ void ObjCStubsSection::initialize() {
}
}

ConcatInputSection *ObjCSelRefsSection::makeSelRef(StringRef methname) {
auto methnameOffset =
in.objcMethnameSection->getStringOffset(methname).outSecOff;

size_t wordSize = target->wordSize;
uint8_t *selrefData = bAlloc().Allocate<uint8_t>(wordSize);
write64le(selrefData, methnameOffset);
ConcatInputSection *objcSelref =
makeSyntheticInputSection(segment_names::data, section_names::objcSelrefs,
S_LITERAL_POINTERS | S_ATTR_NO_DEAD_STRIP,
ArrayRef<uint8_t>{selrefData, wordSize},
/*align=*/wordSize);
objcSelref->live = true;
objcSelref->relocs.push_back({/*type=*/target->unsignedRelocType,
/*pcrel=*/false, /*length=*/3,
/*offset=*/0,
/*addend=*/static_cast<int64_t>(methnameOffset),
/*referent=*/in.objcMethnameSection->isec});
objcSelref->parent = ConcatOutputSection::getOrCreateForInput(objcSelref);
inputSections.push_back(objcSelref);
objcSelref->isFinal = true;
methnameToSelref[CachedHashStringRef(methname)] = objcSelref;
return objcSelref;
}

ConcatInputSection *ObjCSelRefsSection::getSelRef(StringRef methname) {
auto it = methnameToSelref.find(CachedHashStringRef(methname));
if (it == methnameToSelref.end())
return nullptr;
return it->second;
}

ObjCStubsSection::ObjCStubsSection()
: SyntheticSection(segment_names::text, section_names::objcStubs) {
flags = S_ATTR_SOME_INSTRUCTIONS | S_ATTR_PURE_INSTRUCTIONS;
align = config->objcStubsMode == ObjCStubsMode::fast
? target->objcStubsFastAlignment
: target->objcStubsSmallAlignment;
}

bool ObjCStubsSection::isObjCStubSymbol(Symbol *sym) {
return sym->getName().starts_with(symbolPrefix);
}

StringRef ObjCStubsSection::getMethname(Symbol *sym) {
assert(isObjCStubSymbol(sym) && "not an objc stub");
auto name = sym->getName();
StringRef methname = name.drop_front(symbolPrefix.size());
return methname;
}

void ObjCStubsSection::addEntry(Symbol *sym) {
StringRef methname = getMethname(sym);
// We create a selref entry for each unique methname.
if (!methnameToSelref.count(CachedHashStringRef(methname))) {
auto methnameOffset =
in.objcMethnameSection->getStringOffset(methname).outSecOff;

size_t wordSize = target->wordSize;
uint8_t *selrefData = bAlloc().Allocate<uint8_t>(wordSize);
write64le(selrefData, methnameOffset);
auto *objcSelref = makeSyntheticInputSection(
segment_names::data, section_names::objcSelrefs,
S_LITERAL_POINTERS | S_ATTR_NO_DEAD_STRIP,
ArrayRef<uint8_t>{selrefData, wordSize},
/*align=*/wordSize);
objcSelref->live = true;
objcSelref->relocs.push_back(
{/*type=*/target->unsignedRelocType,
/*pcrel=*/false, /*length=*/3,
/*offset=*/0,
/*addend=*/static_cast<int64_t>(methnameOffset),
/*referent=*/in.objcMethnameSection->isec});
objcSelref->parent = ConcatOutputSection::getOrCreateForInput(objcSelref);
inputSections.push_back(objcSelref);
objcSelref->isFinal = true;
methnameToSelref[CachedHashStringRef(methname)] = objcSelref;
}
if (!in.objcSelRefs->getSelRef(methname))
in.objcSelRefs->makeSelRef(methname);

auto stubSize = config->objcStubsMode == ObjCStubsMode::fast
? target->objcStubsFastSize
Expand Down Expand Up @@ -927,9 +940,9 @@ void ObjCStubsSection::writeTo(uint8_t *buf) const {
Defined *sym = symbols[i];

auto methname = getMethname(sym);
auto j = methnameToSelref.find(CachedHashStringRef(methname));
assert(j != methnameToSelref.end());
auto selrefAddr = j->second->getVA(0);
InputSection *selRef = in.objcSelRefs->getSelRef(methname);
assert(selRef != nullptr && "no selref for methname");
auto selrefAddr = selRef->getVA(0);
target->writeObjCMsgSendStub(buf + stubOffset, sym, in.objcStubs->addr,
stubOffset, selrefAddr, objcMsgSend);
}
Expand Down
24 changes: 22 additions & 2 deletions lld/MachO/SyntheticSections.h
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Original file line number Diff line number Diff line change
Expand Up @@ -315,6 +315,27 @@ class StubHelperSection final : public SyntheticSection {
Defined *dyldPrivate = nullptr;
};

class ObjCSelRefsSection final : public SyntheticSection {
public:
ObjCSelRefsSection();
void initialize();

// This SyntheticSection does not do directly write data to the output, it is
// just a placeholder for easily creating SyntheticInputSection's which will
// be inserted into inputSections and handeled by the default writing
// mechanism.
uint64_t getSize() const override { return 0; }
bool isNeeded() const override { return false; }
void writeTo(uint8_t *buf) const override {}

ConcatInputSection *getSelRef(StringRef methname);
ConcatInputSection *makeSelRef(StringRef methname);

private:
llvm::DenseMap<llvm::CachedHashStringRef, ConcatInputSection *>
methnameToSelref;
};

// Objective-C stubs are hoisted objc_msgSend calls per selector called in the
// program. Apple Clang produces undefined symbols to each stub, such as
// '_objc_msgSend$foo', which are then synthesized by the linker. The stubs
Expand All @@ -324,7 +345,6 @@ class StubHelperSection final : public SyntheticSection {
class ObjCStubsSection final : public SyntheticSection {
public:
ObjCStubsSection();
void initialize();
void addEntry(Symbol *sym);
uint64_t getSize() const override;
bool isNeeded() const override { return !symbols.empty(); }
Expand All @@ -338,7 +358,6 @@ class ObjCStubsSection final : public SyntheticSection {

private:
std::vector<Defined *> symbols;
llvm::DenseMap<llvm::CachedHashStringRef, InputSection *> methnameToSelref;
Symbol *objcMsgSend = nullptr;
};

Expand Down Expand Up @@ -794,6 +813,7 @@ struct InStruct {
LazyPointerSection *lazyPointers = nullptr;
StubsSection *stubs = nullptr;
StubHelperSection *stubHelper = nullptr;
ObjCSelRefsSection *objcSelRefs = nullptr;
ObjCStubsSection *objcStubs = nullptr;
UnwindInfoSection *unwindInfo = nullptr;
ObjCImageInfoSection *objCImageInfo = nullptr;
Expand Down
3 changes: 2 additions & 1 deletion lld/MachO/Writer.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -720,7 +720,7 @@ static void addNonWeakDefinition(const Defined *defined) {

void Writer::scanSymbols() {
TimeTraceScope timeScope("Scan symbols");
in.objcStubs->initialize();
in.objcSelRefs->initialize();
for (Symbol *sym : symtab->getSymbols()) {
if (auto *defined = dyn_cast<Defined>(sym)) {
if (!defined->isLive())
Expand Down Expand Up @@ -1359,6 +1359,7 @@ void macho::createSyntheticSections() {
in.got = make<GotSection>();
in.tlvPointers = make<TlvPointerSection>();
in.stubs = make<StubsSection>();
in.objcSelRefs = make<ObjCSelRefsSection>();
in.objcStubs = make<ObjCStubsSection>();
in.unwindInfo = makeUnwindInfoSection();
in.objCImageInfo = make<ObjCImageInfoSection>();
Expand Down