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

Currently ObjCStubsSection is handling both the logic for the
"__objc_stubs" section, as well as the logic for the "__objc_selrefs"
section.
While this is OK for now, it will be an issue for other features that
want to interact with the "__objc_selrefs" section, such as upcoming
relative method lists feature - which will also want to create /
reference entries in the "__objc_selrefs" section.
In this PR we split the logic relating to handling the "__objc_selrefs"
section into a new SyntheticSection (ObjCSelRefsSection). Non-functional
change - neither the behavior nor implementation changes, the interface
is just made more friendly to not have "__objc_selrefs" so bound to
"__objc_stubs".

---------

Co-authored-by: Alex B <[email protected]>

Author: alx32

lld/MachO/SyntheticSections.cpp

@@ -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;
@@ -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
@@ -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);
   }

lld/MachO/SyntheticSections.h

@@ -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
@@ -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(); }
@@ -338,7 +358,6 @@ class ObjCStubsSection final : public SyntheticSection {
 
 private:
   std::vector<Defined *> symbols;
-  llvm::DenseMap<llvm::CachedHashStringRef, InputSection *> methnameToSelref;
   Symbol *objcMsgSend = nullptr;
 };
 
@@ -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;

lld/MachO/Writer.cpp

@@ -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())
@@ -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>();