[lld-macho][nfc] Represent the image loader cache with a ConcatInputSection

[int3]

Summary

We don't need to define any special behavior for this section,
so creating a subclass for it is redundant.

lld/MachO/SyntheticSections.h

public:
   void writeTo(uint8_t *buf) const override;
 
   void setup();
 
   DylibSymbol *stubBinder = nullptr;
   Defined *dyldPrivate = nullptr;
 };
 
-// This section contains space for just a single word, and will be used by dyld
-// to cache an address to the image loader it uses. Note that unlike the other
-// synthetic sections, which are OutputSections, the ImageLoaderCacheSection is
-// an InputSection that gets merged into the __data OutputSection.
-class ImageLoaderCacheSection : public ConcatInputSection {
-public:
-  ImageLoaderCacheSection();
-  uint64_t getSize() const override { return target->wordSize; }
-};
-
 // Note that this section may also be targeted by non-lazy bindings. In
 // particular, this happens when branch relocations target weak symbols.
 class LazyPointerSection : public SyntheticSection {
 public:
   LazyPointerSection();
   uint64_t getSize() const override;
   bool isNeeded() const override;
   void writeTo(uint8_t *buf) const override;
struct InStruct {
   WeakBindingSection *weakBinding = nullptr;
   LazyBindingSection *lazyBinding = nullptr;
   ExportSection *exports = nullptr;
   GotSection *got = nullptr;
   TlvPointerSection *tlvPointers = nullptr;
   LazyPointerSection *lazyPointers = nullptr;
   StubsSection *stubs = nullptr;
   StubHelperSection *stubHelper = nullptr;
-  ImageLoaderCacheSection *imageLoaderCache = nullptr;
   UnwindInfoSection *unwindInfo = nullptr;
+  ConcatInputSection *imageLoaderCache = nullptr;
 };
 
 extern InStruct in;
 extern std::vector<SyntheticSection *> syntheticSections;
 
 void createSyntheticSymbols();
 
 } // namespace macho
 } // namespace lld
 
 #endif

lld/MachO/SyntheticSections.cpp

void StubHelperSection::setup() {
   dyldPrivate =
       make<Defined>("__dyld_private", nullptr, in.imageLoaderCache, 0, 0,
                     /*isWeakDef=*/false,
                     /*isExternal=*/false, /*isPrivateExtern=*/false,
                     /*isThumb=*/false, /*isReferencedDynamically=*/false,
                     /*noDeadStrip=*/false);
 }
 
-ImageLoaderCacheSection::ImageLoaderCacheSection()
-    : ConcatInputSection(segment_names::data, section_names::data) {
-  uint8_t *arr = bAlloc.Allocate<uint8_t>(target->wordSize);
-  memset(arr, 0, target->wordSize);
-  data = {arr, target->wordSize};
-  align = target->wordSize;
-  live = true;
-}
-
 LazyPointerSection::LazyPointerSection()
     : SyntheticSection(segment_names::data, section_names::lazySymbolPtr) {
   align = target->wordSize;
   flags = S_LAZY_SYMBOL_POINTERS;
 }
 
 uint64_t LazyPointerSection::getSize() const {
   return in.stubs->getEntries().size() * target->wordSize;

lld/MachO/Writer.cpp

void macho::createSyntheticSections() {
   in.weakBinding = make<WeakBindingSection>();
   in.lazyBinding = make<LazyBindingSection>();
   in.exports = make<ExportSection>();
   in.got = make<GotSection>();
   in.tlvPointers = make<TlvPointerSection>();
   in.lazyPointers = make<LazyPointerSection>();
   in.stubs = make<StubsSection>();
   in.stubHelper = make<StubHelperSection>();
-  in.imageLoaderCache = make<ImageLoaderCacheSection>();
   in.unwindInfo = makeUnwindInfoSection();
+
+  // This section contains space for just a single word, and will be used by
+  // dyld to cache an address to the image loader it uses.
+  ArrayRef<uint8_t> data{bAlloc.Allocate<uint8_t>(target->wordSize),
+                         target->wordSize};
+  in.imageLoaderCache = make<ConcatInputSection>(
+      segment_names::data, section_names::data, /*file=*/nullptr, data,
+      /*align=*/target->wordSize, /*flags=*/S_REGULAR);
+  // References from dyld are not visible to us, so ensure this section is
+  // always treated as live.
+  in.imageLoaderCache->live = true;
 }
 
 OutputSection *macho::firstTLVDataSection = nullptr;
 
 template void macho::writeResult<LP64>();
 template void macho::writeResult<ILP32>();