[lld] [lld-macho][arm64] implement -objc_stubs_small (PR #78665)

[Ellis Hoag via llvm-commits]

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@@ -809,31 +809,50 @@ void StubHelperSection::setUp() {
 ObjCStubsSection::ObjCStubsSection()
     : SyntheticSection(segment_names::text, section_names::objcStubs) {
   flags = S_ATTR_SOME_INSTRUCTIONS | S_ATTR_PURE_INSTRUCTIONS;
-  align = target->objcStubsAlignment;
+  align = config->objcStubsMode == ObjCStubsMode::fast
+              ? target->objcStubsFastAlignment
+              : target->objcStubsSmallAlignment;
 }
 
 void ObjCStubsSection::addEntry(Symbol *sym) {
   assert(sym->getName().starts_with(symbolPrefix) && "not an objc stub");
   StringRef methname = sym->getName().drop_front(symbolPrefix.size());
   offsets.push_back(
       in.objcMethnameSection->getStringOffset(methname).outSecOff);
+
+  auto stubSize = config->objcStubsMode == ObjCStubsMode::fast
+                      ? target->objcStubsFastSize
+                      : target->objcStubsSmallSize;
   Defined *newSym = replaceSymbol<Defined>(
       sym, sym->getName(), nullptr, isec,
-      /*value=*/symbols.size() * target->objcStubsFastSize,
-      /*size=*/target->objcStubsFastSize,
+      /*value=*/symbols.size() * stubSize,
+      /*size=*/stubSize,
       /*isWeakDef=*/false, /*isExternal=*/true, /*isPrivateExtern=*/true,
       /*includeInSymtab=*/true, /*isReferencedDynamically=*/false,
       /*noDeadStrip=*/false);
   symbols.push_back(newSym);
 }
 
 void ObjCStubsSection::setUp() {
-  Symbol *objcMsgSend = symtab->addUndefined("_objc_msgSend", /*file=*/nullptr,
-                                             /*isWeakRef=*/false);
+  objcMsgSend = symtab->addUndefined("_objc_msgSend", /*file=*/nullptr,
+                                     /*isWeakRef=*/false);
+  if (auto *undefined = dyn_cast<Undefined>(objcMsgSend))
+    treatUndefinedSymbol(*undefined,
+                         "lazy binding (normally in libobjc.dylib)");
   objcMsgSend->used = true;
-  in.got->addEntry(objcMsgSend);
-  assert(objcMsgSend->isInGot());
-  objcMsgSendGotIndex = objcMsgSend->gotIndex;
+  if (config->objcStubsMode == ObjCStubsMode::fast) {
+    in.got->addEntry(objcMsgSend);
+    assert(objcMsgSend->isInGot());
+  } else {
+    assert(config->objcStubsMode == ObjCStubsMode::small);
+    // In line with ld64's behavior, when objc_msgSend is a direct symbol,
+    // we directly reference it.
+    // In other cases, typically when binding in libobjc.dylib,
+    // we generate a stub to invoke objc_msgSend.
+    auto *d = dyn_cast<Defined>(objcMsgSend);
+    if (!d)
+      in.stubs->addEntry(objcMsgSend);
----------------
ellishg wrote:

Can we use `!isa<Defined>(objcMsgSend)` instead?

https://github.com/llvm/llvm-project/pull/78665