[lld/mac] Fix bug in interaction of -dead_strip and -undefined dynamic_lookup

[thakis]

Summary

We lost the used bit on the Undefined when we replaced it with a DylibSymbol
in treatUndefined().

[int3]

lgtm

lld/MachO/Symbols.h

 T *replaceSymbol(Symbol *s, ArgT &&...arg) {
   static_assert(sizeof(T) <= sizeof(SymbolUnion), "SymbolUnion too small");
   static_assert(alignof(T) <= alignof(SymbolUnion),
                 "SymbolUnion not aligned enough");
   assert(static_cast<Symbol *>(static_cast<T *>(nullptr)) == nullptr &&
          "Not a Symbol");
 
   bool isUsedInRegularObj = s->isUsedInRegularObj;
+  bool used = s->used;
   T *sym = new (s) T(std::forward<ArgT>(arg)...);
   sym->isUsedInRegularObj |= isUsedInRegularObj;
+  sym->used |= used;
   return sym;
 }
 
 } // namespace macho
 
 std::string toString(const macho::Symbol &);
 std::string toMachOString(const llvm::object::Archive::Symbol &);
 
 } // namespace lld
 
 #endif

lld/MachO/Writer.cpp

if (isa<DylibSymbol>(sym))
     return true;
   if (const auto *defined = dyn_cast<Defined>(sym))
     return defined->isExternalWeakDef();
   return false;
 }
 
 static void prepareSymbolRelocation(Symbol *sym, const InputSection *isec,
                                     const Reloc &r) {
+  assert(sym->isLive());
   const RelocAttrs &relocAttrs = target->getRelocAttrs(r.type);
 
   if (relocAttrs.hasAttr(RelocAttrBits::BRANCH)) {
     prepareBranchTarget(sym);
   } else if (relocAttrs.hasAttr(RelocAttrBits::GOT)) {
     if (relocAttrs.hasAttr(RelocAttrBits::POINTER) || needsBinding(sym))
       in.got->addEntry(sym);
   } else if (relocAttrs.hasAttr(RelocAttrBits::TLV)) {

lld/test/MachO/dead-strip.s

 # STUBS:  jmp
 # STUBS:  jmp
 # STUBS-NOT:  jmp
 ## An undefined symbol referenced from a dead-stripped function shouldn't
 ## produce a diagnostic:
 # RUN: %lld -lSystem -dead_strip %t/strip-dylib-ref.o %t/dylib.dylib \
 # RUN:     -o %t/strip-dylib-ref -U _ref_undef_fun
 
+## Check that referenced undefs are kept with -undefined dynamic_lookup.
+# RUN: llvm-mc -filetype=obj -triple=x86_64-apple-macos \
+# RUN:     %t/ref-undef.s -o %t/ref-undef.o
+# RUN: %lld -lSystem -dead_strip %t/ref-undef.o \
+# RUN:     -o %t/ref-undef -undefined dynamic_lookup
+# RUN: llvm-objdump --syms --lazy-bind %t/ref-undef | \
+# RUN:     FileCheck --check-prefix=STRIPDYNLOOKUP %s
+# STRIPDYNLOOKUP: SYMBOL TABLE:
+# STRIPDYNLOOKUP:   *UND* _ref_undef_fun
+# STRIPDYNLOOKUP: Lazy bind table:
+# STRIPDYNLOOKUP:   __DATA   __la_symbol_ptr {{.*}} flat-namespace _ref_undef_fun
+
 ## S_ATTR_LIVE_SUPPORT tests.
 # RUN: llvm-mc -filetype=obj -triple=x86_64-apple-macos \
 # RUN:     %t/live-support.s -o %t/live-support.o
 # RUN: %lld -lSystem -dead_strip %t/live-support.o %t/dylib.dylib \
 # RUN:     -U _ref_undef_fun -U _unref_undef_fun -o %t/live-support
 # RUN: llvm-objdump --syms %t/live-support | \
 # RUN:     FileCheck --check-prefix=LIVESUPP --implicit-check-not _unref %s
 # LIVESUPP-LABEL: SYMBOL TABLE:

 ## S_LITERAL_POINTERS flag in order not to confuse llvm-objdump.
 .section __DATA,word_ptrs
 .globl _more_data
 _more_data:
   .quad L._foo4
   .quad L._bar4
 
 .subsections_via_symbols
+
+#--- ref-undef.s
+.globl _main
+_main:
+  callq _ref_undef_fun
+.subsections_via_symbols