[lld-macho] Mark aliased symbols as noDeadStrip

[keith]

Summary

This matches ld64 behavior

[keith]

I didn't investigate the ld64 behavior besides checking that this same test case passes with it

lld/MachO/MarkLive.cpp

232

Left this here since we don't support this one yet

[thakis]

Thanks!

I wonder if N_INDR symbols need this too? Probably not. (@int3)

[int3]

I played around with the test a bit and I realized that ld64 doesn't actually dead-strip _foo if .subsections_via_symbols isn't there (even though _foo and _main are in separate files and therefore separate sections...)

With .subsections_via_symbols added, I noticed that ld64 does actually dead-strip _bar (but not _foo!). I am not really sure that behavior makes sense, presumably an alias exists so that some other library can look up the aliasing symbol name, not the aliased symbol name...

So I think we should add .subsections_via_symbols + add a comment to the test describing this deviation from ld64

I will look into N_INDR shortly...

[int3]

Oh yeah our alias-symbols.s test case already covers the -dead_strip + N_INDR case so we're good

[int3]

lgtm modulo the changes to the test suggested above

[int3]

So I think we should add .subsections_via_symbols + add a comment to the test describing this deviation from ld64

Just for completeness: imitating this behavior isn't the most trivial since we track liveness at the InputSection level rather than at the symbol level. But I don't think it's important to imitate this, so it's all good

[keith]

Thanks for investigating some more, IIUC you're saying merge as is correct?

[int3]

yeah add .subsections_via_symbols and a comment about ld64's behavior to the test, then merge :)

[keith]

Update test with ld64 context

lld/MachO/Driver.cpp

static void referenceStubBinder() {
   // dyld_stub_binder doesn't exist in case it is actually needed.
   symtab->addUndefined("dyld_stub_binder", /*file=*/nullptr, /*isWeak=*/false);
 }
 
 static void createAliases() {
   for (const auto &pair : config->aliasedSymbols) {
     if (const auto &sym = symtab->find(pair.first)) {
       if (const auto &defined = dyn_cast<Defined>(sym)) {
-        symtab->aliasDefined(defined, pair.second, defined->getFile());
+        symtab->aliasDefined(defined, pair.second, defined->getFile())
+            ->noDeadStrip = true;
       } else {
         error("TODO: support aliasing to symbols of kind " +
               Twine(sym->kind()));
       }
     } else {
       warn("undefined base symbol '" + pair.first + "' for alias '" +
            pair.second + "'\n");
     }

lld/MachO/MarkLive.cpp

if (auto *defined = dyn_cast<Defined>(sym)) {
       if (defined->referencedDynamically || defined->noDeadStrip) {
         marker->addSym(defined);
         continue;
       }
 
       // FIXME: When we implement these flags, make symbols from them GC
       // roots:
       // * -reexported_symbol(s_list)
-      // * -alias(-list)
+      // * -alias_list
       // * -init
 
       // In dylibs and bundles and in executables with -export_dynamic,
       // all external functions are GC roots.
       bool externsAreRoots =
           config->outputType != MH_EXECUTE || config->exportDynamic;
       if (externsAreRoots && !defined->privateExtern) {
         marker->addSym(defined);

lld/test/MachO/alias-option.s

 # MISSING-DAG: undefined base symbol '_missing2' for alias '_baz'
 
 # RUN: %lld -alias _foo _main %t/foo.o -o %t/main_rename.o
 # RUN: llvm-nm %t/main_rename.o | FileCheck %s --check-prefix=MAIN
 
 # MAIN: [[#%x,FOO_ADDR:]] T _foo
 # MAIN: [[#FOO_ADDR]]     T _main
 
-# RUN: %lld -alias _foo _bar -alias _main _fake_main %t/main.o %t/foo.o -o %t/multiple.o
+## Verify dead stripping doesn't remove the aliased symbol. This behavior differs
+## from ld64 where it actually does dead strip only the alias, not the original symbol.
+# RUN: %lld -dead_strip -alias _foo _bar -alias _main _fake_main %t/main.o %t/foo.o -o %t/multiple.o
 # RUN: llvm-nm %t/multiple.o | FileCheck %s --check-prefix=MULTIPLE
 
 # MULTIPLE: [[#%x,FOO_ADDR:]]  T _bar
 # MULTIPLE: [[#%x,MAIN_ADDR:]] T _fake_main
 # MULTIPLE: [[#FOO_ADDR]]      T _foo
 # MULTIPLE: [[#MAIN_ADDR]]     T _main
 
 #--- foo.s
+.subsections_via_symbols
 .globl _foo
 _foo:
   ret
 
 #--- main.s
+.subsections_via_symbols
 .globl _main
 _main:
   ret