[lld-macho] Improve hiding of unnamed_addr symbols

[int3]

Summary

Symbols for which canBeOmittedFromSymbolTable() is true should be
treated as private externs. This diff tries to do that by unsetting the
ExportDynamic bit. It seems to mostly work with the FullLTO backend, but
with the ThinLTO backend, the local_unnamed_addr symbols still fail to
be properly hidden. Nonetheless, this is a step in the right direction.

I've documented all the remaining differences between our behavior and
LD64's in the lto-internalized-unnamed-addr.ll test.

See also https://discourse.llvm.org/t/mach-o-lto-handling-of-linkonce-odr-unnamed-addr/60015

[int3]

cc @steven_wu -- you may find the comments in the test file of interest, given the discussion in https://discourse.llvm.org/t/mach-o-lto-handling-of-linkonce-odr-unnamed-addr/60015/

lld/MachO/InputFiles.cpp

case GlobalValue::HiddenVisibility:
     isPrivateExtern = true;
     break;
   case GlobalValue::ProtectedVisibility:
     error(name + " has protected visibility, which is not supported by Mach-O");
     break;
   case GlobalValue::DefaultVisibility:
     break;
   }
+  isPrivateExtern = isPrivateExtern || objSym.canBeOmittedFromSymbolTable();
 
   if (objSym.isCommon())
     return symtab->addCommon(name, &file, objSym.getCommonSize(),
                              objSym.getCommonAlignment(), isPrivateExtern);
 
   return symtab->addDefined(name, &file, /*isec=*/nullptr, /*value=*/0,
                             /*size=*/0, objSym.isWeak(), isPrivateExtern,
                             /*isThumb=*/false,

lld/test/MachO/lto-internalize-unnamed-addr.ll

+; REQUIRES: x86
+; RUN: rm -rf %t; split-file %s %t
+;; This test covers both FullLTO and ThinLTO code paths because we have observed
+;; (unexpected) differences between the two.
+; RUN: llvm-as %t/test.ll -o %t/test.o
+; RUN: llvm-as %t/test2.ll -o %t/test2.o
+; RUN: opt -module-summary %t/test.ll -o %t/test.thinlto.o
+; RUN: opt -module-summary %t/test2.ll -o %t/test2.thinlto.o
+
+; RUN: %lld -lSystem %t/test.o %t/test2.o -o %t/test
+; RUN: llvm-nm -m %t/test | FileCheck %s --check-prefix=LTO
+
+; RUN: %lld -lSystem -dylib %t/test.o %t/test2.o -o %t/test.dylib
+; RUN: llvm-nm -m %t/test.dylib | FileCheck %s --check-prefix=LTO-DYLIB
+
+; RUN: %lld -lSystem %t/test.thinlto.o %t/test2.o -o %t/test.thinlto
+; RUN: llvm-nm -m %t/test.thinlto | FileCheck %s --check-prefix=THINLTO
+
+; RUN: %lld -lSystem -dylib %t/test.thinlto.o %t/test2.o -o %t/test.thinlto.dylib
+; RUN: llvm-nm -m %t/test.thinlto.dylib | FileCheck %s --check-prefix=THINLTO
+
+; LTO-DAG: (__DATA,__data) non-external _global_unnamed
+; LTO-DAG: (__DATA,__data) non-external _local_unnamed
+;; LD64 marks this with (was a private external). IMO both LD64 and LLD should
+;; mark all the other internalized symbols with (was a private external).
+; LTO-DAG: (__TEXT,__const) non-external _local_unnamed_always_const
+; LTO-DAG: (__TEXT,__const) non-external _local_unnamed_const
+;; LD64 doesn't internalize this -- it emits it as a weak external -- which I
+;; think is a missed optimization on its end.
+; LTO-DAG: (__TEXT,__const) non-external _local_unnamed_sometimes_const
+
+;; The output here is largely identical to LD64's, except that the non-external
+;; symbols here are all marked as (was a private external) by LD64. LLD should
+;; follow suit.
+; LTO-DYLIB-DAG: (__DATA,__data) non-external _global_unnamed
+; LTO-DYLIB-DAG: (__DATA,__data) weak external _local_unnamed
+; LTO-DYLIB-DAG: (__TEXT,__const) non-external _local_unnamed_always_const
+; LTO-DYLIB-DAG: (__TEXT,__const) non-external _local_unnamed_const
+; LTO-DYLIB-DAG: (__TEXT,__const) weak external _local_unnamed_sometimes_const
+
+; THINLTO-DAG: (__DATA,__data) non-external (was a private external) _global_unnamed
+; THINLTO-DAG: (__DATA,__data) weak external _local_unnamed
+;; The next two symbols are rendered as non-external (was a private external)
+;; by LD64. This is a missed optimization on LLD's end.
+; THINLTO-DAG: (__TEXT,__const) weak external _local_unnamed_always_const
+; THINLTO-DAG: (__TEXT,__const) weak external _local_unnamed_const
+;; LD64 actually fails to link when the following symbol is included in the test
+;; input, instead producing this error:
+;; reference to bitcode symbol '_local_unnamed_sometimes_const' which LTO has not compiled in '_used' from /tmp/lto.o for architecture x86_64
+; THINLTO-DAG: (__TEXT,__const) weak external _local_unnamed_sometimes_const
+
+;--- test.ll
+target triple = "x86_64-apple-darwin"
+target datalayout = "e-m:o-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+
+@global_unnamed = linkonce_odr unnamed_addr global i8 42
+@local_unnamed_const = linkonce_odr local_unnamed_addr constant i8 42
+@local_unnamed_always_const = linkonce_odr local_unnamed_addr constant i8 42
+@local_unnamed_sometimes_const = linkonce_odr local_unnamed_addr constant i8 42
+@local_unnamed = linkonce_odr local_unnamed_addr global i8 42
+@used = hidden constant [5 x i8*] [i8* @global_unnamed, i8* @local_unnamed,
+  i8* @local_unnamed_const, i8* @local_unnamed_always_const,
+  i8* @local_unnamed_sometimes_const]
[e-mail] = appending global [1 x [5 x i8*]*] [[5 x i8*]* @used]
+
+define void @main() {
+  ret void
+}
+
+;--- test2.ll
+target triple = "x86_64-apple-darwin"
+target datalayout = "e-m:o-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+
+@local_unnamed_always_const = linkonce_odr local_unnamed_addr constant i8 42
+@local_unnamed_sometimes_const = linkonce_odr local_unnamed_addr global i8 42