[lld-macho] Add support for exporting no symbols

[keith]

Summary

As an optimization for ld64 sometimes it can be useful to not export any
symbols for top level binaries that don't need any exports, to do this
you can pass -exported_symbols_list /dev/null, or new with Xcode 14
(ld64 816) there is a -no_exported_symbols flag for the same behavior.
This reproduces this behavior where previously an empty exported symbols
list file would have been ignored.

[int3]

lld/MachO/Driver.cpp

1412

hmm should we make it an error to use -no_exported_symbols with -exported_symbol as well?

1415–1416

tangential & existing code, but this seems kind of redundant to me lol. We're already erroring out, doesn't seem like there's a need to tell the user we're ignoring unexports. (Not sure there's a need to clear the unexported symbols list either, but I suppose it might make any subsequent error messages less confusing.)

1561

IMO this check is a bit confusing because it conflates a perf optimization with an actual difference in behavior. How about having the Config be something like

struct Config {
  ...
  bool explicitExports = false;
  SymbolPatterns exportPatterns; // (can represent both `exportedSymbols` and `unexportedSymbols`)
};

With exportPatterns interpreted differently based on the value of explicitExports. Then this if check becomes if (config->explicitExports) { ...

That said I know this is existing code, and refactoring it can be rightly said to be out of scope :)

[thakis]

lld/MachO/Driver.cpp

1409

(very nit: I find this name a bit hard to read, since empty is also a verb and this reads like "should I empty the exported symbols list" to me)

[oontvoo]

lld/MachO/Driver.cpp

1561

agreed - perhaps get rid of the two unexportedSymbols and exportedSymbols lists and have just one list then a flag to say what the list means?

[keith]

Add explicit config for exported symbols options

[keith]

lld/MachO/Driver.cpp

1415–1416

Removed this, it was silently ignored anyways because of the order of the if statement below. If we find a case where error messages are degraded without it I guess we should add it back with a test showing that case.

1561

So I did the first suggestion here with the explicit exports, I'm not sure if it would be as clear if we joined exportedSymbols and unexportedSymbols but if folks generally prefer that I can change it!

[int3]

Thanks!

lld/MachO/Driver.cpp

1408–1413

no need for { } here and line 1416 below

1415–1416

nice. can we remove the ">>> ignoring unexports" string as well?

[oontvoo]

👍

[keith]

Formatting improvements

lld/MachO/Driver.cpp

if (segName == segment_names::linkEdit)
       error("-segprot cannot be used to change __LINKEDIT's protections");
     config->segmentProtections.push_back({segName, maxProt, initProt});
   }
 
   handleSymbolPatterns(args, config->exportedSymbols, OPT_exported_symbol,
                        OPT_exported_symbols_list);
   handleSymbolPatterns(args, config->unexportedSymbols, OPT_unexported_symbol,
                        OPT_unexported_symbols_list);
-  if (!config->exportedSymbols.empty() && !config->unexportedSymbols.empty()) {
+  bool emptyExportedSymbolsList =
+      args.hasArg(OPT_exported_symbols_list) && config->exportedSymbols.empty();
+  bool exportNoSymbols =
+      emptyExportedSymbolsList || args.hasArg(OPT_no_exported_symbols);
+  if ((!config->exportedSymbols.empty() || exportNoSymbols) &&
+      !config->unexportedSymbols.empty()) {
     error("cannot use both -exported_symbol* and -unexported_symbol* options\n"
           ">>> ignoring unexports");
     config->unexportedSymbols.clear();
   }
 
   // Imitating LD64's:
   // -non_global_symbols_no_strip_list and -non_global_symbols_strip_list can't
   // both be present.
   // But -x can be used with either of these two, in which case, the last arg
   // takes effect.
   // (TODO: This is kind of confusing - considering disallowing using them
if (config->timeTraceEnabled)
     referenceStubBinder();
 
     // FIXME: should terminate the link early based on errors encountered so
     // far?
 
     createSyntheticSections();
     createSyntheticSymbols();
 
-    if (!config->exportedSymbols.empty()) {
+    if (!config->exportedSymbols.empty() || exportNoSymbols) {
       parallelForEach(symtab->getSymbols(), [](Symbol *sym) {
         if (auto *defined = dyn_cast<Defined>(sym)) {
           StringRef symbolName = defined->getName();
           if (config->exportedSymbols.match(symbolName)) {
             if (defined->privateExtern) {
               if (defined->weakDefCanBeHidden) {
                 // weak_def_can_be_hidden symbols behave similarly to
                 // private_extern symbols in most cases, except for when

lld/MachO/Options.td

 def exported_symbol : Separate<["-"], "exported_symbol">,
     MetaVarName<"<symbol>">,
     HelpText<"<symbol> remains global, while others become private externs">,
     Group<grp_resolve>;
 def exported_symbols_list : Separate<["-"], "exported_symbols_list">,
     MetaVarName<"<file>">,
     HelpText<"Symbols specified in <file> remain global, while others become private externs">,
     Group<grp_resolve>;
+def no_exported_symbols : Flag<["-"], "no_exported_symbols">,
+    HelpText<"Don't export any symbols from the binary, useful for main executables that don't have plugins">,
+    Group<grp_resolve>;
 def unexported_symbol : Separate<["-"], "unexported_symbol">,
     MetaVarName<"<symbol>">,
     HelpText<"Global <symbol> becomes private extern">,
     Group<grp_resolve>;
 def unexported_symbols_list : Separate<["-"], "unexported_symbols_list">,
     MetaVarName<"<file>">,
     HelpText<"Global symbols specified in <file> become private externs">,
     Group<grp_resolve>;

lld/test/MachO/export-options.s

 # REQUIRES: x86
 
 # RUN: rm -rf %t; split-file %s %t
 # RUN: llvm-mc -filetype=obj -triple=x86_64-apple-macos %t/default.s -o %t/default.o
 # RUN: llvm-mc -filetype=obj -triple=x86_64-apple-macos %t/lazydef.s -o %t/lazydef.o
 # RUN: llvm-ar --format=darwin rcs %t/lazydef.a %t/lazydef.o
 
 ## Check that mixing exported and unexported symbol options yields an error
 # RUN: not %lld -dylib %t/default.o -o /dev/null \
 # RUN:         -exported_symbol a -unexported_symbol b 2>&1 | \
 # RUN:     FileCheck --check-prefix=CONFLICT %s
+#
+# RUN: not %lld -dylib %t/default.o -o /dev/null \
+# RUN:         -exported_symbols_list /dev/null -unexported_symbol b 2>&1 | \
+# RUN:     FileCheck --check-prefix=CONFLICT %s
+#
+# RUN: not %lld -dylib %t/default.o -o /dev/null \
+# RUN:         -no_exported_symbols -unexported_symbol b 2>&1 | \
+# RUN:     FileCheck --check-prefix=CONFLICT %s
 
 # CONFLICT: error: cannot use both -exported_symbol* and -unexported_symbol* options
 # CONFLICT-NEXT: >>> ignoring unexports
 
 ## Check that an exported literal name with no symbol definition yields an error
 ## but that an exported glob-pattern with no matching symbol definition is OK
 # RUN: not %lld -dylib %t/default.o -o /dev/null \
 # RUN:         -exported_symbol absent_literal \

 # RUN: %no-fatal-warnings-lld -dylib -exported_symbol "_foo" %t/autohide.o \
 # RUN:   -dead_strip %t/weak-private-extern.o -o %t/exported-hidden
 # RUN: llvm-nm -m %t/exported-hidden | FileCheck %s --check-prefix=AUTOHIDE-PRIVATE-DEAD-STRIP
 
 # EXP-AUTOHIDE: T _foo
 # AUTOHIDE-PRIVATE: error: cannot export hidden symbol _foo
 # AUTOHIDE-PRIVATE-DEAD-STRIP: (__TEXT,__text) non-external (was a private external) _foo
 
+## Test not exporting any symbols
+# RUN: %lld -dylib %t/symdefs.o -o %t/noexports -exported_symbols_list /dev/null
+# RUN: llvm-objdump --macho --exports-trie %t/noexports | FileCheck --check-prefix=NOEXPORTS %s
+# RUN: %lld -dylib %t/symdefs.o -o %t/noexports -no_exported_symbols
+# RUN: llvm-objdump --macho --exports-trie %t/noexports | FileCheck --check-prefix=NOEXPORTS %s
+
+# NOEXPORTS-NOT: globby_also
+# NOEXPORTS-NOT: globby_only
+# NOEXPORTS-NOT: literal_also
+# NOEXPORTS-NOT: literal_only
+
 #--- default.s
 
 .globl _keep_globl, _hide_globl
 _keep_globl:
   retq
 _hide_globl:
   retq
 .private_extern _private_extern