[lld/mac] Implement -u flag

[thakis]

Summary

Since we emit diagnostics for undefineds in Writer::scanRelocations()
and symbols referenced by -u flags aren't referenced by any relocations,
this needs some manual code (similar to the entry point).

[MaskRay]

lld/test/MachO/u.s

22

Would be nice to check the interaction with .weak_reference

[thakis]

lld/test/MachO/u.s

22

As far as I understand, .weak_reference only affects which opcodes are written to the import bind FSA description and doesn't interact with symbol lookup. Are you thinking of -U instead of -u maybe? Or do you mean a test that checks if a -u resolved against a dylib pulls in the dylib (independent of weak_ref)?

[int3]

Since we emit diagnostics for undefineds in Writer::scanRelocations()

I've been wondering whether this is the right way to do it... in particular, we end up emitting one error message per relocation to an undefined, even if they all point to the same undefined. I made a brief attempt to scan the symbol table instead, but that creates spurious errors for things like .globl unused_and_undefined. I suppose we could always just dedup the symbols when scanning through the relocations though. Do you know how LLD-ELF handles this?

lld/test/MachO/u.s

22

yeah I don't see how weak_reference would need to be tested here either

[thakis]

In D94371#2493861, @int3 wrote:

Since we emit diagnostics for undefineds in Writer::scanRelocations()

I've been wondering whether this is the right way to do it... in particular, we end up emitting one error message per relocation to an undefined, even if they all point to the same undefined. I made a brief attempt to scan the symbol table instead, but that creates spurious errors for things like .globl unused_and_undefined. I suppose we could always just dedup the symbols when scanning through the relocations though. Do you know how LLD-ELF handles this?

COFF goes through the symbol table. ELF is relocation-based (IIRC it used to not be but something moved it to be relocation based.) I made it dedup diags in D63344 (later slightly refined in D77017).

[MaskRay]

For ELF, both linkers (GNU ld,gold,LLD) and loaders (glibc,musl,FreeBSD rtld,etc) report undefined symbols only when relocations are present.

[int3]

Thanks for the pointers to the ELF diffs! I guess we'll implement deduplication in a similar way in the future.

lld/test/MachO/u.s

32

nit: extra line

lld/MachO/Config.h

struct Configuration {
   PlatformInfo platform;
   UndefinedSymbolTreatment undefinedSymbolTreatment =
       UndefinedSymbolTreatment::error;
   llvm::MachO::HeaderFileType outputType;
   std::vector<llvm::StringRef> systemLibraryRoots;
   std::vector<llvm::StringRef> librarySearchPaths;
   std::vector<llvm::StringRef> frameworkSearchPaths;
   std::vector<llvm::StringRef> runtimePaths;
+  std::vector<Symbol *> explicitUndefineds;
   llvm::DenseMap<llvm::StringRef, SymbolPriorityEntry> priorities;
 };
 
 // The symbol with the highest priority should be ordered first in the output
 // section (modulo input section contiguity constraints). Using priority
 // (highest first) instead of order (lowest first) has the convenient property
 // that the default-constructed zero priority -- for symbols/sections without a
 // user-defined order -- naturally ends up putting them at the end of the

lld/MachO/Driver.cpp

bool macho::link(ArrayRef<const char *> argsArr, bool canExitEarly,
   }
 
   config = make<Configuration>();
   symtab = make<SymbolTable>();
   target = createTargetInfo(args);
 
   config->entry = symtab->addUndefined(args.getLastArgValue(OPT_e, "_main"),
                                        /*isWeakRef=*/false);
+  for (auto *arg : args.filtered(OPT_u))
+    config->explicitUndefineds.push_back(
+        symtab->addUndefined(arg->getValue(), /*isWeakRef=*/false));
   config->outputFile = args.getLastArgValue(OPT_o, "a.out");
   config->installName =
       args.getLastArgValue(OPT_install_name, config->outputFile);
   config->headerPad = args::getHex(args, OPT_headerpad, /*Default=*/32);
   config->headerPadMaxInstallNames =
       args.hasArg(OPT_headerpad_max_install_names);
   config->printEachFile = args.hasArg(OPT_t);
   config->printWhyLoad = args.hasArg(OPT_why_load);
bool macho::link(ArrayRef<const char *> argsArr, bool canExitEarly,
   StringRef orderFile = args.getLastArgValue(OPT_order_file);
   if (!orderFile.empty())
     parseOrderFile(orderFile);
 
   if (config->outputType == MH_EXECUTE && isa<Undefined>(config->entry)) {
     error("undefined symbol: " + toString(*config->entry));
     return false;
   }
+  // FIXME: This prints symbols that are undefined both in input files and
+  // via -u flag twice.
+  for (const auto *undefined : config->explicitUndefineds) {
+    if (isa<Undefined>(undefined)) {
+      error("undefined symbol: " + toString(*undefined) +
+            "\n>>> referenced by flag -u " + toString(*undefined));
+      return false;
+    }
+  }
 
   createSyntheticSections();
   symtab->addDSOHandle(in.header);
 
   for (opt::Arg *arg : args.filtered(OPT_sectcreate)) {
     StringRef segName = arg->getValue(0);
     StringRef sectName = arg->getValue(1);
     StringRef fileName = arg->getValue(2);

lld/MachO/Options.td

def alias_list : Separate<["-"], "alias_list">,
      Group<grp_resolve>;
 def flat_namespace : Flag<["-"], "flat_namespace">,
      HelpText<"Resolve symbols from all dylibs, both direct & transitive. Do not record source libraries: dyld must re-search at runtime and use the first definition found">,
      Flags<[HelpHidden]>,
      Group<grp_resolve>;
 def u : Separate<["-"], "u">,
      MetaVarName<"<symbol>">,
      HelpText<"Require that <symbol> be defined for the link to succeed">,
-     Flags<[HelpHidden]>,
      Group<grp_resolve>;
 def U : Separate<["-"], "U">,
      MetaVarName<"<symbol>">,
      HelpText<"Allow <symbol> to have no definition">,
      Flags<[HelpHidden]>,
      Group<grp_resolve>;
 def undefined : Separate<["-"], "undefined">,
      MetaVarName<"<treatment>">,

lld/test/MachO/u.s

+# REQUIRES: x86
+# RUN: rm -rf %t
+# RUN: split-file %s %t
+
+# RUN: llvm-mc -filetype=obj -triple=x86_64-apple-macos -o %t/foo.o %t/foo.s
+# RUN: llvm-ar csr  %t/lib.a %t/foo.o
+
+# RUN: llvm-mc -filetype=obj -triple=x86_64-apple-macos -o %t/main.o %t/main.s
+
+# RUN: %lld %t/main.o %t/lib.a -o /dev/null -why_load | \
+# RUN:     FileCheck %s --check-prefix=NOFOO --allow-empty
+
+# RUN: %lld %t/main.o %t/lib.a -u _foo -o /dev/null -why_load | \
+# RUN:     FileCheck %s --check-prefix=FOO
+
+# RUN: not %lld %t/main.o %t/lib.a -u _asdf -o /dev/null 2>&1 | \
+# RUN:     FileCheck %s --check-prefix=UNDEF
+
+# NOFOO-NOT: _foo forced load of lib.a(foo.o)
+# FOO: _foo forced load of lib.a(foo.o)
+# UNDEF: error: undefined symbol: _asdf
+
+#--- foo.s
+.globl _foo
+_foo:
+  ret
+
+#--- main.s
+.globl _main
+_main:
+  ret
+