[LLD] [COFF] Add -exclude-all-symbols for MinGW

[mstorsjo]

Summary

Also hook up the option in the MinGW driver.

Sent as one single review item for simplicity, but I can commit as two separate commits for COFF/MinGW.

[ruiu]

COFF/Driver.cpp

1613 ↗

(On Diff #187323)

Shouldn't this be Config->MinGW && Config->DLL && ...? IIUC this is a mingw extension, and we don't want to run the code below if not mingw.

[mstorsjo]

COFF/Driver.cpp

1613 ↗

(On Diff #187323)

We could do that as well. As far as I can see, it would only make a difference for the case when passing -export-all-symbols but not -lldmingw. The former is also an LLD private option only used by the MinGW frontend, so it doesn't really matter either way, but I can make it that way (I agree that the compound condition might be less confusing that way).

[ruiu]

COFF/Driver.cpp

1613 ↗

(On Diff #187323)

I think my preference is to make code that is expected to run only in MinGW mode to start with if (Config->MinGW) so that a reader can easily identify MinGW-specific features as such. Currently -export-all-symbols can be used without -lldmingw, but that's something no one cares, and I'd think for clarity -export-all-symbols` should be significant only when -lldmingw is also given.

[mstorsjo]

Changed the condition as requested.

[ruiu]

LGTM

lld/trunk/COFF/Driver.cpp

void LinkerDriver::link(ArrayRef<const char *> ArgsArr) {
   if (Args.hasFlag(OPT_safeseh, OPT_safeseh_no, false)) {
     for (ObjFile *File : ObjFile::Instances)
       if (!File->hasSafeSEH())
         error("/safeseh: " + File->getName() + " is not compatible with SEH");
     if (errorCount())
       return;
   }
 
-  // In MinGW, all symbols are automatically exported if no symbols
-  // are chosen to be exported.
-  if (Config->DLL && ((Config->MinGW && Config->Exports.empty()) ||
-                      Args.hasArg(OPT_export_all_symbols))) {
+  // In MinGW, if no symbols are chosen to be exported, then all symbols are
+  // automatically exported by default. This behavior can be forced by the
+  // -export-all-symbols option, so that it happens even when exports are
+  // explicitly specified. The automatic behavior can be disabled using the
+  // -exclude-all-symbols option, so that lld-link behaves like link.exe rather
+  // than MinGW in the case that nothing is explicitly exported.
+  if (Config->MinGW && Config->DLL &&
+      ((Config->Exports.empty() && !Args.hasArg(OPT_exclude_all_symbols)) ||
     Exporter.initSymbolExcludes();
 
     Symtab->forEachSymbol([=](Symbol *S) {
       auto *Def = dyn_cast<Defined>(S);
       if (!Exporter.shouldExport(Def))
         return;
       Export E;
       E.Name = Def->getName();

lld/trunk/COFF/Options.td

 defm tsaware  : B<"tsaware",
                   "Create Terminal Server aware executable (default)",
                   "Create non-Terminal Server aware executable">;
 
 def help : F<"help">;
 def help_q : Flag<["/?", "-?"], "">, Alias<help>;
 
 // LLD extensions
+def exclude_all_symbols : F<"exclude-all-symbols">;
 def export_all_symbols : F<"export-all-symbols">;
 def kill_at : F<"kill-at">;
 def lldmingw : F<"lldmingw">;
 def output_def : Joined<["/", "-"], "output-def:">;
 def pdb_source_path : P<"pdbsourcepath",
                         "Base path used to make relative source file path absolute in PDB">;
 def rsp_quoting : Joined<["--"], "rsp-quoting=">,
   HelpText<"Quoting style for response files, 'windows' (default) or 'posix'">;

lld/trunk/test/COFF/exclude-all.s

+# REQUIRES: x86
+
+# RUN: llvm-mc -triple=i686-windows-gnu %s -filetype=obj -o %t.obj
+
+# RUN: lld-link -lldmingw -exclude-all-symbols -dll -out:%t.dll -entry:DllMainCRTStartup@12 %t.obj
+# RUN: llvm-readobj -coff-exports %t.dll | FileCheck %s -check-prefix=NO-EXPORTS
+
+# NO-EXPORTS-NOT: Name:
+
+.global _foobar
+.global _DllMainCRTStartup@12
+.global _dataSym
+.text
+_DllMainCRTStartup@12:
+  ret
+_foobar:
+  ret
+.data
+_dataSym:
+  .int 4
+
+# Test specifying -exclude-all-symbols, on an object file that contains
+# dllexport directive for some of the symbols. In this case, the dllexported
+# symbols are still exported.
+
+# RUN: yaml2obj < %p/Inputs/export.yaml > %t.obj
+#
+# RUN: lld-link -out:%t.dll -dll %t.obj -lldmingw -exclude-all-symbols -output-def:%t.def
+# RUN: llvm-readobj -coff-exports %t.dll | FileCheck -check-prefix=DLLEXPORT %s
+
+# DLLEXPORT: Name: exportfn3