[LLD] [COFF] Add a separate option for allowing duplicate weak symbols

The MinGW mode (enabled with the flag -lldmingw) does allow duplicate
weak symbols. A test in
compiler-rt/test/profile/Windows/coverage-weak-lld.cpp does currently
enable the -lldmingw flag in an MSVC context, in order to deal with
duplicate weak symbols.

Add a new, separate, lld specific flag for enabling this. In MinGW
mode, this is enabled by default, otherwise it is disabled.

This allows making the MinGW mode more restrictive in adding
libpaths from the surrounding environment; in MinGW mode, all
libpaths are passed explicitly by the compiler driver to the
linker, which is attempted in https://reviews.llvm.org/D144084.

Author: mstorsjo

lld/COFF/Config.h

@@ -314,6 +314,7 @@ struct Configuration {
   bool stdcallFixup = false;
   bool writeCheckSum = false;
   EmitKind emit = EmitKind::Obj;
+  bool allowDuplicateWeak = false;
 };
 
 } // namespace lld::coff

lld/COFF/Driver.cpp

@@ -1993,6 +1993,9 @@ void LinkerDriver::linkerMain(ArrayRef<const char *> argsArr) {
   config->stdcallFixup =
       args.hasFlag(OPT_stdcall_fixup, OPT_stdcall_fixup_no, config->mingw);
   config->warnStdcallFixup = !args.hasArg(OPT_stdcall_fixup);
+  config->allowDuplicateWeak =
+      args.hasFlag(OPT_lld_allow_duplicate_weak,
+                   OPT_lld_allow_duplicate_weak_no, config->mingw);
 
   if (args.hasFlag(OPT_inferasanlibs, OPT_inferasanlibs_no, false))
     warn("ignoring '/inferasanlibs', this flag is not supported");

lld/COFF/InputFiles.cpp

@@ -81,7 +81,7 @@ static void checkAndSetWeakAlias(COFFLinkerContext &ctx, InputFile *f,
       // of another symbol emitted near the weak symbol.
       // Just use the definition from the first object file that defined
       // this weak symbol.
-      if (ctx.config.mingw)
+      if (ctx.config.allowDuplicateWeak)
         return;
       ctx.symtab.reportDuplicate(source, f);
     }

lld/COFF/Options.td

@@ -232,6 +232,7 @@ defm demangle : B<"demangle",
 def include_optional : Joined<["/", "-", "/?", "-?"], "includeoptional:">,
     HelpText<"Add symbol as undefined, but allow it to remain undefined">;
 def kill_at : F<"kill-at">;
+defm lld_allow_duplicate_weak : B_priv<"lld-allow-duplicate-weak">;
 def lldemit : P<"lldemit", "Specify output type">;
 def lldmingw : F<"lldmingw">;
 def noseh : F<"noseh">;

lld/test/COFF/gnu-weak.test

@@ -1,4 +1,9 @@
 RUN: lld-link -lldmingw %S/Inputs/gnu-weak.o %S/Inputs/gnu-weak2.o -out:%t.exe
+RUN: lld-link -lld-allow-duplicate-weak %S/Inputs/gnu-weak.o %S/Inputs/gnu-weak2.o -out:%t.exe
+RUN: not lld-link %S/Inputs/gnu-weak.o %S/Inputs/gnu-weak2.o -out:%t.exe 2>&1 | FileCheck %s --check-prefix=DEFAULT-ERROR
+
+DEFAULT-ERROR: error: duplicate symbol: weakfunc
+
 
 GNU ld can handle several definitions of the same weak symbol, and
 unless there is a strong definition of it, it just picks the first