LLD COFF: Add support for /force:multiple option

[troughton]

Summary

This patch adds support for linking with multiple definitions to LLD's COFF driver, in line with link.exe's /force:multiple option.

[troughton]

Could you please either take a look or suggest a reviewer?

[ruiu]

COFF/SymbolTable.cpp

205

Then you don't need to change the function name.

265

Looks like this is the only place where you are using this function. I'd inline it.

[troughton]

Address review comments by inlining errorOrWarnMultiple.

[ruiu]

LGTM

COFF/SymbolTable.cpp

329

We don't use auto unless an actual type is obvious:

auto -> std::string

Variable names should start with an uppercase letter and should be concise:

message -> Msg

[troughton]

Specify std::string over auto and change message to Msg.

[troughton]

If this looks fine, could you please merge it in? I’m not sure how the merge process is handled for LLVM.

[ruiu]

If you have a commit access, you can do that yourself, but since you don't have one, I'll do that for you.

[ruiu]

It needs a test, but I'll write that since the test should be small.

COFF/Config.h

   bool DoICF = true;
   bool TailMerge;
   bool Relocatable = true;
-  bool Force = false;
+  bool ForceMultiple = false;
+  bool ForceUnresolved = false;
   bool Debug = false;
   bool DebugDwarf = false;
   bool DebugGHashes = false;

COFF/Driver.cpp

 
   // Handle /force or /force:unresolved
   if (Args.hasArg(OPT_force, OPT_force_unresolved))
-    Config->Force = true;
+    Config->ForceUnresolved = true;
 
+  // Handle /force or /force:multiple
+  if (Args.hasArg(OPT_force, OPT_force_multiple))
+    Config->ForceMultiple = true;
+  
   // Handle /debug
   if (Args.hasArg(OPT_debug, OPT_debug_dwarf, OPT_debug_ghash)) {
     Config->Debug = true;

COFF/Options.td

 def wholearchive_flag : F<"wholearchive">;
 
 def force : F<"force">,
+    HelpText<"Allow undefined and multiply defined symbols when creating executables">;
+def force_unresolved : F<"force:unresolved">, 
     HelpText<"Allow undefined symbols when creating executables">;
-def force_unresolved : F<"force:unresolved">;
+def force_multiple : F<"force:multiple">, 
+    HelpText<"Allow multiply defined symbols when creating executables">;
 defm WX : B<"WX", "Treat warnings as errors", "Don't treat warnings as errors">;
 
 defm allowbind : B<"allowbind", "Enable DLL binding (default)",

COFF/SymbolTable.cpp

 }
 
 static void errorOrWarn(const Twine &S) {
-  if (Config->Force)
+  if (Config->ForceUnresolved)
     warn(S);
   else
     error(S);

 
     // Remaining undefined symbols are not fatal if /force is specified.
     // They are replaced with dummy defined symbols.
-    if (Config->Force)
+    if (Config->ForceUnresolved)
       replaceSymbol<DefinedAbsolute>(Sym, Name, 0);
     Undefs.insert(Sym);
   }

 }
 
 void SymbolTable::reportDuplicate(Symbol *Existing, InputFile *NewFile) {
-  error("duplicate symbol: " + toString(*Existing) + " in " +
-        toString(Existing->getFile()) + " and in " + toString(NewFile));
+  auto message = "duplicate symbol: " + toString(*Existing) + " in " +
+                 toString(Existing->getFile()) + " and in " +
+                 toString(NewFile);
+    
+  if (Config->ForceMultiple)
+    warn(message);
+  else
+    error(message);
 }
 
 Symbol *SymbolTable::addAbsolute(StringRef N, COFFSymbolRef Sym) {