[MinGW] Support the -u/--undefined option.

This is implemented by the lld-link option -include:, just like
--require-defined. Contrary to --require-defined, the -u/--undefined
option allows the symbol to remain undefined in the end.

This should fix PR42121.

Differential Revision: https://reviews.llvm.org/D62876

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@362882 91177308-0d34-0410-b5e6-96231b3b80d8

2 files changed, 6 insertions, 0 deletions

diff --git a/MinGW/Driver.cpp b/MinGW/Driver.cpp
index 36dde8d05..acab0d4a7 100644
--- a/MinGW/Driver.cpp
+++ b/MinGW/Driver.cpp
@@ -269,6 +269,8 @@ bool mingw::link(ArrayRef<const char *> ArgsArr, raw_ostream &Diag) {
 
   for (auto *A : Args.filtered(OPT_require_defined))
     Add("-include:" + StringRef(A->getValue()));
+  for (auto *A : Args.filtered(OPT_undefined))
+    Add("-includeoptional:" + StringRef(A->getValue()));
 
   std::vector<StringRef> SearchPaths;
   for (auto *A : Args.filtered(OPT_L)) {
diff --git a/MinGW/Options.td b/MinGW/Options.td
index 601492d83..242ae2552 100644
--- a/MinGW/Options.td
+++ b/MinGW/Options.td
@@ -46,6 +46,10 @@ def strip_all: F<"strip-all">,
     HelpText<"Omit all symbol information from the output binary">;
 def strip_debug: F<"strip-debug">,
     HelpText<"Omit all debug information, but keep symbol information">;
+def undefined: S<"u">,
+    HelpText<"Include symbol in the link, if available">;
+def undefined_long: S<"undefined">, Alias<undefined>;
+def undefined_eq: J<"undefined=">, Alias<undefined>;
 def whole_archive: F<"whole-archive">,
     HelpText<"Include all object files for following archives">;
 def v: Flag<["-"], "v">, HelpText<"Display the version number">;