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

[mstorsjo]

Summary

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.

[rnk]

lgtm

MinGW/Options.td

61–62

Interesting, I've always interpreted it as -require-defined.

[ruiu]

This is LGTM, but I believe you will have to implement -u in a more correct way. IIRC, I implemented -u as -require-defined in the ELF linker first. It seemed to be working fine until we found something weird, investigated it and then found that that was caused by an incorrect implementation of -u. So, if it is not too hard, you probably should implement it in the right way.

[orgads]

Solves the problem for me. Thanks!

[mstorsjo]

Mimic the real -u option behavior properly.

[ruiu]

LGTM

MinGW/Driver.cpp

bool mingw::link(ArrayRef<const char *> ArgsArr, raw_ostream &Diag) {
 
   if (Args.getLastArgValue(OPT_m) == "i386pe")
     Add("-alternatename:__image_base__=___ImageBase");
   else
     Add("-alternatename:__image_base__=__ImageBase");
 
   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)) {
     SearchPaths.push_back(A->getValue());
     Add("-libpath:" + StringRef(A->getValue()));
   }
 
   StringRef Prefix = "";

MinGW/Options.td

 def output_def: S<"output-def">, HelpText<"Output def file">;
 def shared: F<"shared">, HelpText<"Build a shared object">;
 def subs: S<"subsystem">, HelpText<"Specify subsystem">;
 def stack: S<"stack">;
 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">;
 def verbose: F<"verbose">, HelpText<"Verbose mode">;
 def version: F<"version">, HelpText<"Display the version number and exit">;
 def require_defined: S<"require-defined">,
     HelpText<"Force symbol to be added to symbol table as an undefined one">;
 def require_defined_eq: J<"require-defined=">, Alias<require_defined>;
 
 // LLD specific options
 def _HASH_HASH_HASH : Flag<["-"], "###">,
     HelpText<"Print (but do not run) the commands to run for this compilation">;
 def appcontainer: F<"appcontainer">, HelpText<"Set the appcontainer flag in the executable">;
 def mllvm: S<"mllvm">;
 def pdb: S<"pdb">, HelpText<"Output PDB debug info file, chosen implicitly if the argument is empty">;
 def pdb_eq: J<"pdb=">, Alias<pdb>;
 def Xlink : J<"Xlink=">, MetaVarName<"<arg>">,
     HelpText<"Pass <arg> to the COFF linker">;

test/MinGW/driver.test

 RUN: ld.lld -### foo.o -m i386pep --Map bar.map | FileCheck -check-prefix=MAP %s
 RUN: ld.lld -### foo.o -m i386pep -Map=bar.map | FileCheck -check-prefix=MAP %s
 RUN: ld.lld -### foo.o -m i386pep --Map=bar.map | FileCheck -check-prefix=MAP %s
 MAP: -lldmap:bar.map
 
 RUN: ld.lld -### foo.o -m i386pe -require-defined _foo --require-defined _bar -require-defined=_baz --require-defined=_foo2 | FileCheck -check-prefix=REQUIRE-DEFINED %s
 REQUIRE-DEFINED: -include:_foo -include:_bar -include:_baz -include:_foo2
 
+RUN: ld.lld -### foo.o -m i386pe -u _foo --undefined _bar -undefined=_baz --undefined=_foo2 | FileCheck -check-prefix=UNDEFINED %s
+UNDEFINED: -includeoptional:_foo -includeoptional:_bar -includeoptional:_baz -includeoptional:_foo2
+
 RUN: ld.lld -### -m i386pep foo.o -Llibpath | FileCheck -check-prefix LIBPATH %s
 LIBPATH: -libpath:libpath
 
 RUN: ld.lld -### -m i386pep foo.o --no-insert-timestamp | FileCheck -check-prefix NOTIMESTAMP %s
 RUN: ld.lld -### -m i386pep foo.o --insert-timestamp --no-insert-timestamp | FileCheck -check-prefix NOTIMESTAMP %s
 NOTIMESTAMP: -timestamp:0
 RUN: ld.lld -### -m i386pep foo.o --no-insert-timestamp --insert-timestamp | FileCheck -check-prefix TIMESTAMP %s
 TIMESTAMP-NOT: -timestamp:0