[LLD] [COFF] Allow specifying export forwarding in a def file

[mstorsjo]

Summary

Previously this was only supported when specified on the command line or in directives.

[mstorsjo]

Ping

[ruiu]

Is this a feature supported by the MS linker?

[mstorsjo]

In D46244#1088633, @ruiu wrote:

Is this a feature supported by the MS linker?

Yes, link.exe supports this.

[ruiu]

COFF/Driver.cpp

631

Can you add a comment?

[mstorsjo]

COFF/Driver.cpp

631

Sure - I can add one like this:

In simple cases, only Name is set. Renamed exports are parsed and set as "ExtName = Name". If Name has the form "OtherDll.Func", it shouldn't be a normal exported function but a forward to another DLL instead.

[ruiu]

COFF/Driver.cpp

631

Sounds good! I'd also mention that this is a MS linker feature.

[mstorsjo]

Added the requested comment.

[mstorsjo]

COFF/Driver.cpp

631

It's not only a MS linker feature, GNU ld.bfd also supports the same, so I didn't add this comment.

I started poking all this when I wanted to fix the misconception as I'm fixing in D46245, which all boiled down to the comment "// GNU dlltool accepts both = and ==." The thing is that GNU tools do support both = and ==, but they're two completely different things, while the = case is the exact same as in link.exe. Due to this misunderstanding, we've been overloading the same concept with different behaviours between MinGW and MSVC cases, even though it's actually much less messy.

[mstorsjo]

Ping, anything else on this one?

[ruiu]

COFF/Driver.cpp

631

By MS feature, I meant I'd make it clear that this is not MinGW-only feature. I'd think this is valuable information to add, as we've added a few MinGW-specific rules.

[mstorsjo]

Extended the comment as requested, mentioning that it's supported by both MS and GNU.

[mstorsjo]

Ping @ruiu

[ruiu]

LGTM

COFF/Driver.cpp

if (M.MinorImageVersion)
     Config->MinorImageVersion = M.MinorImageVersion;
   if (M.MajorOSVersion)
     Config->MajorOSVersion = M.MajorOSVersion;
   if (M.MinorOSVersion)
     Config->MinorOSVersion = M.MinorOSVersion;
 
   for (COFFShortExport E1 : M.Exports) {
     Export E2;
+    // In simple cases, only Name is set. Renamed exports are parsed
+    // and set as "ExtName = Name". If Name has the form "OtherDll.Func",
+    // it shouldn't be a normal exported function but a forward to another
+    // DLL instead. This is supported by both MS and GNU linkers.
+    if (E1.ExtName != E1.Name && StringRef(E1.Name).contains('.')) {
+      E2.Name = E1.ExtName;
+      E2.ForwardTo = E1.Name;
+      Config->Exports.push_back(E2);
+      continue;
+    }
     E2.Name = Saver.save(E1.Name);
     if (E1.isWeak())
       E2.ExtName = Saver.save(E1.ExtName);
     E2.Ordinal = E1.Ordinal;
     E2.Noname = E1.Noname;
     E2.Data = E1.Data;
     E2.Private = E1.Private;
     E2.Constant = E1.Constant;

test/COFF/export.test

 SYMTAB: __imp_exportfn2 in export.test.tmp.DLL
 SYMTAB: exportfn2 in export.test.tmp.DLL
 SYMTAB: __imp_exportfn3 in export.test.tmp.DLL
 SYMTAB: exportfn3 in export.test.tmp.DLL
 
 # RUN: lld-link /out:%t.dll /dll %t.obj /export:foo=kernel32.foobar
 # RUN: llvm-objdump -p %t.dll | FileCheck -check-prefix=FORWARDER %s
 
+# RUN: echo "EXPORTS foo=kernel32.foobar" > %t.def
+# RUN: lld-link /out:%t.dll /dll %t.obj /def:%t.def
+# RUN: llvm-objdump -p %t.dll | FileCheck -check-prefix=FORWARDER %s
+
 FORWARDER: Export Table:
 FORWARDER:  DLL name: export.test.tmp.dll
 FORWARDER:  Ordinal base: 0
 FORWARDER:  Ordinal      RVA  Name
 FORWARDER:        0        0
 FORWARDER:        1   0x1010  exportfn
 FORWARDER:        2           foo (forwarded to kernel32.foobar)