[LLD][COFF] Support GNU style == aliases

[aykevl]

Summary

D46245 added support for this in llvm-libtool, but while lld-link can also create .lib files from .def files it didn't support aliases.

I compared the Inputs/library.def test against the output from llvm-libtool and it matches, except for the fact that lld-link reorders functions for some reason.

I have also verified that this fixes a bug I was running into while trying to compile .def files to .lib files in MinGW-w64 (using lld-link instead of llvm-libtool).

[mstorsjo]

This looks ok to me, but there's a couple unclear cases:

• While this works for creating an import library out of a def file only, analoguously to llvm-dlltool, one can't create similar aliases while actually linking (doing that requires a couple more tweaks in a couple places, at least).
• While this can replace some use cases of llvm-dlltool, for i386, there's nothing (yet) here that corresponds to dlltool's flag -k. (This is used for mingw-w64's system def files for i386, it's vital for producing the right kind of import library for stdcall functions.)

[aykevl]

In D113365#3116356, @mstorsjo wrote:

• While this works for creating an import library out of a def file only, analoguously to llvm-dlltool, one can't create similar aliases while actually linking (doing that requires a couple more tweaks in a couple places, at least).

Can you elaborate? With this fix I was able to link Windows binaries that referenced these aliases (using the MinWG lld driver). The linker was able to resolve them to the correct, non-weak versions.

• While this can replace some use cases of llvm-dlltool, for i386, there's nothing (yet) here that corresponds to dlltool's flag -k. (This is used for mingw-w64's system def files for i386, it's vital for producing the right kind of import library for stdcall functions.)

Thanks, good to know! Right now I'm not targeting i386 so that's not a problem for me.

[mstorsjo]

In D113365#3121497, @aykevl wrote:

In D113365#3116356, @mstorsjo wrote:

• While this works for creating an import library out of a def file only, analoguously to llvm-dlltool, one can't create similar aliases while actually linking (doing that requires a couple more tweaks in a couple places, at least).

Can you elaborate? With this fix I was able to link Windows binaries that referenced these aliases (using the MinWG lld driver). The linker was able to resolve them to the correct, non-weak versions.

I’m not talking about linking against the created import library, I’m talking about linking the actual dll that the import library is for.

Example:

$ cat mylib.c 
void func1(void) {}
$ cat mylib.def 
EXPORTS
func1
aliasname == func1
$ clang -c mylib.c
$ lld-link mylib.o -out:mylib.dll -dll -noentry -def:mylib.def -implib:mylib.lib
lld-link: error: <root>: undefined symbol: aliasname
$ lld-link -def:mylib.def -implib:mylib.lib -machine:x64 

Thus, the def with an alias works when you create an implib from the def file only, but you can't use the same def file when you actually produce your DLL.

lld/COFF/Config.h

 static const auto ARM64 = llvm::COFF::IMAGE_FILE_MACHINE_ARM64;
 static const auto ARMNT = llvm::COFF::IMAGE_FILE_MACHINE_ARMNT;
 static const auto I386 = llvm::COFF::IMAGE_FILE_MACHINE_I386;
 
 // Represents an /export option.
 struct Export {
   StringRef name;       // N in /export:N or /export:E=N
   StringRef extName;    // E in /export:E=N
+  StringRef aliasTarget; // GNU specific: N in "alias == N"
   Symbol *sym = nullptr;
   uint16_t ordinal = 0;
   bool noname = false;
   bool data = false;
   bool isPrivate = false;
   bool constant = false;
 
   // If an export is a form of /export:foo=dllname.bar, that means
   // that foo should be exported as an alias to bar in the DLL.
   // forwardTo is set to "dllname.bar" part. Usually empty.
   StringRef forwardTo;
   StringChunk *forwardChunk = nullptr;
 
   // True if this /export option was in .drectves section.
   bool directives = false;
   StringRef symbolName;
   StringRef exportName; // Name in DLL
 
   bool operator==(const Export &e) {
     return (name == e.name && extName == e.extName &&
+            aliasTarget == e.aliasTarget &&
             ordinal == e.ordinal && noname == e.noname &&
             data == e.data && isPrivate == e.isPrivate);
   }
 };
 
 enum class DebugType {
   None  = 0x0,
   CV    = 0x1,  /// CodeView

lld/COFF/Driver.cpp

 
 static void createImportLibrary(bool asLib) {
   std::vector<COFFShortExport> exports;
   for (Export &e1 : config->exports) {
     COFFShortExport e2;
     e2.Name = std::string(e1.name);
     e2.SymbolName = std::string(e1.symbolName);
     e2.ExtName = std::string(e1.extName);
+    e2.AliasTarget = std::string(e1.aliasTarget);
     e2.Ordinal = e1.ordinal;
     e2.Noname = e1.noname;
     e2.Data = e1.data;
     e2.Private = e1.isPrivate;
     e2.Constant = e1.constant;
     exports.push_back(e2);
   }
 
if (!e1.ExtName.empty() && e1.ExtName != e1.Name &&
         StringRef(e1.Name).contains('.')) {
       e2.name = saver.save(e1.ExtName);
       e2.forwardTo = saver.save(e1.Name);
       config->exports.push_back(e2);
       continue;
     }
     e2.name = saver.save(e1.Name);
     e2.extName = saver.save(e1.ExtName);
+    e2.aliasTarget = saver.save(e1.AliasTarget);
     e2.ordinal = e1.Ordinal;
     e2.noname = e1.Noname;
     e2.data = e1.Data;
     e2.isPrivate = e1.Private;
     e2.constant = e1.Constant;
     config->exports.push_back(e2);
   }
 }

lld/test/COFF/Inputs/library.def

 LIBRARY library
 EXPORTS
   function
+  alias == function
   data DATA
   constant CONSTANT

lld/test/COFF/lib.test

 # RUN: lld-link /machine:x64 /def:%S/Inputs/library.def /out:%t.lib
 # RUN: llvm-nm %t.lib | FileCheck %s
 
+CHECK: 00000000 a @comp.id
+CHECK: 00000000 a @feat.00
+CHECK: 00000000 W alias
+CHECK:          U function
+
+CHECK: 00000000 a @comp.id
+CHECK: 00000000 a @feat.00
+CHECK: 00000000 W __imp_alias
+CHECK:          U __imp_function
+
 CHECK: 00000000 R __imp_constant
 CHECK: 00000000 R constant
 
 CHECK: 00000000 D __imp_data
 
 CHECK: 00000000 T __imp_function
 CHECK: 00000000 T function