[llvm-dlltool] Fix renamed imports without a separate regular import entry (#98229)

Normally, when doing renamed imports, we do this by providing a
weak alias, towards another regular import, for the symbol we
want to actually import. In a def file, this looks like this:

    regularfunc
    renamedfunc == regularfunc

However, if we want to link against a function in a DLL, where we
(intentionally) don't provide a regular import for that symbol
with the name in its DLL, doing the renamed import with a weak
alias doesn't work, as there's no symbol that the weak alias can
point towards.

We can't make up such an import either, as we may intentionally
not want to provide a regular import for that name.

This situation can either be resolved by using the "long" import
library format (as e.g. produced by GNU dlltool), or by using the
new short import library name type "export as".

This patch implements it by using the "export as" name type.

When producing a renamed import, defer emitting it until all regular
imports have been produced. If the renamed import refers to a
symbol that does exist as a regular import entry, produce a
weak alias, just as before. (This implementation also avoids needing
to know whether the symbol that the alias points towards actually
is prefixed or not, too.)

If the renamed import points at a symbol that isn't otherwise
available (or is available as a renamed symbol itself), generate
an "export as" import entry.

This name type is new, and is normally used in ARM64EC import
libraries, but can also be used for other architectures.

lld/test/COFF/lib.test

@@ -1,6 +1,14 @@
 # RUN: lld-link /machine:x64 /def:%S/Inputs/library.def /out:%t.lib
 # RUN: llvm-nm %t.lib | FileCheck %s
 
+CHECK: 00000000 R __imp_constant
+CHECK: 00000000 R constant
+
+CHECK: 00000000 D __imp_data
+
+CHECK: 00000000 T __imp_function
+CHECK: 00000000 T function
+
 CHECK: 00000000 a @comp.id
 CHECK: 00000000 a @feat.00
 CHECK: 00000000 W alias
@@ -11,11 +19,3 @@ 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
-

llvm/include/llvm/Object/COFFImportFile.h

@@ -135,11 +135,10 @@ struct COFFShortExport {
 /// linking both ARM64EC and pure ARM64 objects, and the linker will pick only
 /// the exports relevant to the target platform. For non-hybrid targets,
 /// the NativeExports parameter should not be used.
-Error writeImportLibrary(StringRef ImportName, StringRef Path,
-                         ArrayRef<COFFShortExport> Exports,
-                         COFF::MachineTypes Machine, bool MinGW,
-                         ArrayRef<COFFShortExport> NativeExports = std::nullopt,
-                         bool AddUnderscores = true);
+Error writeImportLibrary(
+    StringRef ImportName, StringRef Path, ArrayRef<COFFShortExport> Exports,
+    COFF::MachineTypes Machine, bool MinGW,
+    ArrayRef<COFFShortExport> NativeExports = std::nullopt);
 
 } // namespace object
 } // namespace llvm

llvm/lib/Object/COFFImportFile.cpp

@@ -12,6 +12,8 @@
 
 #include "llvm/Object/COFFImportFile.h"
 #include "llvm/ADT/ArrayRef.h"
+#include "llvm/ADT/SmallVector.h"
+#include "llvm/ADT/StringMap.h"
 #include "llvm/ADT/Twine.h"
 #include "llvm/Object/Archive.h"
 #include "llvm/Object/ArchiveWriter.h"
@@ -657,8 +659,7 @@ NewArchiveMember ObjectFactory::createWeakExternal(StringRef Sym,
 Error writeImportLibrary(StringRef ImportName, StringRef Path,
                          ArrayRef<COFFShortExport> Exports,
                          MachineTypes Machine, bool MinGW,
-                         ArrayRef<COFFShortExport> NativeExports,
-                         bool AddUnderscores) {
+                         ArrayRef<COFFShortExport> NativeExports) {
 
   MachineTypes NativeMachine = Machine;
   if (isArm64EC(Machine)) {
@@ -680,6 +681,13 @@ Error writeImportLibrary(StringRef ImportName, StringRef Path,
 
   auto addExports = [&](ArrayRef<COFFShortExport> Exp,
                         MachineTypes M) -> Error {
+    StringMap<std::string> RegularImports;
+    struct Deferred {
+      std::string Name;
+      ImportType ImpType;
+      const COFFShortExport *Export;
+    };
+    SmallVector<Deferred, 0> Renames;
     for (const COFFShortExport &E : Exp) {
       if (E.Private)
         continue;
@@ -724,15 +732,11 @@ Error writeImportLibrary(StringRef ImportName, StringRef Path,
         else if (Name == E.ImportName)
           NameType = IMPORT_NAME;
         else {
-          StringRef Prefix = "";
-          if (Machine == IMAGE_FILE_MACHINE_I386 && AddUnderscores)
-            Prefix = "_";
-
-          if (ImportType == IMPORT_CODE)
-            Members.push_back(OF.createWeakExternal(
-                (Prefix + E.ImportName).str(), Name, false, M));
-          Members.push_back(OF.createWeakExternal((Prefix + E.ImportName).str(),
-                                                  Name, true, M));
+          Deferred D;
+          D.Name = Name;
+          D.ImpType = ImportType;
+          D.Export = &E;
+          Renames.push_back(D);
           continue;
         }
       } else {
@@ -754,9 +758,25 @@ Error writeImportLibrary(StringRef ImportName, StringRef Path,
         }
       }
 
+      RegularImports[applyNameType(NameType, Name)] = Name;
       Members.push_back(OF.createShortImport(Name, E.Ordinal, ImportType,
                                              NameType, ExportName, M));
     }
+    for (const auto &D : Renames) {
+      auto It = RegularImports.find(D.Export->ImportName);
+      if (It != RegularImports.end()) {
+        // We have a regular import entry for a symbol with the name we
+        // want to reference; produce an alias pointing at that.
+        StringRef Symbol = It->second;
+        if (D.ImpType == IMPORT_CODE)
+          Members.push_back(OF.createWeakExternal(Symbol, D.Name, false, M));
+        Members.push_back(OF.createWeakExternal(Symbol, D.Name, true, M));
+      } else {
+        Members.push_back(OF.createShortImport(D.Name, D.Export->Ordinal,
+                                               D.ImpType, IMPORT_NAME_EXPORTAS,
+                                               D.Export->ImportName, M));
+      }
+    }
     return Error::success();
   };
 

llvm/lib/ToolDrivers/llvm-dlltool/DlltoolDriver.cpp

@@ -243,9 +243,8 @@ int llvm::dlltoolDriverMain(llvm::ArrayRef<const char *> ArgsArr) {
   }
 
   std::string Path = std::string(Args.getLastArgValue(OPT_l));
-  if (!Path.empty() &&
-      writeImportLibrary(OutputFile, Path, Exports, Machine,
-                         /*MinGW=*/true, NativeExports, AddUnderscores))
+  if (!Path.empty() && writeImportLibrary(OutputFile, Path, Exports, Machine,
+                                          /*MinGW=*/true, NativeExports))
     return 1;
   return 0;
 }

llvm/test/tools/llvm-dlltool/coff-decorated.def

@@ -7,7 +7,7 @@ EXPORTS
 CdeclFunction
 StdcallFunction@4
 @FastcallFunction@4
-StdcallAlias@4==StdcallFunction@4
+StdcallAlias@4==StdcallFunction
 ??_7exception@@6B@
 StdcallExportName@4=StdcallInternalFunction@4
 OtherStdcallExportName@4=CdeclInternalFunction

llvm/test/tools/llvm-dlltool/coff-weak-exports.def

@@ -5,6 +5,9 @@
 
 LIBRARY test.dll
 EXPORTS
+AltTestFunction
+AltTestFunction2
+AltTestData
 TestFunction==AltTestFunction
 TestData DATA == AltTestData
 ; When creating an import library, the DLL internal function name of
@@ -17,6 +20,14 @@ ImpLibName2 = Implementation2 == AltTestFunction2
 ; matter for the import library
 ImpLibName3 = kernel32.Sleep
 
+; CHECK:      T AltTestFunction
+; CHECK-NEXT: T __imp_AltTestFunction
+; CHECK:      T AltTestFunction2
+; CHECK-NEXT: T __imp_AltTestFunction2
+; CHECK:      T ImpLibName
+; CHECK-NEXT: T __imp_ImpLibName
+; CHECK:      T ImpLibName3
+; CHECK-NEXT: T __imp_ImpLibName3
 ; CHECK:      U AltTestFunction
 ; CHECK-NEXT: W TestFunction
 ; CHECK:      U __imp_AltTestFunction
@@ -24,14 +35,10 @@ ImpLibName3 = kernel32.Sleep
 ; CHECK-NOT:  W TestData
 ; CHECK:      U __imp_AltTestData
 ; CHECK-NEXT: W __imp_TestData
-; CHECK:      T ImpLibName
-; CHECK-NEXT: T __imp_ImpLibName
 ; CHECK:      U AltTestFunction2
 ; CHECK-NEXT: W ImpLibName2
 ; CHECK:      U __imp_AltTestFunction2
 ; CHECK-NEXT: W __imp_ImpLibName2
-; CHECK:      T ImpLibName3
-; CHECK-NEXT: T __imp_ImpLibName3
 
 ; ARCH-NOT: unknown arch
 

llvm/test/tools/llvm-dlltool/renaming.def

@@ -0,0 +1,39 @@
+; RUN: llvm-dlltool -k -m i386 --input-def %s --output-lib %t.a
+; RUN: llvm-readobj %t.a | FileCheck %s
+; RUN: llvm-nm %t.a | FileCheck %s -check-prefix=CHECK-NM
+
+LIBRARY test.dll
+EXPORTS
+
+symbolname == actualimport
+
+dataname DATA == actualdata
+
+_wcstok == wcstok
+wcstok == wcstok_s
+
+; CHECK-NM-NOT: actualimport
+; CHECK-NM-NOT: actualdata
+
+; CHECK:      Type: code
+; CHECK-NEXT: Name type: export as
+; CHECK-NEXT: Export name: actualimport
+; CHECK-NEXT: Symbol: __imp__symbolname
+; CHECK-NEXT: Symbol: _symbolname
+
+; CHECK:      Type: data
+; CHECK-NEXT: Name type: export as
+; CHECK-NEXT: Export name: actualdata
+; CHECK-NEXT: Symbol: __imp__dataname
+
+; CHECK:      Type: code
+; CHECK-NEXT: Name type: export as
+; CHECK-NEXT: Export name: wcstok
+; CHECK-NEXT: Symbol: __imp___wcstok
+; CHECK-NEXT: Symbol: __wcstok
+
+; CHECK:      Type: code
+; CHECK-NEXT: Name type: export as
+; CHECK-NEXT: Export name: wcstok_s
+; CHECK-NEXT: Symbol: __imp__wcstok
+; CHECK-NEXT: Symbol: _wcstok