[Arm64EC] Copy import descriptors to the EC Map (llvm#84834)

As noted in <llvm#78537>, MSVC
places import descriptors in both the EC and regular map - that PR moved
the descriptors to ONLY the regular map, however this causes linking
errors when linking as Arm64EC:

```
bcryptprimitives.lib(bcryptprimitives.dll) : error LNK2001: unresolved external symbol __IMPORT_DESCRIPTOR_bcryptprimitives (EC Symbol)
```

This change copies import descriptors from the regular map to the EC
map, which fixes this linking error.

llvm/include/llvm/Object/COFFImportFile.h

@@ -26,6 +26,12 @@
 namespace llvm {
 namespace object {
 
+constexpr std::string_view ImportDescriptorPrefix = "__IMPORT_DESCRIPTOR_";
+constexpr std::string_view NullImportDescriptorSymbolName =
+    "__NULL_IMPORT_DESCRIPTOR";
+constexpr std::string_view NullThunkDataPrefix = "\x7f";
+constexpr std::string_view NullThunkDataSuffix = "_NULL_THUNK_DATA";
+
 class COFFImportFile : public SymbolicFile {
 private:
   enum SymbolIndex { ImpSymbol, ThunkSymbol, ECAuxSymbol, ECThunkSymbol };

llvm/lib/Object/ArchiveWriter.cpp

@@ -677,6 +677,13 @@ static bool isECObject(object::SymbolicFile &Obj) {
   return false;
 }
 
+bool isImportDescriptor(StringRef Name) {
+  return Name.starts_with(ImportDescriptorPrefix) ||
+         Name == StringRef{NullImportDescriptorSymbolName} ||
+         (Name.starts_with(NullThunkDataPrefix) &&
+          Name.ends_with(NullThunkDataSuffix));
+}
+
 static Expected<std::vector<unsigned>> getSymbols(SymbolicFile *Obj,
                                                   uint16_t Index,
                                                   raw_ostream &SymNames,
@@ -704,6 +711,10 @@ static Expected<std::vector<unsigned>> getSymbols(SymbolicFile *Obj,
       if (Map == &SymMap->Map) {
         Ret.push_back(SymNames.tell());
         SymNames << Name << '\0';
+        // If EC is enabled, then the import descriptors are NOT put into EC
+        // objects so we need to copy them to the EC map manually.
+        if (SymMap->UseECMap && isImportDescriptor(Name))
+          SymMap->ECMap[Name] = Index;
       }
     } else {
       Ret.push_back(SymNames.tell());

llvm/lib/Object/COFFImportFile.cpp

@@ -108,7 +108,7 @@ template <class T> static void append(std::vector<uint8_t> &B, const T &Data) {
 }
 
 static void writeStringTable(std::vector<uint8_t> &B,
-                             ArrayRef<const std::string> Strings) {
+                             ArrayRef<const std::string_view> Strings) {
   // The COFF string table consists of a 4-byte value which is the size of the
   // table, including the length field itself.  This value is followed by the
   // string content itself, which is an array of null-terminated C-style
@@ -171,9 +171,6 @@ static Expected<std::string> replace(StringRef S, StringRef From,
   return (Twine(S.substr(0, Pos)) + To + S.substr(Pos + From.size())).str();
 }
 
-static const std::string NullImportDescriptorSymbolName =
-    "__NULL_IMPORT_DESCRIPTOR";
-
 namespace {
 // This class constructs various small object files necessary to support linking
 // symbols imported from a DLL.  The contents are pretty strictly defined and
@@ -192,8 +189,9 @@ class ObjectFactory {
 public:
   ObjectFactory(StringRef S, MachineTypes M)
       : NativeMachine(M), ImportName(S), Library(llvm::sys::path::stem(S)),
-        ImportDescriptorSymbolName(("__IMPORT_DESCRIPTOR_" + Library).str()),
-        NullThunkSymbolName(("\x7f" + Library + "_NULL_THUNK_DATA").str()) {}
+        ImportDescriptorSymbolName((ImportDescriptorPrefix + Library).str()),
+        NullThunkSymbolName(
+            (NullThunkDataPrefix + Library + NullThunkDataSuffix).str()) {}
 
   // Creates an Import Descriptor.  This is a small object file which contains a
   // reference to the terminators and contains the library name (entry) for the

llvm/test/tools/llvm-lib/arm64ec-implib.test

@@ -16,6 +16,8 @@ ARMAP-NEXT: #funcexp in test.dll
 ARMAP-NEXT: #mangledfunc in test.dll
 ARMAP-NEXT: ?test_cpp_func@@$$hYAHPEAX@Z in test.dll
 ARMAP-NEXT: ?test_cpp_func@@YAHPEAX@Z in test.dll
+ARMAP-NEXT: __IMPORT_DESCRIPTOR_test in test.dll
+ARMAP-NEXT: __NULL_IMPORT_DESCRIPTOR in test.dll
 ARMAP-NEXT: __imp_?test_cpp_func@@YAHPEAX@Z in test.dll
 ARMAP-NEXT: __imp_aux_?test_cpp_func@@YAHPEAX@Z in test.dll
 ARMAP-NEXT: __imp_aux_expname in test.dll
@@ -28,6 +30,7 @@ ARMAP-NEXT: __imp_mangledfunc in test.dll
 ARMAP-NEXT: expname in test.dll
 ARMAP-NEXT: funcexp in test.dll
 ARMAP-NEXT: mangledfunc in test.dll
+ARMAP-NEXT: test_NULL_THUNK_DATA in test.dll
 
 RUN: llvm-readobj test.lib | FileCheck -check-prefix=READOBJ %s
 
@@ -107,6 +110,8 @@ EXPAS-ARMAP-NEXT: #func1 in test.dll
 EXPAS-ARMAP-NEXT: #func2 in test.dll
 EXPAS-ARMAP-NEXT: #func3 in test.dll
 EXPAS-ARMAP-NEXT: #func4 in test.dll
+EXPAS-ARMAP-NEXT: __IMPORT_DESCRIPTOR_test in test.dll
+EXPAS-ARMAP-NEXT: __NULL_IMPORT_DESCRIPTOR in test.dll
 EXPAS-ARMAP-NEXT: __imp_aux_func1 in test.dll
 EXPAS-ARMAP-NEXT: __imp_aux_func2 in test.dll
 EXPAS-ARMAP-NEXT: __imp_aux_func3 in test.dll
@@ -121,6 +126,7 @@ EXPAS-ARMAP-NEXT: func1 in test.dll
 EXPAS-ARMAP-NEXT: func2 in test.dll
 EXPAS-ARMAP-NEXT: func3 in test.dll
 EXPAS-ARMAP-NEXT: func4 in test.dll
+EXPAS-ARMAP-NEXT: test_NULL_THUNK_DATA in test.dll
 
 EXPAS-READOBJ:      File: test.dll
 EXPAS-READOBJ-NEXT: Format: COFF-import-file-ARM64EC