COFF: Handle undefined symbols starting with __imp_ in a special way.

MSVC linker is able to link an object file created from the following code.
Note that __imp_hello is not defined anywhere.

  void hello() { printf("Hello\n"); }
  extern void (*__imp_hello)();
  int main() { __imp_hello(); }

Function symbols exported from DLLs are automatically mangled by appending
__imp_ prefix, so they have two names (original one and with the prefix).
This "feature" seems to simulate that behavior even for non-DLL symbols.

This is in my opnion very odd feature. Even MSVC linker warns if you use this.
I'm adding that anyway for the sake of compatibiltiy.

llvm-svn: 240620

Author: rui314

lld/COFF/SymbolTable.cpp

@@ -70,17 +70,26 @@ bool SymbolTable::reportRemainingUndefines() {
     auto *Undef = dyn_cast<Undefined>(Sym->Body);
     if (!Undef)
       continue;
+    StringRef Name = Undef->getName();
     if (SymbolBody *Alias = Undef->getWeakAlias()) {
       Sym->Body = Alias->getReplacement();
       if (!isa<Defined>(Sym->Body)) {
         // Aliases are yet another symbols pointed by other symbols
         // that could also remain undefined.
-        llvm::errs() << "undefined symbol: " << Undef->getName() << "\n";
+        llvm::errs() << "undefined symbol: " << Name << "\n";
         Ret = true;
       }
       continue;
     }
-    llvm::errs() << "undefined symbol: " << Undef->getName() << "\n";
+    // If we can resolve a symbol by removing __imp_ prefix, do that.
+    // This odd rule is for compatibility with MSVC linker.
+    if (Name.startswith("__imp_")) {
+      if (Defined *Imp = find(Name.substr(strlen("__imp_")))) {
+        Sym->Body = Imp;
+        continue;
+      }
+    }
+    llvm::errs() << "undefined symbol: " << Name << "\n";
     Ret = true;
   }
   return Ret;

lld/test/COFF/locally-imported.test

@@ -0,0 +1,41 @@
+# RUN: yaml2obj < %s > %t.obj
+# RUN: lld -flavor link2 /out:%t.exe %t.obj
+
+# The linker automatically removes __imp_ prefix if it helps to resolve all symbols.
+
+---
+header:
+  Machine:         IMAGE_FILE_MACHINE_AMD64
+  Characteristics: []
+sections:
+  - Name:            .text
+    Characteristics: [ IMAGE_SCN_CNT_CODE, IMAGE_SCN_MEM_EXECUTE, IMAGE_SCN_MEM_READ ]
+    Alignment:       4
+    SectionData:     B82A000000C3
+symbols:
+  - Name:            .text
+    Value:           0
+    SectionNumber:   1
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_NULL
+    StorageClass:    IMAGE_SYM_CLASS_STATIC
+    SectionDefinition:
+      Length:          6
+      NumberOfRelocations: 0
+      NumberOfLinenumbers: 0
+      CheckSum:        0
+      Number:          0
+      Selection:       IMAGE_COMDAT_SELECT_ANY
+  - Name:            mainCRTStartup
+    Value:           0
+    SectionNumber:   1
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_FUNCTION
+    StorageClass:    IMAGE_SYM_CLASS_EXTERNAL
+  - Name:            __imp_mainCRTStartup
+    Value:           0
+    SectionNumber:   0
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_FUNCTION
+    StorageClass:    IMAGE_SYM_CLASS_EXTERNAL
+...