Look for an entry point function if /nodefaultlib is given.

[ruiu]

Summary

Fixes https://bugs.llvm.org/show_bug.cgi?id=38018

[thakis]

Thanks!

COFF/Driver.cpp

 StringRef LinkerDriver::mangle(StringRef Sym) {
   assert(Config->Machine != IMAGE_FILE_MACHINE_UNKNOWN);
   if (Config->Machine == I386)
     return Saver.save("_" + Sym);
   return Sym;
 }
 
 // Windows specific -- find default entry point name.
+//
+// There are four different entry point functions for Windows executables,
+// each of which corresponds to a user-defined "main" function. This function
+// infers an entry point from a user-defined "main" function.
 StringRef LinkerDriver::findDefaultEntry() {
+  // As a special case, if /nodefaultlib is given, we directly look for an
+  // entry point. This is because, if no default library is linked, users
+  // need to define an entry point instead of a "main".
+  if (Config->NoDefaultLibAll) {
+    for (StringRef S : {"mainCRTStartup", "wmainCRTStartup",
+                        "WinMainCRTStartup", "wWinMainCRTStartup"}) {
+      StringRef Entry = Symtab->findMangle(S);
+      if (!Entry.empty() && !isa<Undefined>(Symtab->find(Entry)))
+        return mangle(S);
+    }
+    return "";
+  }
+
   // User-defined main functions and their corresponding entry points.
   static const char *Entries[][2] = {
       {"main", "mainCRTStartup"},
       {"wmain", "wmainCRTStartup"},
       {"WinMain", "WinMainCRTStartup"},
       {"wWinMain", "wWinMainCRTStartup"},
   };
   for (auto E : Entries) {

test/COFF/entry-inference3.test

+# RUN: yaml2obj < %s > %t.obj
+# RUN: not lld-link /out:%t.exe %t.obj /verbose /nodefaultlib > %t.log 2>&1
+# RUN: FileCheck %s < %t.log
+
+# CHECK: Entry name inferred: mainCRTStartup
+
+--- !COFF
+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
+  - Name:            mainCRTStartup
+    Value:           0
+    SectionNumber:   1
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_NULL
+    StorageClass:    IMAGE_SYM_CLASS_EXTERNAL
+...