lld-link: Several tweaks to default entry point selection.

Three related changes:

1. link.exe uses the presence of main and wmain to decide if it should call
   mainCRTStartup or wmainCRTStartup, even if /nodefaultlib is passed. For
   compatibility, remove FindMain logic.

2. Default to the non-wide entrypoint if main is not found. This has two effects:

2a. In normal links, lld-link now prints

        lld-link: error: undefined symbol: _main
        >>> referenced by f:\dd\vctools\crt\vcstartup\src\startup\exe_common.inl:78
        >>>               libcmt.lib(exe_main.obj):("int __cdecl invoke_main(void)" (?invoke_main@@yahxz))
        >>> referenced by f:\dd\vctools\crt\vcstartup\src\startup\exe_common.inl:283
        >>>               libcmt.lib(exe_main.obj):("int __cdecl __scrt_common_main_seh(void)" (?__scrt_common_main_seh@@yahxz))

    instead of

        lld-link: error: entry point must be defined

    This is arguably a better error message, since it now mentions that _main is
    missing. (This matches link.exe's diagnostic in this case.)

2b. With /nodefautlib, we now default to mainCRTStartup if no main() is
    present, again matching link.exe. This makes r337407 obsolete.

This means if you have a cc file containing both mainCRTStartup and
wmainCRTStartup and you pass /nodefaultlib /subsystem:console, lld-link will
now call mainCRTStartup, matching link.exe

3. Print a warning if both main and wmain are present, similar to link.exe's
   LNK4067.

Differential Revision: https://reviews.llvm.org/D52832

llvm-svn: 343698

lld/COFF/Driver.cpp

@@ -430,26 +430,28 @@ StringRef LinkerDriver::findDefaultEntry() {
   assert(Config->Subsystem != IMAGE_SUBSYSTEM_UNKNOWN &&
          "must handle /subsystem before calling this");
 
-  // 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".
-  bool FindMain = !Config->NoDefaultLibAll;
   if (Config->Subsystem == IMAGE_SUBSYSTEM_WINDOWS_GUI) {
-    if (findUnderscoreMangle(FindMain ? "WinMain" : "WinMainCRTStartup"))
-      return mangle("WinMainCRTStartup");
-    if (findUnderscoreMangle(FindMain ? "wWinMain" : "wWinMainCRTStartup"))
-      return mangle("wWinMainCRTStartup");
-  }
-  if (findUnderscoreMangle(FindMain ? "main" : "mainCRTStartup"))
-    return mangle("mainCRTStartup");
-  if (findUnderscoreMangle(FindMain ? "wmain" : "wmainCRTStartup"))
-    return mangle("wmainCRTStartup");
-  return "";
+    if (findUnderscoreMangle("wWinMain")) {
+      if (!findUnderscoreMangle("WinMain"))
+        return mangle("wWinMainCRTStartup");
+      warn("found both wWinMain and WinMain; using latter");
+    }
+    return mangle("WinMainCRTStartup");
+  }
+  if (findUnderscoreMangle("wmain")) {
+    if (!findUnderscoreMangle("main"))
+      return mangle("wmainCRTStartup");
+    warn("found both wmain and main; using latter");
+  }
+  return mangle("mainCRTStartup");
 }
 
 WindowsSubsystem LinkerDriver::inferSubsystem() {
   if (Config->DLL)
     return IMAGE_SUBSYSTEM_WINDOWS_GUI;
+  // Note that link.exe infers the subsystem from the presence of these
+  // functions even if /entry: or /nodefaultlib are passed which causes them
+  // to not be called.
   bool HaveMain = findUnderscoreMangle("main");
   bool HaveWMain = findUnderscoreMangle("wmain");
   bool HaveWinMain = findUnderscoreMangle("WinMain");

lld/test/COFF/entry-inference.test

@@ -1,18 +1,26 @@
 # RUN: sed -e s/ENTRYNAME/main/ %s | yaml2obj > %t.obj
 # RUN: not lld-link /out:%t.exe %t.obj > %t.log 2>&1
 # RUN: FileCheck -check-prefix=MAIN %s < %t.log
+# RUN: not lld-link /nodefaultlib /out:%t.exe %t.obj > %t.log 2>&1
+# RUN: FileCheck -check-prefix=MAIN %s < %t.log
 
 # RUN: sed s/ENTRYNAME/wmain/ %s | yaml2obj > %t.obj
 # RUN: not lld-link /out:%t.exe %t.obj > %t.log 2>&1
 # RUN: FileCheck -check-prefix=WMAIN %s < %t.log
+# RUN: not lld-link /nodefaultlib /out:%t.exe %t.obj > %t.log 2>&1
+# RUN: FileCheck -check-prefix=WMAIN %s < %t.log
 
 # RUN: sed s/ENTRYNAME/WinMain/ %s | yaml2obj > %t.obj
 # RUN: not lld-link /out:%t.exe %t.obj > %t.log 2>&1
 # RUN: FileCheck -check-prefix=WINMAIN %s < %t.log
+# RUN: not lld-link /nodefaultlib /out:%t.exe %t.obj > %t.log 2>&1
+# RUN: FileCheck -check-prefix=WINMAIN %s < %t.log
 
 # RUN: sed s/ENTRYNAME/wWinMain/ %s | yaml2obj > %t.obj
 # RUN: not lld-link /out:%t.exe %t.obj > %t.log 2>&1
 # RUN: FileCheck -check-prefix=WWINMAIN %s < %t.log
+# RUN: not lld-link /nodefaultlib /out:%t.exe %t.obj > %t.log 2>&1
+# RUN: FileCheck -check-prefix=WWINMAIN %s < %t.log
 
 # MAIN:     error: <root>: undefined symbol: mainCRTStartup
 # WMAIN:    error: <root>: undefined symbol: wmainCRTStartup

lld/test/COFF/entry-inference4.test

@@ -14,10 +14,22 @@
 # RUN: not lld-link /subsystem:console /out:%t.exe %t.obj > %t.log 2>&1
 # RUN: FileCheck -check-prefix=WMAIN %s < %t.log
 
-# MAIN:     error: <root>: undefined symbol: mainCRTStartup
-# WMAIN:    error: <root>: undefined symbol: wmainCRTStartup
-# WINMAIN:  error: <root>: undefined symbol: WinMainCRTStartup
-# WWINMAIN: error: <root>: undefined symbol: wWinMainCRTStartup
+# RUN: sed 's/ENTRY1/wmain/;s/ENTRY2/main/' %s | yaml2obj > %t.obj
+# RUN: not lld-link /out:%t.exe %t.obj > %t.log 2>&1
+# RUN: FileCheck -check-prefix=MAINWMAIN %s < %t.log
+
+# RUN: sed 's/ENTRY1/wWinMain/;s/ENTRY2/WinMain/' %s | yaml2obj > %t.obj
+# RUN: not lld-link /out:%t.exe %t.obj > %t.log 2>&1
+# RUN: FileCheck -check-prefix=WINMAINWWINMAIN %s < %t.log
+
+# MAIN:            error: <root>: undefined symbol: mainCRTStartup
+# WMAIN:           error: <root>: undefined symbol: wmainCRTStartup
+# MAINWMAIN:       warning: found both wmain and main; using latter
+# MAINWMAIN:       error: <root>: undefined symbol: mainCRTStartup
+# WINMAIN:         error: <root>: undefined symbol: WinMainCRTStartup
+# WWINMAIN:        error: <root>: undefined symbol: wWinMainCRTStartup
+# WINMAINWWINMAIN: warning: found both wWinMain and WinMain; using latter
+# WINMAINWWINMAIN: error: <root>: undefined symbol: WinMainCRTStartup
 
 --- !COFF
 header: