Consider the entrypoint when determining exports

The export table is determined by looping over the objects included on
the command line. If the main symbol is included in a library (e.g.
libcamlrun or libasmrun) then there may not be an object on the
commandline which causes it, and any transitive dependencies, to be
linked.

The entrypoint symbol for the Cygwin/mingw-w64/MSVC toolchains is now
resolved, and may cause additional objects to be linked.

Author: dra27

CHANGES

@@ -20,8 +20,9 @@ Next version
 - GPR#156: Allow choosing an alternative linker and manifest tool at runtime
   with -use-linker=<cmd> and -use-mt=<cmd>.
   (Antonin Décimo, review by David Allsopp)
-- GPR#146: For mingw-w64, select crt2u.o instead of crt2.o if -link -municode
-  is specified (David Allsopp)
+- GPR#146: Take the entrypoint (main, wmainCRTStartup, etc.) into account when
+  determining the modules which will be linked. For mingw-w64, select crt2u.o
+  instead of crt2.o if -link -municode is specified (David Allsopp)
 
 Version 0.43
 - GPR#108: Add -lgcc_s to Cygwin's link libraries, upstreaming a patch from the

reloc.ml

@@ -715,7 +715,7 @@ let needed imported defined resolve_alias resolve_alternate obj =
     StrSet.empty
     obj.symbols
 
-let build_dll link_exe output_file files exts extra_args =
+let build_dll link_exe output_file files exts extra_args_string =
   let main_pgm = link_exe <> `DLL in
 
   (* fully resolve filenames, eliminate duplicates *)
@@ -997,6 +997,53 @@ let build_dll link_exe output_file files exts extra_args =
           link_obj (Printf.sprintf "%s(%s)" libname objname) obj)
   in
 
+  let entrypoint =
+    if not main_pgm then
+      None
+    else
+      match !toolchain with
+      | `CYGWIN64 ->
+          Some "main"
+      | `MINGW | `MINGW64 -> begin
+          let entry_point s =
+            String.length s > 7 && String.sub s 0 7 = "-Wl,-e,"
+          in
+          try
+            let s = List.find entry_point !extra_args in
+            Some (String.sub s 7 (String.length s - 7))
+          with Not_found ->
+            Some "mainCRTStartup"
+        end
+      | `MSVC | `MSVC64 -> begin
+          let entry_point s =
+            String.length s > 7 && String.lowercase_ascii (String.sub s 0 7) = "/entry:"
+          in
+          try
+            let s = List.find entry_point !extra_args in
+            Some (String.sub s 7 (String.length s - 7))
+          with Not_found ->
+            if !subsystem = "windows" then
+              Some "WinMainCRTStartup"
+            else
+              Some "mainCRTStartup"
+        end
+      | `LIGHTLD | `GNAT | `GNAT64 ->
+          None
+  in
+  let () =
+    match entrypoint with
+    | None -> ()
+    | Some entrypoint ->
+        try
+          let (libname, objname, _) as o = defined_in entrypoint in
+          if !explain then
+            Printf.printf "%s(%s) because of entrypoint %s\n%!" libname objname
+                                                                entrypoint;
+          link_libobj o
+        with Not_found ->
+          if !explain then
+            Printf.printf "Entrypoint %s not found\n%!" entrypoint
+  in
   let redirect = Hashtbl.create 16 in
   List.iter
     (fun (fn, obj) ->
@@ -1132,22 +1179,22 @@ let build_dll link_exe output_file files exts extra_args =
            being an empty file. *)
         let c = open_out implib in output_string c "x"; close_out c;
         let _impexp = add_temp (Filename.chop_suffix implib ".lib" ^ ".exp") in
-        let extra_args =
-          if !custom_crt then "/nodefaultlib:LIBCMT /nodefaultlib:MSVCRT " ^ extra_args
-          else "msvcrt.lib " ^ extra_args
+        let extra_args_string =
+          if !custom_crt then "/nodefaultlib:LIBCMT /nodefaultlib:MSVCRT " ^ extra_args_string
+          else "msvcrt.lib " ^ extra_args_string
         in
 
-        let extra_args =
-          if !machine = `x64 then (Printf.sprintf "/base:%s " !base_addr) ^ extra_args else extra_args
+        let extra_args_string =
+          if !machine = `x64 then (Printf.sprintf "/base:%s " !base_addr) ^ extra_args_string else extra_args_string
         in
 
-        let extra_args =
+        let extra_args_string =
           (* FlexDLL doesn't process .voltbl sections correctly, so don't allow the linker
              to process them. *)
           let command = link ^ " | findstr EMITVOLATILEMETADATA > NUL" in
           if Sys.command command = 0 then
-            "/EMITVOLATILEMETADATA:NO " ^ extra_args
-          else extra_args
+            "/EMITVOLATILEMETADATA:NO " ^ extra_args_string
+          else extra_args_string
         in
 
         (* Flexdll requires that all images (main programs and all the DLLs) are
@@ -1175,7 +1222,7 @@ let build_dll link_exe output_file files exts extra_args =
           (Filename.quote output_file)
           !subsystem
           files descr
-          extra_args
+          extra_args_string
     | `CYGWIN64 ->
         let def_file =
           if main_pgm then ""
@@ -1197,7 +1244,7 @@ let build_dll link_exe output_file files exts extra_args =
           descr
           files
           def_file
-          extra_args
+          extra_args_string
     | `MINGW | `MINGW64 | `GNAT | `GNAT64 ->
         let def_file =
           if main_pgm then ""
@@ -1221,7 +1268,7 @@ let build_dll link_exe output_file files exts extra_args =
           files
           def_file
           (if !implib then "-Wl,--out-implib=" ^ Filename.quote (Filename.chop_extension output_file ^ ".a") else "")
-          extra_args
+          extra_args_string
     | `LIGHTLD ->
         no_merge_manifest := true;
         let ld = Option.value !Cmdline.use_linker ~default:"ld" in
@@ -1234,7 +1281,7 @@ let build_dll link_exe output_file files exts extra_args =
           descr
           files
           (if !implib then "--out-implib " ^ Filename.quote (Filename.chop_extension output_file ^ ".a") else "")
-          extra_args
+          extra_args_string
   in
   debug ~dry_mode 1 "+ %s" cmd;
   if not !dry_mode then begin