d: Fix order of precedence for -defaultlib and -debuglib

The order of precedence used by the upstream reference compiler for
determining what library to link against is:
- No library if -nophoboslib or -fno-druntime was seen.
- The library passed to -debuglib if -g was also seen.
- The library passed to -defaultlib
- The in-tree libgphobos library.

This aligns the D language driver to follow the same rules.

gcc/d/ChangeLog:

	* d-spec.cc (need_phobos): Remove.
	(lang_specific_driver): Replace need_phobos with phobos_library.
	Reorder -debuglib and -defaultlib to have precedence over libphobos.
	(lang_specific_pre_link): Remove test for need_phobos.

gcc/d/ChangeLog

+2020-04-24  Iain Buclaw  <ibuclaw@gdcproject.org>
+
+	* d-spec.cc (need_phobos): Remove.
+	(lang_specific_driver): Replace need_phobos with phobos_library.
+	Reorder -debuglib and -defaultlib to have precedence over libphobos.
+	(lang_specific_pre_link): Remove test for need_phobos.
+
 2020-04-19  Iain Buclaw  <ibuclaw@gdcproject.org>
 
 	PR d/94609

gcc/d/d-spec.cc

@@ -61,10 +61,6 @@ enum phobos_action
 
 static phobos_action phobos_library = PHOBOS_DEFAULT;
 
-/* If true, use the standard D runtime library when linking with
-   standard libraries.  */
-static bool need_phobos = true;
-
 /* If true, do load libgphobos.spec even if not needed otherwise.  */
 static bool need_spec = false;
 
@@ -151,13 +147,15 @@ lang_specific_driver (cl_decoded_option **in_decoded_options,
 	  break;
 
 	case OPT_nophoboslib:
-	  need_phobos = false;
+	  phobos_library = PHOBOS_NOLINK;
 	  args[i] |= SKIPOPT;
 	  break;
 
 	case OPT_fdruntime:
 	  if (!value)
-	    need_phobos = false;
+	    phobos_library = PHOBOS_NOLINK;
+	  else
+	    phobos_library = PHOBOS_LINK;
 	  break;
 
 	case OPT_defaultlib_:
@@ -165,7 +163,6 @@ lang_specific_driver (cl_decoded_option **in_decoded_options,
 	    free (CONST_CAST (char *, defaultlib));
 	  if (arg != NULL)
 	    {
-	      need_phobos = false;
 	      args[i] |= SKIPOPT;
 	      defaultlib = XNEWVEC (char, strlen (arg));
 	      strcpy (CONST_CAST (char *, defaultlib), arg);
@@ -177,7 +174,6 @@ lang_specific_driver (cl_decoded_option **in_decoded_options,
 	    free (CONST_CAST (char *, debuglib));
 	  if (arg != NULL)
 	    {
-	      need_phobos = false;
 	      args[i] |= SKIPOPT;
 	      debuglib = XNEWVEC (char, strlen (arg));
 	      strcpy (CONST_CAST (char *, debuglib), arg);
@@ -314,10 +310,11 @@ lang_specific_driver (cl_decoded_option **in_decoded_options,
 #endif
 
   /* Make sure to have room for the trailing NULL argument.
-     - needstdcxx might add `-lstdcxx'
+     - need_stdcxx might add `-lstdcxx'
      - libphobos adds `-Bstatic -lphobos -Bdynamic'
      - only_source adds 1 more arg, also maybe add `-o'.  */
-  num_args = argc + need_stdcxx + shared_libgcc + need_phobos * 4 + 2;
+  num_args = argc + need_stdcxx + shared_libgcc
+    + (phobos_library != PHOBOS_NOLINK) * 4 + 2;
   new_decoded_options = XNEWVEC (cl_decoded_option, num_args);
 
   i = 0;
@@ -409,60 +406,51 @@ lang_specific_driver (cl_decoded_option **in_decoded_options,
     }
 
   /* Add `-lgphobos' if we haven't already done so.  */
-  if (phobos_library != PHOBOS_NOLINK && need_phobos)
+  if (phobos_library != PHOBOS_NOLINK)
     {
       /* Default to static linking.  */
       if (phobos_library != PHOBOS_DYNAMIC)
 	phobos_library = PHOBOS_STATIC;
 
 #ifdef HAVE_LD_STATIC_DYNAMIC
-      if (phobos_library == PHOBOS_DYNAMIC && static_link)
-	{
-	  generate_option (OPT_Wl_, LD_DYNAMIC_OPTION, 1, CL_DRIVER,
-			   &new_decoded_options[j]);
-	  j++;
-	}
-      else if (phobos_library == PHOBOS_STATIC && !static_link)
+      if (phobos_library == PHOBOS_STATIC && !static_link)
 	{
 	  generate_option (OPT_Wl_, LD_STATIC_OPTION, 1, CL_DRIVER,
-			   &new_decoded_options[j]);
-	  j++;
+			   &new_decoded_options[j++]);
 	}
 #endif
-
-      generate_option (OPT_l,
-		       saw_profile_flag ? LIBPHOBOS_PROFILE : LIBPHOBOS, 1,
-		       CL_DRIVER, &new_decoded_options[j]);
-      added_libraries++;
-      j++;
-
-#ifdef HAVE_LD_STATIC_DYNAMIC
-      if (phobos_library == PHOBOS_DYNAMIC && static_link)
+      /* Order of precedence in determining what library to link against is:
+	 - `-l<lib>' from `-debuglib=<lib>' if `-g' was also seen.
+	 - `-l<lib>' from `-defaultlib=<lib>'.
+	 - `-lgphobos' unless `-nophoboslib' or `-fno-druntime' was seen.  */
+      if (debuglib && saw_debug_flag)
 	{
-	  generate_option (OPT_Wl_, LD_STATIC_OPTION, 1, CL_DRIVER,
-			   &new_decoded_options[j]);
-	  j++;
+	  generate_option (OPT_l, debuglib, 1, CL_DRIVER,
+			   &new_decoded_options[j++]);
+	  added_libraries++;
 	}
-      else if (phobos_library == PHOBOS_STATIC && !static_link)
+      else if (defaultlib)
+	{
+	  generate_option (OPT_l, defaultlib, 1, CL_DRIVER,
+			   &new_decoded_options[j++]);
+	  added_libraries++;
+	}
+      else
+	{
+	  generate_option (OPT_l,
+			   saw_profile_flag ? LIBPHOBOS_PROFILE : LIBPHOBOS, 1,
+			   CL_DRIVER, &new_decoded_options[j++]);
+	  added_libraries++;
+	}
+
+#ifdef HAVE_LD_STATIC_DYNAMIC
+      if (phobos_library == PHOBOS_STATIC && !static_link)
 	{
 	  generate_option (OPT_Wl_, LD_DYNAMIC_OPTION, 1, CL_DRIVER,
-			   &new_decoded_options[j]);
-	  j++;
+			   &new_decoded_options[j++]);
 	}
 #endif
     }
-  else if (saw_debug_flag && debuglib)
-    {
-      generate_option (OPT_l, debuglib, 1, CL_DRIVER,
-		       &new_decoded_options[j++]);
-      added_libraries++;
-    }
-  else if (defaultlib)
-    {
-      generate_option (OPT_l, defaultlib, 1, CL_DRIVER,
-		       &new_decoded_options[j++]);
-      added_libraries++;
-    }
 
   if (saw_libcxx)
     new_decoded_options[j++] = *saw_libcxx;
@@ -492,7 +480,7 @@ lang_specific_driver (cl_decoded_option **in_decoded_options,
 int
 lang_specific_pre_link (void)
 {
-  if ((phobos_library != PHOBOS_NOLINK && need_phobos) || need_spec)
+  if ((phobos_library != PHOBOS_NOLINK) || need_spec)
     do_spec ("%:include(libgphobos.spec)");
 
   return 0;