Silence warning in LTO mode on VxWorks

The link phase is always partial (-r) for VxWorks in kernel mode, which
means that it uses incremental LTO linking by default (-flinker-output=rel).
But in this mode the LTO plugin outputs a warning if one of the object files
involved in the link does not contain LTO bytecode, before switching to
nolto-rel mode.  We do not do repeated incremental linking for VxWorks so
silence the warning.

lto-plugin/
	* lto-plugin.c: Document -linker-output-auto-notlo-rel option.
	(linker_output_set): Change type to bool.
	(linker_output_known): Likewise.
	(linker_output_auto_nolto_rel): New variable.
	(all_symbols_read_handler): Take it into account.
	<LDPO_REL>: Do not issue the warning if it is set.
	(process_option): Process -linker-output-auto-notlo-rel.
	(cleanup_handler): Remove unused variable.
	(onload) <LDPT_LINKER_OUTPUT>: Adjust to above type change.
gcc/
	* gcc.c (LTO_PLUGIN_SPEC): Define if not already.
	(LINK_PLUGIN_SPEC): Execute LTO_PLUGIN_SPEC.
	* config/vxworks.h (LTO_PLUGIN_SPEC): Define.

gcc/ChangeLog

@@ -1,3 +1,9 @@
+2020-05-05  Eric Botcazou  <ebotcazou@adacore.com>
+
+	* gcc.c (LTO_PLUGIN_SPEC): Define if not already.
+	(LINK_PLUGIN_SPEC): Execute LTO_PLUGIN_SPEC.
+	* config/vxworks.h (LTO_PLUGIN_SPEC): Define.
+
 2020-05-05  Eric Botcazou  <ebotcazou@adacore.com>
 
 	* gimplify.c (gimplify_init_constructor): Do not put the constructor

gcc/config/vxworks.h

@@ -273,3 +273,11 @@ extern void vxworks_asm_out_destructor (rtx symbol, int priority);
 #undef DWARF_GNAT_ENCODINGS_DEFAULT
 #define DWARF_GNAT_ENCODINGS_DEFAULT \
   (TARGET_VXWORKS7 ? DWARF_GNAT_ENCODINGS_MINIMAL : DWARF_GNAT_ENCODINGS_ALL)
+
+/* The default configuration of incremental LTO linking (-flinker-output=rel)
+   warns if an object file included in the link does not contain LTO bytecode,
+   because in this case the output will not contain it either, thus preventing
+   further incremental LTO linking.  We do not do repeated incremental linking
+   so silence the warning (instead of passing -flinker-output=nolto-rel).  */
+#undef LTO_PLUGIN_SPEC
+#define LTO_PLUGIN_SPEC "%{!mrtp:-plugin-opt=-linker-output-auto-notlo-rel}"

gcc/gcc.c

@@ -944,6 +944,10 @@ proper position among the other output files.  */
 # endif
 #endif
 
+#ifndef LTO_PLUGIN_SPEC
+#define LTO_PLUGIN_SPEC ""
+#endif
+
 /* Conditional to test whether the LTO plugin is used or not.
    FIXME: For slim LTO we will need to enable plugin unconditionally.  This
    still cause problems with PLUGIN_LD != LD and when plugin is built but
@@ -968,6 +972,7 @@ proper position among the other output files.  */
     -plugin %(linker_plugin_file) \
     -plugin-opt=%(lto_wrapper) \
     -plugin-opt=-fresolution=%u.res \
+    " LTO_PLUGIN_SPEC "\
     %{flinker-output=*:-plugin-opt=-linker-output-known} \
     %{!nostdlib:%{!nodefaultlibs:%:pass-through-libs(%(link_gcc_c_sequence))}} \
     }" PLUGIN_COND_CLOSE

lto-plugin/ChangeLog

@@ -1,3 +1,15 @@
+2020-05-05  Eric Botcazou  <ebotcazou@adacore.com>
+
+	* lto-plugin.c: Document -linker-output-auto-notlo-rel option.
+	(linker_output_set): Change type to bool.
+	(linker_output_known): Likewise.
+	(linker_output_auto_nolto_rel): New variable.
+	(all_symbols_read_handler): Take it into account.
+	<LDPO_REL>: Do not issue the warning if it is set.
+	(process_option): Process -linker-output-auto-notlo-rel.
+	(cleanup_handler): Remove unused variable.
+	(onload) <LDPT_LINKER_OUTPUT>: Adjust to above type change.
+
 2020-04-28  H.J. Lu  <hongjiu.lu@intel.com>
 
 	PR bootstrap/94739

lto-plugin/lto-plugin.c

@@ -32,6 +32,9 @@ along with this program; see the file COPYING3.  If not see
    -nop: Instead of running lto-wrapper, pass the original to the plugin. This
    only works if the input files are hybrid. 
    -linker-output-known: Do not determine linker output
+   -linker-output-auto-notlo-rel: Switch from rel to nolto-rel mode without
+   warning.  This is used on systems like VxWorks (kernel) where the link is
+   always partial and repeated incremental linking is generally not used.
    -sym-style={none,win32,underscore|uscore}
    -pass-through  */
 
@@ -195,8 +198,9 @@ static bool verbose;
 static char nop;
 static char *resolution_file = NULL;
 static enum ld_plugin_output_file_type linker_output;
-static int linker_output_set;
+static bool linker_output_set;
-static int linker_output_known;
+static bool linker_output_known;
+static bool linker_output_auto_nolto_rel;
 static const char *link_output_name = NULL;
 
 /* The version of gold being used, or -1 if not gold.  The number is
@@ -709,9 +713,10 @@ use_original_files (void)
 static enum ld_plugin_status
 all_symbols_read_handler (void)
 {
+  const unsigned num_lto_args
+    = num_claimed_files + lto_wrapper_num_args + 2
+      + !linker_output_known + !linker_output_auto_nolto_rel;
   unsigned i;
-  unsigned num_lto_args = num_claimed_files + lto_wrapper_num_args + 2
-    	   + !linker_output_known;
   char **lto_argv;
   const char *linker_output_str = NULL;
   const char **lto_arg_ptr;
@@ -743,9 +748,10 @@ all_symbols_read_handler (void)
 	case LDPO_REL:
 	  if (non_claimed_files)
 	    {
-	      message (LDPL_WARNING, "incremental linking of LTO and non-LTO "
+	      if (!linker_output_auto_nolto_rel)
-		       "objects; using -flinker-output=nolto-rel which will "
+		message (LDPL_WARNING, "incremental linking of LTO and non-LTO"
-		       "bypass whole program optimization");
+			 " objects; using -flinker-output=nolto-rel which will"
+			 " bypass whole program optimization");
 	      linker_output_str = "-flinker-output=nolto-rel";
 	    }
 	  else
@@ -1291,8 +1297,10 @@ static void
 process_option (const char *option)
 {
   if (strcmp (option, "-linker-output-known") == 0)
-    linker_output_known = 1;
+    linker_output_known = true;
-  if (strcmp (option, "-debug") == 0)
+  else if (strcmp (option, "-linker-output-auto-notlo-rel") == 0)
+    linker_output_auto_nolto_rel = true;
+  else if (strcmp (option, "-debug") == 0)
     debug = true;
   else if ((strcmp (option, "-v") == 0)
            || (strcmp (option, "--verbose") == 0))
@@ -1390,7 +1398,7 @@ onload (struct ld_plugin_tv *tv)
 	  break;
 	case LDPT_LINKER_OUTPUT:
 	  linker_output = (enum ld_plugin_output_file_type) p->tv_u.tv_val;
-	  linker_output_set = 1;
+	  linker_output_set = true;
 	  break;
 	case LDPT_OUTPUT_NAME:
 	  /* We only use this to make user-friendly temp file names.  */