Always default to DWARF2_DEBUG if not specified, warn about deprecated STABS

This makes defaults.h choose DWARF2_DEBUG if PREFERRED_DEBUGGING_TYPE
is not specified by the target and errors out if DWARF DWARF is not supported.

It also makes us warn when STABS is enabled but not the preferred
debugging type and removes the corresponding diagnostic from the Ada frontend.
The warnings are pruned from the testsuite output via prune_gcc_output.

The following target configurations now explicitely default to STABS:
 pdp11-*-*   pdp11 is a.out, dwarf support is difficult
 hppa[12]*-*-hpux10*  does not support DWARF
 hppa[12]*-*-hpux11*  likewise
note that the hppa configs have been deprecated.

Targets with DWARF support will now see
> ./cc1 -quiet t.c -gstabs
t.c: warning: STABS debugging information is obsolete and not supported anymore

that is, -gstabs will still generate STABS but use will be diagnosed
on targets where DWARF is available.

I have built all targets from contrib/config-list.mk to make sure we
don't run into the #error and the following makes the STABS usage
explicit for pdp11 and hppa with SOM.

This completes the series of deprecating STABS for GCC 12.

2021-09-21  Richard Biener  <rguenther@suse.de>

gcc/
	* defaults.h (PREFERRED_DEBUGGING_TYPE): Choose DWARF2_DEBUG
	when not set.
	* toplev.c (process_options): Warn when STABS debugging is
	enabled but not the preferred format.
	* config/pa/som.h (PREFERRED_DEBUGGING_TYPE): Define to
	DBX_DEBUG.
	* config/pdp11/pdp11.h (PREFERRED_DEBUGGING_TYPE): Likewise.

gcc/ada/
	* gcc-interface/misc.c (gnat_post_options): Do not warn
	about DBX_DEBUG use here.

gcc/testsuite/
	* lib/prune.exp: Prune STABS obsoletion message.

6 files changed, 20 insertions, 30 deletions

diff --git a/gcc/ada/gcc-interface/misc.c b/gcc/ada/gcc-interface/misc.c
index d073570..2caa83f 100644
--- a/gcc/ada/gcc-interface/misc.c
+++ b/gcc/ada/gcc-interface/misc.c
@@ -274,12 +274,6 @@ gnat_post_options (const char **pfilename ATTRIBUTE_UNUSED)
   if (!OPTION_SET_P (flag_diagnostics_show_caret))
     global_dc->show_caret = false;
 
-  /* Warn only if STABS is not the default: we don't want to emit a warning if
-     the user did not use a -gstabs option.  */
-  if (PREFERRED_DEBUGGING_TYPE != DBX_DEBUG && write_symbols == DBX_DEBUG)
-    warning (0, "STABS debugging information for Ada is obsolete and not "
-		"supported anymore");
-
   /* Copy global settings to local versions.  */
   gnat_encodings = global_options.x_gnat_encodings;
   optimize = global_options.x_optimize;
diff --git a/gcc/config/pa/som.h b/gcc/config/pa/som.h
index 05cc315..36a1122 100644
--- a/gcc/config/pa/som.h
+++ b/gcc/config/pa/som.h
@@ -21,6 +21,10 @@ along with GCC; see the file COPYING3.  If not see
 #undef TARGET_SOM
 #define TARGET_SOM 1
 
+/* With SOM we can only do STABS.  */
+#undef PREFERRED_DEBUGGING_TYPE
+#define PREFERRED_DEBUGGING_TYPE DBX_DEBUG
+
 /* We do not use BINCL stabs in SOM.
    ??? If it does not hurt, we probably should to avoid useless divergence
    from other embedded stabs implementations.  */
diff --git a/gcc/config/pdp11/pdp11.h b/gcc/config/pdp11/pdp11.h
index 9bc5e08..91a8ce7 100644
--- a/gcc/config/pdp11/pdp11.h
+++ b/gcc/config/pdp11/pdp11.h
@@ -54,6 +54,9 @@ along with GCC; see the file COPYING3.  If not see
 
 #define DBX_DEBUGGING_INFO
 
+#undef PREFERRED_DEBUGGING_TYPE
+#define PREFERRED_DEBUGGING_TYPE DBX_DEBUG
+
 #define TARGET_40_PLUS		(TARGET_40 || TARGET_45)
 #define TARGET_10		(! TARGET_40_PLUS)
 
diff --git a/gcc/defaults.h b/gcc/defaults.h
index 04d9bb1..bb68d0d 100644
--- a/gcc/defaults.h
+++ b/gcc/defaults.h
@@ -899,33 +899,14 @@ see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
 #define DEFAULT_GDB_EXTENSIONS 1
 #endif
 
-/* If more than one debugging type is supported, you must define
-   PREFERRED_DEBUGGING_TYPE to choose the default.  */
-
-#if 1 < (defined (DBX_DEBUGGING_INFO) \
-         + defined (DWARF2_DEBUGGING_INFO) + defined (XCOFF_DEBUGGING_INFO) \
-         + defined (VMS_DEBUGGING_INFO))
+/* Default to DWARF2_DEBUGGING_INFO.  Legacy targets can choose different
+   by defining PREFERRED_DEBUGGING_TYPE.  */
 #ifndef PREFERRED_DEBUGGING_TYPE
-#error You must define PREFERRED_DEBUGGING_TYPE
-#endif /* no PREFERRED_DEBUGGING_TYPE */
-
-/* If only one debugging format is supported, define PREFERRED_DEBUGGING_TYPE
-   here so other code needn't care.  */
-#elif defined DBX_DEBUGGING_INFO
-#define PREFERRED_DEBUGGING_TYPE DBX_DEBUG
-
-#elif defined DWARF2_DEBUGGING_INFO || defined DWARF2_LINENO_DEBUGGING_INFO
+#if defined DWARF2_DEBUGGING_INFO || defined DWARF2_LINENO_DEBUGGING_INFO
 #define PREFERRED_DEBUGGING_TYPE DWARF2_DEBUG
-
-#elif defined VMS_DEBUGGING_INFO
-#define PREFERRED_DEBUGGING_TYPE VMS_AND_DWARF2_DEBUG
-
-#elif defined XCOFF_DEBUGGING_INFO
-#define PREFERRED_DEBUGGING_TYPE XCOFF_DEBUG
-
 #else
-/* No debugging format is supported by this target.  */
-#define PREFERRED_DEBUGGING_TYPE NO_DEBUG
+#error You must define PREFERRED_DEBUGGING_TYPE if DWARF is not supported
+#endif
 #endif
 
 #ifndef FLOAT_LIB_COMPARE_RETURNS_BOOL
diff --git a/gcc/testsuite/lib/prune.exp b/gcc/testsuite/lib/prune.exp
index fac212e..1314248 100644
--- a/gcc/testsuite/lib/prune.exp
+++ b/gcc/testsuite/lib/prune.exp
@@ -93,6 +93,9 @@ proc prune_gcc_output { text } {
     # Ignore dsymutil warning (tool bug is actually linker)
     regsub -all "(^|\n)\[^\n\]*could not find object file symbol for symbol\[^\n\]*" $text "" text
 
+    # Ignore stabs obsoletion warnings
+    regsub -all "(^|\n)\[^\n\]*\[Ww\]arning: STABS debugging information is obsolete and not supported anymore\[^\n\]*" $text "" text
+
     # If dg-enable-nn-line-numbers was provided, then obscure source-margin
     # line numbers by converting them to "NN" form.
     set text [maybe-handle-nn-line-numbers $text]
diff --git a/gcc/toplev.c b/gcc/toplev.c
index 4f574a5..cb4f8c4 100644
--- a/gcc/toplev.c
+++ b/gcc/toplev.c
@@ -1435,6 +1435,11 @@ process_options (bool no_backend)
       && ctf_debug_info_level == CTFINFO_LEVEL_NONE)
     write_symbols = NO_DEBUG;
 
+  /* Warn if STABS debug gets enabled and is not the default.  */
+  if (PREFERRED_DEBUGGING_TYPE != DBX_DEBUG && (write_symbols & DBX_DEBUG))
+    warning (0, "STABS debugging information is obsolete and not "
+	     "supported anymore");
+
   if (write_symbols == NO_DEBUG)
     ;
 #if defined(DBX_DEBUGGING_INFO)