Introduce DWARF_VERSION_DEFAULT and redefine for VxWorks

This change introduces a target overridable macro to replace
the hardcoded value used to initialize dwarf_version from common.opt.

The main advantage compared to special code in a target
override_options hook is that redefinitions by target config files
are visible by both the compiler proper and by the driver, which
might refer to dwarf_version in ASM_DEBUG_SPECs and friends.

This is useful at least on VxWorks, where we usually need to
default to dwarf 4 or even 2 to accommodate non-gdb debuggers
provided by the environment, including for assembly sources
used in libgcc for some ports (witnessed with lse.S on aarch64).

2022-02-28  Olivier Hainque  <hainque@adacore.com>

gcc/
	* defaults.h (DWARF_DEFAULT_VERSION): Define if not
	defined already.
	* common.opt (gdwarf-): Use it.
	* doc/tm.texi.in (DWARF_DEFAULT_VERSION): Document.
	* doc/tm.texi: Update accordingly.

	* config/vxworks.h (DWARF_DEFAULT_VERSION): Redefine.
	* config/vxworks.cc: Remove code setting dwarf_version, now
	handled by the DWARF_DEFAULT_VERSION redefinition.

6 files changed, 25 insertions, 13 deletions

diff --git a/gcc/common.opt b/gcc/common.opt
index 58dc1a0..3a97e67 100644
--- a/gcc/common.opt
+++ b/gcc/common.opt
@@ -3317,7 +3317,7 @@ Common Driver JoinedOrMissing Negative(gdwarf-)
 Generate debug information in default version of DWARF format.
 
 gdwarf-
-Common Driver Joined UInteger Var(dwarf_version) Init(5)
+Common Driver Joined UInteger Var(dwarf_version) Init(DWARF_VERSION_DEFAULT)
 Generate debug information in DWARF v2 (or later) format.
 
 gdwarf32
diff --git a/gcc/config/vxworks.cc b/gcc/config/vxworks.cc
index 9d25873..f0c0df2 100644
--- a/gcc/config/vxworks.cc
+++ b/gcc/config/vxworks.cc
@@ -164,16 +164,10 @@ vxworks_override_options (void)
   if (flag_pic > 0 && !TARGET_VXWORKS_RTP)
     error ("PIC is only supported for RTPs");
 
-  /* VxWorks comes with non-gdb debuggers which only support strict
-     dwarf up to certain version.  Default dwarf control to friendly
-     values for these.  */
-
+  /* VxWorks comes with non-gdb debuggers which only support strict dwarf
+     up to certain versions, as controlled by DWARF_VERSION_DEFAULT.  */
   if (!OPTION_SET_P (dwarf_strict))
     dwarf_strict = 1;
-
-  if (!OPTION_SET_P (dwarf_version))
-    dwarf_version = VXWORKS_DWARF_VERSION_DEFAULT;
-
 }
 
 /* We don't want to use library symbol __clear_cache on SR0640.  Avoid
diff --git a/gcc/config/vxworks.h b/gcc/config/vxworks.h
index 84a9c93..d6ac83b 100644
--- a/gcc/config/vxworks.h
+++ b/gcc/config/vxworks.h
@@ -376,11 +376,11 @@ extern void vxworks_asm_out_destructor (rtx symbol, int priority);
   vxworks_emit_call_builtin___clear_cache
 extern void vxworks_emit_call_builtin___clear_cache (rtx begin, rtx end);
 
-/* Default dwarf control values, for non-gdb debuggers that come with
-   VxWorks.  */
+/* Default dwarf control values, accounting for non-gdb debuggers that come
+   with VxWorks.  */
 
-#undef VXWORKS_DWARF_VERSION_DEFAULT
-#define VXWORKS_DWARF_VERSION_DEFAULT (TARGET_VXWORKS7 ? 4 : 2)
+#undef DWARF_VERSION_DEFAULT
+#define DWARF_VERSION_DEFAULT (TARGET_VXWORKS7 ? 4 : 2)
 
 #undef DWARF_GNAT_ENCODINGS_DEFAULT
 #define DWARF_GNAT_ENCODINGS_DEFAULT \
diff --git a/gcc/defaults.h b/gcc/defaults.h
index 953605c..376687d 100644
--- a/gcc/defaults.h
+++ b/gcc/defaults.h
@@ -1441,6 +1441,12 @@ see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
 #define DWARF_GNAT_ENCODINGS_DEFAULT DWARF_GNAT_ENCODINGS_GDB
 #endif
 
+/* When generating dwarf info, the default standard version we'll honor
+   and advertise in absence of -gdwarf-<N> on the command line.  */
+#ifndef DWARF_VERSION_DEFAULT
+#define DWARF_VERSION_DEFAULT 5
+#endif
+
 #ifndef USED_FOR_TARGET
 /* Done this way to keep gengtype happy.  */
 #if BITS_PER_UNIT == 8
diff --git a/gcc/doc/tm.texi b/gcc/doc/tm.texi
index 7590924..110f8df 100644
--- a/gcc/doc/tm.texi
+++ b/gcc/doc/tm.texi
@@ -3294,6 +3294,12 @@ someone decided it was a good idea to use that register number to
 terminate the stack backtrace.  New ports should avoid this.
 @end defmac
 
+@defmac DWARF_VERSION_DEFAULT
+A C expression whose value is the default dwarf standard version we'll honor
+and advertise when generating dwarf debug information, in absence of
+an explicit @option{-gdwarf-@var{version}} option on the command line.
+@end defmac
+
 @deftypefn {Target Hook} void TARGET_DWARF_HANDLE_FRAME_UNSPEC (const char *@var{label}, rtx @var{pattern}, int @var{index})
 This target hook allows the backend to emit frame-related insns that
 contain UNSPECs or UNSPEC_VOLATILEs.  The DWARF 2 call frame debugging
diff --git a/gcc/doc/tm.texi.in b/gcc/doc/tm.texi.in
index 21b849e..501ddf1 100644
--- a/gcc/doc/tm.texi.in
+++ b/gcc/doc/tm.texi.in
@@ -2575,6 +2575,12 @@ someone decided it was a good idea to use that register number to
 terminate the stack backtrace.  New ports should avoid this.
 @end defmac
 
+@defmac DWARF_VERSION_DEFAULT
+A C expression whose value is the default dwarf standard version we'll honor
+and advertise when generating dwarf debug information, in absence of
+an explicit @option{-gdwarf-@var{version}} option on the command line.
+@end defmac
+
 @hook TARGET_DWARF_HANDLE_FRAME_UNSPEC
 
 @hook TARGET_DWARF_POLY_INDETERMINATE_VALUE