Deprecating cc0 (and consequently cc0 targets)

[Jeff Law <law at redhat dot com>]

At the register allocation BOF during the Cauldron someone (I forget
who) raised the question of when/how do we get rid of reload.

The first step in that process is to drop support for cc0.  cc0 is a
horribly antiquated mechanism for describing how to handle condition
codes.  It has numerous limitations, the most problematical being the
requirement that the cc0 setter and cc0 user must be consecutive in the
IL.  This requirement bleeds all over the compiler resulting in code
that is harder to understand and maintain.  I'm fairly confident this
code is broken in various ways, particularly WRT exceptions.

So this message is serving as official notice that we are *deprecating*
all cc0 support in gcc-10.  We are not removing any code or targets at
this time -- removals would happen during the gcc-11 cycle.


avr
cris
h8300
m68k
vax
cr16

This is based on actually looking at the backend patterns.
backends.html indicates the mn103 port would be affected, but I think
it's just out-of-date.  Similarly it fails to mention cr16, but cr16
clearly has affected patterns (I'll address that separately)

This patch deprecates the affected targets.  With some magic folks can
continue to build them.  However, if nobody steps forward to convert
them from cc0 to MODE_CC those targets will be removed during the gcc-11
cycle.

If someone is interested in possibly doing a conversion, I would
strongly recommend reviewing the best documentation we have on the subject:

https://gcc.gnu.org/wiki/CC0Transition

I worked with my son about a year ago to transition the v850 to MODE_CC
using that page as a guideline.  We also have a partial transition for
the H8/300 port (not far enough to even build anything yet).  I had
hoped we would finish the H8/300 port last year and would have tackled
another, but events have gotten in the way.

The transition isn't terribly hard, but does take time because of the
number of patterns that have to be changed.

Let the discussion begin...


Jeff

diff --git a/gcc/config.gcc b/gcc/config.gcc
index 69d0a024d85..0c1637e8be1 100644
--- a/gcc/config.gcc
+++ b/gcc/config.gcc
@@ -248,6 +248,12 @@ md_file=
 # Obsolete configurations.
 case ${target} in
   tile*-*-*				\
+  avr*-*-*				\
+  h8300*-*-*				\
+  cris*-*-*				\
+  m68k*-*-*				\
+  vax*-*-*				\
+  cr16*-*-*				\
  )
     if test "x$enable_obsolete" != xyes; then
       echo "*** Configuration ${target} is obsolete." >&2
@@ -273,7 +279,6 @@ case ${target} in
  | arm*-*-uclinux*			\
  | i[34567]86-go32-*			\
  | i[34567]86-*-go32*			\
- | m68k-*-uclinuxoldabi*		\
  | mips64orion*-*-rtems*		\
  | pdp11-*-bsd				\
  | powerpc*-*-linux*paired*		\
@@ -294,7 +299,6 @@ case ${target} in
  | *-*-solaris2.[0-9].*			\
  | *-*-solaris2.10*			\
  | *-*-sysv*				\
- | vax-*-vms*				\
  )
 	echo "*** Configuration ${target} not supported" 1>&2
 	exit 1