gcc.dg/sms-antideps.c fails on IA64 (modulo scheduling problem)

[Steve Ellcey <sje at cup dot hp dot com>]

I noticed that gcc.dg/sms-antideps.c is failing on my IA64 Linux and
HP-UX platforms.  The failure is:

x.c: In function 'foo':
x.c:25: internal compiler error: in gen_sub2_insn, at optabs.c:4640
Please submit a full bug report,
with preprocessed source if appropriate.
See <URL:http://gcc.gnu.org/bugs.html> for instructions.

When I enter gen_sub2_insn the operands I have are:

	(reg:DI 332 ar.lc) and (const_int 0 [0x0])

It is generate_prolog_epilog (called by sms_schedule) in modulo-sched.c
that is trying to generate this subtract.  First off:  why am I
subracting zero?  Shouldn't generate_prolog_epilog skip the subtraction
if last_stage is zero?

I wind up dying because count_reg in this case is "ar.lc" (application
register, loop counter) and this register on the IA64 does not allow the
subtraction operation on it.  All I can do with this register is to do a
move into it to initialize it and then use and decrement it in counted
loops (the doloop_end insn).  How do I explain to the modulo scheduler
that it cannot do this optimization?  Not doing the subtract when
last_stage is zero might make this one test case work but it would leave
open the larger problem of possibly generating a subtract instruction on
a register (count_reg) that does not allow that operation.

Ideas or advise (or patches)?

Steve Ellcey
sje@cup.hp.com