Hi all,

A pet project of mine is to get to the point where backend rtx costs functions won't have
to handle rtxes that don't match down to any patterns/expanders we have. Or at least limit such cases.
A case dealt with in this patch is QImode PLUS. We don't actually generate or handle these anywhere in
the arm backend *except* in sync.md where, for example, atomic_<sync_optab><mode> matches:
(set (match_operand:QHSD 0 "mem_noofs_operand" "+Ua")
     (unspec_volatile:QHSD
       [(syncop:QHSD (match_dup 0)
          (match_operand:QHSD 1 "<atomic_op_operand>" "<atomic_op_str>"))
        (match_operand:SI 2 "const_int_operand")]        ;; model
       VUNSPEC_ATOMIC_OP))

Here QHSD can contain QImode and HImode while syncop can be PLUS.
Now immediately during splitting in arm_split_atomic_op we convert that
QImode PLUS into an SImode one, so we never actually generate any kind of QImode add operations
(how would we? we don't have define_insns for such things) but the RTL optimisers will get a hold
of the UNSPEC_VOLATILE in the meantime and ask for it's cost (for example, cse when building libatomic).
Currently we don't handle UNSPEC_VOLATILE (VUNSPEC_ATOMIC_OP) so the arm rtx costs function just recurses
into the QImode PLUS that I'd like to avoid.
This patch stops that by passing the VUNSPEC_ATOMIC_OP into arm_unspec_cost and handling it there
(very straightforwardly just returning COSTS_N_INSNS (2); there's no indication that we want to do anything
smarter here) and stopping the recursion.

This is a small step in the direction of not having to care about obviously useless rtxes in the backend.
The astute reader might notice that in sync.md we also have the pattern atomic_fetch_<sync_optab><mode>
which expands to/matches this:
(set (match_operand:QHSD 0 "s_register_operand" "=&r")
     (match_operand:QHSD 1 "mem_noofs_operand" "+Ua"))
    (set (match_dup 1)
     (unspec_volatile:QHSD
       [(syncop:QHSD (match_dup 1)
          (match_operand:QHSD 2 "<atomic_op_operand>" "<atomic_op_str>"))
        (match_operand:SI 3 "const_int_operand")]        ;; model
       VUNSPEC_ATOMIC_OP))


Here the QImode PLUS is in a PARALLEL together with the UNSPEC, so it might have rtx costs called on it
as well. This will always be a (plus (reg) (mem)) rtx, which is unlike any other normal rtx we generate
in the arm backend. I'll try to get a patch to handle that case, but I'm still thinking on how to best
do that.

Tested arm-none-eabi, I didn't see any codegen differences in some compiled codebases.

Ok for trunk?

P.S. I know that expmed creates all kinds of irregular rtxes and asks for their costs. I'm hoping to clean that
up at some point...

2015-04-20  Kyrylo Tkachov  <kyrylo.tkachov@arm.com>

     * config/arm/arm.c (arm_new_rtx_costs): Handle UNSPEC_VOLATILE.
     (arm_unspec_cost): Allos UNSPEC_VOLATILE.  Do not recurse inside
     unknown unspecs.