Hi all,

On arm we don't have a dedicated instruction that corresponds to a CTZ rtx but we synthesise it
with an RBIT instruction followed by a CLZ. This is currently done at expand time.
However, I'd like to push that step until after reload and keep the CTZ rtx as a single whole in
the early RTL optimisers.  This better expresses the semantics of the operation as a whole, since
the RBIT operation is represented as an UNSPEC anyway and so will not see the benefits of combine,
and a CTZ-specific optimisation that is implemented in patch 3/3 of this series won't be triggered
if the expression is broken up into an UNSPEC and a CLZ.

Therefore this patch changes the expander to expand to a CTZ rtx and split it after reload into
an RBIT + CLZ to allow sched2 to schedule them apart if it deems necessary.
This patch enables the optimisation in patch 3/3 where the appropriate test is added.

Bootstrapped and tested on arm-none-linux-gnueabihf.

Ok for trunk?

Thanks,
Kyrill

2016-05-26  Kyrylo Tkachov  <kyrylo.tkachov@arm.com>

     PR middle-end/37780
     * config/arm/arm.md (ctzsi2): Convert to define_insn_and_split.