ifcvt limitations?

[Jeff Law]

On 06/02/2015 09:57 AM, Kyrill Tkachov wrote:
> I'm stuck on noce_process_if_block (in ifcvt.c) and what I think is a
> restriction that the THEN-block contents have to be only a single set
> insn. This fails on aarch64 because we get an extra zero_extend.
>
> In particular, the following check in noce_process_if_block triggers:
>    insn_a = first_active_insn (then_bb);
>    if (! insn_a
>        || insn_a != last_active_insn (then_bb, FALSE)
>        || (set_a = single_set (insn_a)) == NULL_RTX)
>      return FALSE;
>
> Is there any particular reason why the code shouldn't be able to handle
> arbitrarily large contents
> in then_bb (within a sane limit)?
It's just never been implemented or tested per this comment in 
noce_process_if_block.

   /* We're looking for patterns of the form

      (1) if (...) x = a; else x = b;
      (2) x = b; if (...) x = a;
      (3) if (...) x = a;   // as if with an initial x = x.

      The later patterns require jumps to be more expensive.

      ??? For future expansion, look for multiple X in such patterns.  */

I think folks would look favorably upon removing that limitation, 
obviously with some kind of cost checking.

Jeff