Re: ifcvt limitations?

[Kyrill Tkachov <kyrylo dot tkachov at foss dot arm dot com>]

On 02/06/15 17:50, Jeff Law wrote:
> 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.

Thanks, I've made some progress towards making it more aggressive.
A question since I'm in the area...
noce_try_cmove_arith that I've been messing around with has this code:

  /* A conditional move from two memory sources is equivalent to a
     conditional on their addresses followed by a load.  Don't do this
     early because it'll screw alias analysis.  Note that we've
     already checked for no side effects.  */
  /* ??? FIXME: Magic number 5.  */
  if (cse_not_expected
      && MEM_P (a) && MEM_P (b)
      && MEM_ADDR_SPACE (a) == MEM_ADDR_SPACE (b)
      && if_info->branch_cost >= 5)


Any ideas on where the rationale for that 5 came from?
I see it's been there since the very introduction of ifcvt.c
I'd like to replace it with something more sane, maybe even remove it?

Thanks,
Kyrill

> Jeff