Re: fwprop fix for PR79405

[Richard Biener <richard.guenther@gmail.com>]

On Thu, Feb 16, 2017 at 8:41 PM, Bernd Schmidt <bschmidt@redhat.com> wrote:
> We have two registers being assigned to each other:
>
>  (set (reg 213) (reg 209))
>  (set (reg 209) (reg 213))
>
> These being the only definitions, we are happy to forward propagate reg 209
> for reg 213 into a third insn, making a new use for reg 209. We are then
> happy to forward propagate reg 213 for it in the same insn... ending up in
> an infinite loop.
>
> I don't really see an elegant way to prevent this, so the following just
> tries to detect the situation (and more general ones) by brute force.
> Bootstrapped and tested on x86_64-linux, verified that the test passes with
> a ppc cross, ok?

But isn't the issue that we are walking "all uses" (in random order) rather than
only processing each stmt once?  That is,

  /* Go through all the uses.  df_uses_create will create new ones at the
     end, and we'll go through them as well.

     Do not forward propagate addresses into loops until after unrolling.
     CSE did so because it was able to fix its own mess, but we are not.  */

  for (i = 0; i < DF_USES_TABLE_SIZE (); i++)
    {
      df_ref use = DF_USES_GET (i);
      if (use)
        if (DF_REF_TYPE (use) == DF_REF_REG_USE
            || DF_REF_BB (use)->loop_father == NULL
            /* The outer most loop is not really a loop.  */
            || loop_outer (DF_REF_BB (use)->loop_father) == NULL)
          forward_propagate_into (use);
    }

if that were simply walking all instructions, doing forward_propagat_into on
each use on an instruction we'd avoid the cycle (because we stop propagating).

Because when propagating DF_USES_TABLE changes.

Richard.


>
>
> Bernd
>