[PR tree-optimization/71691] Fix unswitching in presence of maybe-undef SSA_NAMEs (take 2)

[Jeff Law]

On 12/20/2016 07:16 AM, Richard Biener wrote:
>
> it should be already set as we use visited_ssa as "was it ever on the worklist",
> so maybe renaming it would be a good thing as well.
>
> +             if (TREE_CODE (name) == SSA_NAME)
> +               {
> +                 /* If an SSA has already been seen, this may be a loop.
> +                    Fail conservatively.  */
> +                 if (bitmap_bit_p (visited_ssa, SSA_NAME_VERSION (name)))
> +                   return false;
>
> so to me "conservative" is returning true, not false.
Agreed.

>
> +                 bitmap_set_bit (visited_ssa, SSA_NAME_VERSION (name));
> +                 worklist.safe_push (name);
>
> but for loops we can just continue and ignore this use.  And bitmap_set_bit
> returns whether it set a bit, thus
>
>                 if (bitmap_set_bit (visited_ssa, SSA_NAME_VERSION (name)))
>                   worklist.safe_push (name);
>
> should work?
What if we're in an irreducible region?


>
> In principle the thing is sound but I'd like to be able to pass in a bitmap of
> known maybe-undefined/must-defined SSA names to have a cache for
> multiple invocations from within a pass (like this unswitching case).
So that means keeping another bitmap for things positively identified as 
defined, then saving it for later invocations.  So maybe some of my work 
+ some of his melded together.  (mine computes once for the entire FN 
and saves the result, so converting it to on-demand saving the result 
shouldn't be terrible).


>
> Only unswitching on conditions that post-dominate the loop entry might be a
> simpler solution for the PR in question.  OTOH this may disable too much
> unswitching in practice.
It might be worth experimentation.

Jeff