Re: [PATCH] Loop unswitching: support gswitch statements.

["Martin Liška" <mliska@suse.cz>]

On 11/30/21 12:17, Richard Biener wrote:
> On Mon, Nov 29, 2021 at 1:45 PM Martin Liška <mliska@suse.cz> wrote:
>>
>> On 11/26/21 09:12, Richard Biener wrote:
>>> On Wed, Nov 24, 2021 at 3:32 PM Martin Liška <mliska@suse.cz> wrote:
>>>>
>>>> On 11/24/21 15:14, Martin Liška wrote:
>>>>> It likely miscompiles gcc.dg/loop-unswitch-5.c, working on that..
>>>>
>>>> Fixed that in the updated version.
>>>
>>> Function level comments need updating it seems.
>>
>> I've done that.
>>
>>>
>>> +static unsigned
>>> +evaluate_insns (class loop *loop,  basic_block *bbs,
>>> +               predicate_vector &predicate_path,
>>> +               auto_bb_flag &reachable_flag)
>>> +{
>>> +  auto_vec<basic_block> worklist (loop->num_nodes);
>>> +  worklist.quick_push (bbs[0]);
>>> ...
>>>
>>> so when adding gswitch support the easiest way to make
>>>
>>> +      FOR_EACH_EDGE (e, ei, bb->succs)
>>> +       {
>>> ...
>>> +           {
>>> +             worklist.safe_push (dest);
>>> +             dest->flags |= reachable_flag;
>>>
>>> work is when the gcond/gswitch simplification would mark
>>> outgoing edges as (non-)executable.  For gswitch this
>>> could be achieved by iterating over the case labels and
>>> intersecting that with the range while for gcond it's a
>>> matter of setting an edge flag instead of returning true/false.
>>
>> Exactly, it can be quite naturally added to the current patch.
>>
>>> I'd call the common function evaluate_control_stmt_using_entry_checks
>>> or so and invoke it on the last stmt of a block with >= 2 outgoing
>>> edges.
>>
>> Yes, I'll do it for the gswitch support patch.
>>
>>>
>>> We still seem to do the simplification work twice, once for costing
>>> and once for transform, but that's OK for now I guess.
>>>
>>> I think you want to clear_aux_for_blocks at the end of the pass.
>>
>> Called that.
>>
>>>
>>> Otherwise I like it - it seems you have some TODO around cost
>>> modeling.  Did you try to do gswitch support ontop as I suggested
>>> to see if the general structure keeps working?
>>
>> I vanished and tested the patch. No, I don't have the gswitch support patch
>> as the current patch was reworked a few times.
>>
>> Can we please progress and have installed the suggested patch?
> 
> I'd like to see the gswitch support - that's what was posted before stage3
> close, this patch on its own doesn't seem worth pushing for.  That said,
> I have some comments below (and the already raised ones about how
> things might need to change with gswitch support).  Is it so difficult to
> develop gswitch support as a separate change ontop of this?

I'm going to work on that in the upcoming days. When are you leaving for the Christmas
holidays :P ?

> 
>> Patch can bootstrap on x86_64-linux-gnu and survives regression tests.
> 
> +#include <utility>
> 
> that's included unconditionally by system.h

Good.

> 
> +/* The type represents a predicate path leading to a basic block.  */
> +typedef auto_vec<std::pair<unswitch_predicate *, bool>> predicate_vector;
> 
> +static bool tree_unswitch_single_loop (class loop *, int,
> +                                      predicate_vector &predicate_path,
> 
> I think we don't want to pass auto_vec by reference, instead auto_vec should
> decay to vec<> when passed around.

Ok.

> 
> +  unswitch_predicate *predicate = new unswitch_predicate (cond, lhs);
> +  if (irange::supports_type_p (TREE_TYPE (lhs)) && CONSTANT_CLASS_P (rhs))
> +    {
> +      ranger->range_on_edge (predicate->true_range, edge_true, lhs);
> +      predicate->false_range = predicate->true_range;
> 
> -  return cond;
> +      if (!predicate->false_range.varying_p ()
> +         && !predicate->false_range.undefined_p ())
> +       predicate->false_range.invert ();
> +    }
> 
> is that correct?  I would guess range_on_edge, for
> 
>     if (a > 10)
>       if (a < 15)
>          /* true */
>       else
>          /* false */
> 
> figures [11, 14] on the true edge of if (a < 15) (considered the
> unswitch predicate),
> inverting that yields [0, 10] u [15, +INF] but that's at least
> sub-optimal for the
> else range.  I think we want to call range_on_edge again to determine the range
> on the else branch?

No, as shown in the previous emails, Ranger is CFG sensitive and we should rely
on our irange merging.

> 
>   }
> 
> -/* Simplifies COND using checks in front of the entry of the LOOP.  Just very
> -   simplish (sufficient to prevent us from duplicating loop in unswitching
> -   unnecessarily).  */
> +static void
> +combine_range (predicate_vector &predicate_path, tree index, irange
> &path_range)
> +{
> 
> unless I misread the patch combine_range misses a comment.
> 
> +evaluate_control_stmt_using_entry_checks (gimple *stmt,
> +                                         predicate_vector &predicate_path)
>   {
> 
> so this function for ranger does combine all predicates on the predicate_path
> but for the symbolic evaluation it looks at the last predicate only?

Yes.

>  I guess
> that's because other predicate simplification opportunities are applied already,
> correct?

Exactly.

>  But doesn't that mean that the combine_range could be done once
> when we build the predicate vector instead of for each stmt?  I'm just
> looking at the difference in treating both cases - if we first analyze the whole
> unswitching path (including all recursions) then we'd have to simplify all
> opportunities at once, so iterating over all predicates would make sense.
> Still merging ranges when pushing the to the predicate vector rather than
> for each stmt would make sense?  We'd then have at most one predicate
> supported by irange for a specific lhs on the vector?

Yes, we can do better. I have a patch that appends to the path, and merges
with the previous irange that has equal LHS. Doing that, we can only first the
last predicate for a LHS and this one would have precise range (merged with all previous).

> 
> +      if (EDGE_COUNT (bb->succs) == 2)
> +       {
> +         gcond *cond = dyn_cast<gcond *> (last_stmt (bb));
> +         if (cond != NULL)
> +           {
> 
> since gcond always has two edges the edge count test is redundant
> and removing it allows to indent less ;)

Lemme work on the gswitch support now.

Martin

> 
> Richard.
> 
> 
>> Ready to be installed?
>> Thanks,
>> Martin
>>
>>>
>>> Thanks,
>>> Richard.
>>>
>>>>
>>>> Martin