[PATCH] Fix PR77937

Richard Biener richard.guenther@gmail.com
Mon Oct 17 08:27:00 GMT 2016


On Fri, Oct 14, 2016 at 3:51 PM, Bill Schmidt
<wschmidt@linux.vnet.ibm.com> wrote:
> Hi Richard,
>
>> On Oct 14, 2016, at 4:19 AM, Richard Biener <richard.guenther@gmail.com> wrote:
>>
>> On Thu, Oct 13, 2016 at 5:38 PM, Bill Schmidt
>> <wschmidt@linux.vnet.ibm.com> wrote:
>>> The previous patch for
>>> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77937 is necessary, but not
>>> sufficient in all cases.  It allows -1 to be used with a pointer
>>> increment, which we really do not want given that this is generally not
>>> profitable.  Disable this case for now.  We can add logic later to
>>> estimate the cost for the rare case where it can be useful.
>>>
>>> Bootstrapped and tested on powerpc64le-unknown-linux-gnu with no
>>> regressions, committed.
>>
>> Huh, I wonder what is special about -1 here.  Do we handle -2?
>
> We do, subject to a little more stringent cost modeling later on, because it
> requires introducing a multiply by the increment.  We have some special
> case code for -1 that introduces a MINUS_EXPR, but that breaks for
> pointer arithmetic.

Ah, ok.  Fine then.

> I am working on a better fix for this as part of the work for PR77916, which
> exposes a related problem elsewhere in the code.  The current fix is a
> stopgap until I can get that work completed.  For -1 we prefer a negate
> over a multiply when we have pointer types and can't use minus, and need
> to properly model that in the cost calculation.

Note that RTL expansion will turn this into a minus again so I dont' think you
need any cost adjustment here.  It's just that GIMPLE doesnt' have a
POINTER_MINUS_EXPR...
(RTL just has plus and minus, nothing special for pointers).

Richard.

> Bill
>
>>
>> Richard.
>>
>>> Thanks,
>>> Bill
>>>
>>>
>>> 2016-10-13  Bill Schmidt  <wschmidt@linux.vnet.ibm.com>
>>>
>>>        PR tree-optimization/77937
>>>        * gimple-ssa-strength-reduction.c (analyze_increments): Set cost
>>>        to infinite when we have a pointer with an increment of -1.
>>>
>>>
>>> Index: gcc/gimple-ssa-strength-reduction.c
>>> ===================================================================
>>> --- gcc/gimple-ssa-strength-reduction.c (revision 241120)
>>> +++ gcc/gimple-ssa-strength-reduction.c (working copy)
>>> @@ -2818,6 +2818,11 @@ analyze_increments (slsr_cand_t first_dep, machine
>>>               || (incr == -1
>>>                   && !POINTER_TYPE_P (first_dep->cand_type)))
>>>        incr_vec[i].cost = COST_NEUTRAL;
>>> +
>>> +      /* FIXME: We don't handle pointers with a -1 increment yet.
>>> +         They are usually unprofitable anyway.  */
>>> +      else if (incr == -1 && POINTER_TYPE_P (first_dep->cand_type))
>>> +       incr_vec[i].cost = COST_INFINITE;
>>>
>>>       /* FORNOW: If we need to add an initializer, give up if a cast from
>>>         the candidate's type to its stride's type can lose precision.
>>>
>>>
>>
>



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