[PATCH] Fix PR77937

[Bill Schmidt]

> On Oct 17, 2016, at 3:27 AM, Richard Biener <richard.guenther@gmail.com> wrote:
> 
> 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).

I came to the same conclusion over the weekend.  So I think the real fix
will be pretty simple now.  I keep getting pulled off to other things, but should
have something for review in the next couple of days.

Thanks!
Bill

> 
> 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.