On Fri, Oct 14, 2016 at 3:51 PM, Bill Schmidt
> Hi Richard,
>> On Oct 14, 2016, at 4:19 AM, Richard Biener <richard.guent...@gmail.com>
>> On Thu, Oct 13, 2016 at 5:38 PM, Bill Schmidt
>> <wschm...@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
(RTL just has plus and minus, nothing special for pointers).
>>> 2016-10-13 Bill Schmidt <wschm...@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.