On Thu, Oct 13, 2016 at 5:38 PM, Bill Schmidt
> 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?
> 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.