> On Oct 17, 2016, at 3:27 AM, Richard Biener <richard.guent...@gmail.com> > wrote: > > On Fri, Oct 14, 2016 at 3:51 PM, Bill Schmidt > <wschm...@linux.vnet.ibm.com> wrote: >> Hi Richard, >> >>> On Oct 14, 2016, at 4:19 AM, Richard Biener <richard.guent...@gmail.com> >>> wrote: >>> >>> 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 > 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 <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.
