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). 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. >>> >>> >> >
