Re: [PATCH] Fix PR77937

[Bill Schmidt <wschmidt at linux dot vnet dot ibm dot com>]

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.

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.

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.
>> 
>> 
>