Re: [PATCH] Fix PR 52631 (VN does not use simplified expression for lookup)

[Richard Guenther <richard dot guenther at gmail dot com>]

On Mon, Aug 20, 2012 at 6:49 AM, Andrew Pinski
<andrew.pinski@caviumnetworks.com> wrote:
> On Wed, Jul 25, 2012 at 4:39 AM, Richard Guenther
> <richard.guenther@gmail.com> wrote:
>> On Tue, Jul 24, 2012 at 5:50 PM, Andrew Pinski
>> <andrew.pinski@caviumnetworks.com> wrote:
>>> Hi,
>>>   Before tuples was introduced, VN used to lookup the simplified
>>> expression to see if it was available already and use that instead of
>>> the non simplified one.  This patch adds the support back to VN to do
>>> exactly that.
>>>
>>> OK? Bootstrapped and tested on x86_64-linux-gnu with no regressions.
>>
>> I think this should be done for all RHS and SSA name LHS, not only
>> for UNARY/BINARY/TERNARY - because even for SINGLE rhs we
>> can end up simplifying (for REALPART_EXPR for example which we
>> handle as nary, too).  I think we constrain try_to_simplify enough
>> so that
>>
>> +                     /* First try to lookup the simplified expression. */
>> +                     if (simplified && valid_gimple_rhs_p (simplified))
>> +                       {
>> +                         tree result = vn_nary_op_lookup (simplified, NULL);
>> +                         if (result)
>> +                           {
>> +                             changed = set_ssa_val_to (lhs, result);
>> +                             goto done;
>> +                           }
>> +                         changed = set_ssa_val_to (lhs, lhs);
>> +                         vn_nary_op_insert (simplified, lhs);
>> +                       }
>>                   switch (get_gimple_rhs_class (code))
>>                     {
>>                     case GIMPLE_UNARY_RHS:
>>                     case GIMPLE_BINARY_RHS:
>> ...
>>
>> should work.  As you also insert the simplified variant I think we really
>> (finally) want to have a valid_nary_op routine rather than relying on
>> valid_gimple_rhs_p which is way too generic.
>
> I don't see valid_gimple_rhs_p being that generic as it checks to make
> sure the operands of the gimple are valid.
> Maybe I am missing something here though.

valid_gimple_rhs_p checks what it says.  But what we want to know is whether
the rhs is valid for a SCCVN NARY.  Those are not the same.

Richard.

> Thanks,
> Andrew Pinski
>
>
>>
>> Thanks,
>> Richard.
>>
>>> Thanks,
>>> Andrew Pinski
>>>
>>> ChangeLog:
>>>
>>>         * tree-ssa-sccvn.c (visit_use): Look up the simplified
>>>         expression before visting the original one.
>>>
>>>         * gcc.dg/tree-ssa/ssa-fre-9.c: Update the number of
>>>         eliminatations that happen.