Re: predicates on expressions ?

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

On Mon, Jul 14, 2014 at 10:52 PM, Prathamesh Kulkarni
<bilbotheelffriend@gmail.com> wrote:
> On Mon, Jul 14, 2014 at 6:35 PM, Richard Biener
> <richard.guenther@gmail.com> wrote:
>> On Mon, Jul 14, 2014 at 12:07 PM, Prathamesh Kulkarni
>> <bilbotheelffriend@gmail.com> wrote:
>>> I was wondering if it was a good idea to implement
>>> predicate on expressions ?
>>>
>>> Sth like:
>>> (match_and_simplify
>>>   (op (op2:predicate @0))
>>>   transform)
>>>
>>> instead of:
>>> (match_and_simplify
>>>   (op (op2@1 @0))
>>>   if (predicate (@1))
>>>   transform)
>>>
>>> When predicate is simple as just being a macro/function,
>>> we could use this style and when the predicate is more complex
>>> resort to using if-expr (or write the predicate as macro in gimple-match-head.c
>>> and use the macro in pattern instead ...)
>>>
>>> Example:
>>> we could rewrite the pattern
>>> (match_and_simplify
>>>   (plus:c @0 (negate @1))
>>>   if (!TYPE_SATURATING (type))
>>>   (minus @0 @1))
>>>
>>> to
>>>
>>> (match_and_simplify
>>>   (plus:c:NOT_TYPE_SATURATING_P @0 (negate @1))
>>>   (minus @0 @1))
>>>
>>> with NOT_TYPE_SATURATING_P predicate defined
>>> appropriately in gimple-match-head.c
>>>
>>> However I am not entirely sure if adding predicates on expressions
>>> would be very useful....
>>
>> Well.  I think there are two aspects to this.  First is pattern
>> readability where I think that the if-expr form is more readable.
>> Second is the ability to do less work in the code generated
>> from the decision tree.
>>
>> For example most of the patterns from associate_plusminus
>> still miss the !TYPE_SATURATING && !FLOAT_TYPE_P &&
>> !FIXED_POINT_TYPE_P if-expr.  That is, we'd have
>>
>> /* (A +- B) - A -> +-B.  */
>> (match_and_simplify
>>   (minus (plus @0 @1) @0)
>>   if (!TYPE_SATURATING (type)
>>       && !FLOAT_TYPE_P (type) && !FIXED_POINT_TYPE_P (type))
>>   @1)
>> (match_and_simplify
>>   (minus (minus @0 @1) @0)
>>   if (!TYPE_SATURATING (type)
>>       && !FLOAT_TYPE_P (type) && !FIXED_POINT_TYPE_P (type))
>>   (negate @1))
>> /* (A +- B) -+ B -> A.  */
>> (match_and_simplify
>>   (minus (plus @0 @1) @1)
>>   if (!TYPE_SATURATING (type)
>>       && !FLOAT_TYPE_P (type) && !FIXED_POINT_TYPE_P (type))
>>   @0)
>> (match_and_simplify
>>   (plus:c (minus @0 @1) @1)
>>   if (!TYPE_SATURATING (type)
>>       && !FLOAT_TYPE_P (type) && !FIXED_POINT_TYPE_P (type))
>>   @0)
>>
>> with code-generation checking the if-expr after matching.  And
>> with using expression predicates we'd be able to check the
>> predicate when matching the outermost 'minus' and "CSE"
>> the predicate check for the first three patterns.  Runtime-wise
>> it depends on whether there is a point to back-track to.
>>
>> I would say it's more interesting to support
>>
>> if (!TYPE_SATURATING (type) && !FLOAT_TYPE_P (type) &&
>> !FiXED_POINT_TYPE_P (type))
>>    (match_and_simplify ....)
>>    (match_and_simplify ....)
>>    ....
>>
>> and treat this if-expression like a predicate on the outermost
>> expression.  That's getting both benefits
>> (bah, the free-form if-expr makes it ugly, what do we use as
>> grouping syntax?  I guess wrapping the whole thing in ()s,
>> similar to (for ...)).
> Um, I was wondering instead of defining new syntax
> if it would be better to make genmatch detect common if-expr
> and hoist them ? I suppose we could compare if-expr's lexicographically ?
>
> However I guess having some syntax to group common if-expr patterns
> explicitly would
> avoid the need for writing the if-expr in each pattern.

Yeah, the main motiviation is to make the patterns itself easier to read
and group them by boiler-plate if-exprs.

> For now should we go with free-form if ?

I'd say

(if (!TYPE_SATURATING (type) ....)
  <patterns...>
)

thus wrap the if inside ()s.  Otherwise there would be no way to
"end" an if.

> If desired, we could change syntax later to
> something else (only parsing code need change, the rest would be in place).
> If we change the syntax for outer-if, for consistency should we also
> change syntax of inner if ?

Probably yes, let's wrap the inner if inside ()s as well.

Code-generation-wise we should record a vector of if-exprs and thus
evaluate the outer ifs at the same place we evaluate inner if-exprs.

Thanks,
Richard.

> Thanks and Regards,
> Prathamesh
>>
>> Richard.
>>
>>> Thanks and Regards,
>>> Prathamesh