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Re: [match-and-simplify] express conversions in transform ?


On Thu, Aug 21, 2014 at 11:57 AM, Richard Biener
<richard.guenther@gmail.com> wrote:
> On Thu, Aug 21, 2014 at 10:42 AM, Prathamesh Kulkarni
> <bilbotheelffriend@gmail.com> wrote:
>> On Tue, Aug 19, 2014 at 4:35 PM, Richard Biener
>> <richard.guenther@gmail.com> wrote:
>>> On Tue, Aug 19, 2014 at 12:18 PM, Prathamesh Kulkarni
>>> <bilbotheelffriend@gmail.com> wrote:
>>>> I was wondering how to write transform that involves conversions
>>>> (without using c_expr) ?
>>>> for eg:
>>>> (T1)(~(T2) X)  -> ~(T1) X
>>>> // T1, T2 has same precision and X is an integral type not narrower than T1, T2
>>>>
>>>> this could be written as:
>>>> (simplify
>>>>   (convert (bit_not (convert@0 integral_op_p@1)))
>>>>   if-expr
>>>>   transform)
>>>>
>>>> How would the transform be written ?
>>>> with c_expr we could probably write as: (is that correct?)
>>>> (bit_not { fold_convert (type, @1); })
>>>
>>> No, unfortunately while that's correct for GENERIC it doesn't work
>>> for GIMPLE code-gen.  c_exprs are required to evaluate to
>>> "atomics", that is, non-expressions.
>>>
>>>> I was wondering whether we should have an explicit support for
>>>> conversions (something equivalent of "casting") ?
>>>> sth like:
>>>> (bit_not (cast type @0))
>>>
>>> Indeed simply writing
>>>
>>>    (bit_not (convert @1))
>>>
>>> will make the code-generator "convert" @0 to its own type
>>> (I knew the hack to simply use the first operands type is not
>>> going to work in all cases... ;).  Other conversion operators
>>> include FIX_TRUNC_EXPR (float -> int), FLOAT_EXPR
>>> (int -> float).
>>>
>>> For most cases it works by accident as the outermost expression
>>> knows its type via 'type'.
>>>
>>> In the cases where we need to specify a type I'd like to avoid
>>> making the type-converting operators take an additional operand.
>>> Instead can we make it use a operator flag-with-argument?  Like
>>>
>>>   (bit_not (convert:type @1))
>>>
>>> or for the type of a capture (convert:t@2 @1).
>>>
>>> We can also put in some more intelligence in automatically
>>> determining the type to convert to.  In your example we
>>> know the bit_not is of type 'type' and this has to match
>>> the type of its operands thus the 'convert' needs to convert
>>> to 'type' as well.  I'm sure there are cases we can't
>>> auto-compute though, like chains of conversions (but not sure
>>> they matter in practice).
>>>
>>> So let's try that - Micha will like more intelligence in the generator ;)
>>>
>>> I suggest to add a predicate to the generator conversion_op_p
>>> to catch the various converting operators and then assert that
>>> during code-gen we are going to create it with 'type' (outermost
>>> type), not TREE_TYPE (ops[0]).  Then start from the cases
>>> we want to support and special-case (which requires passing
>>> down the type of the enclosing expression).
>> Um sorry, I am not sure if I understood that correctly.
>> How would conversion_op_p know when to use 'type' and not TREE_TYPE (ops[0]) ?
>> conversion_op_p would be used somewhat similar to in the attached
>> patch (illustrate only) ?
>
> Yeah, kind of.  Attached is my try which handles
>
>    (bit_not (convert (plus @0 (convert @1))))
>
> fine (converting @1 to the type of @0) but obviously not
>
>    (convert (convert @1))
>
> which is impossible to auto-guess and not
>
>   (bit_not (convert (plus (convert @0) @1)))
>
> though that can be made work by first generating code for @1
> and then for (convert @0) so it can access the type of @1
> to convert to its type.
>
> I'll give the patch proper testing and will install it.

And if we really need sth like (convert (convert @0)) then
this can either use a c-expr or we can invent sth like
(convert (match-type @1 (convert @0)))

that is, add a "fake" binary operation to guess the type.

Richard.

> Thanks,
> Richard.
>
>> Thanks,
>> Prathamesh
>>>
>>> Thanks,
>>> Richard.
>>>
>>>> in case we want to cast to a type of another capture
>>>> we could use:
>>>> (foo (cast (type @0) @1)) ?  // cast @1 to @0's type
>>>> here type would be an operator that extracts type of the capture.
>>>>
>>>> Another use of having type as an operator I guess would be for
>>>> type-matching (not sure if it's useful).
>>>> for instance:
>>>> (foo (bar @0 @1) @(type @0)@1)
>>>> that checks if @0, @1 have same types.
>>>>
>>>> I was wondering on the same lines if we could introduce new
>>>> keyword "precision" analogous to type ?
>>>> (precision @0) would be short-hand for
>>>> TYPE_PRECISION (TREE_TYPE (@0))
>>>>
>>>> So far I found couple patterns in fold_unary_loc with conversions
>>>> in transform:
>>>> (T1)(X p+ Y) into ((T1)X p+ Y)
>>>> (T1)(~(T2) X) -> ~(T1) X
>>>> (T1) (X * Y) -> (T1) X * (T1) Y
>>>>
>>>> Thanks,
>>>> Prathamesh


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