Re: writing patterns

[Richard Sandiford <rdsandiford at googlemail dot com>]

Richard Biener <richard.guenther@gmail.com> writes:
> On Wed, Jul 30, 2014 at 12:49 PM, Prathamesh Kulkarni
> <bilbotheelffriend@gmail.com> wrote:
>> Hi,
>>    Sorry to ask a stupid question, but I am having issues writing patterns
>> involving casts. I am trying to write patterns from simplify_rotate.
>>
>> Could you show me how to write a patterns that involve
>> casts ?
>> for eg:
>> ((T) ((T2) X << CNT1)) + ((T) ((T2) X >> CNT2))     iff CNT1 + CNT2 == B
>> T -> some unsigned type with bitsize B, and some type T2 wider than T.
>> How to express this in the pattern ?
>
> [copying gcc@ because of the syntax stuff]
>
> for example with (leaving captures as the appear in the pattern above)
>
> (match_and_simplify
>    (plus (convert@2 (lshift (convert@0 X) CNT1))
>            (convert (rshift (convert@1 X) CNT2)))
>     /* Types T2 have to match */
>    (if (types_compatible_p (TREE_TYPE (@0), TREE_TYPE (@1))
>         /* Type T should be unsigned.  */
>        && TYPE_UNSIGNED (TREE_TYPE (@2))
>        /* T2 should be wider than T.  */
>        && TYPE_PRECISION (TREE_TYPE (@0)) > TYPE_PRECISION (TREE_TYPE (@2))
>        /* CNT1 + CNT2 == B */
>        && wi::eq_p (TYPE_PRECISION (TREE_TYPE (@2)),
>                            wi::add (CNT1, CNT2))))
>    (lrotate CNT1))
>
> which suggests that we may want to add some type capturing / matching
> so we can maybe write
>
>   (plus (convert@T (lshift (convert@T2 X) CNT1))
>           (convert (rshift (convert@T2 X) CNT2)))
>   (if (/* T2s will be matched automagically */
>        && TYPE_UNSIGNED (@T)
>        && TYPE_PRECISION (@T2) > TYPE_PRECISION (@T)
>        && wi::eq_p (TYPE_PRECISION (@T), wi::add (CNT1, CNT2))))
>
> which is less to type and supports requiring matching types.  Maybe
> require @T[0-9]+ here thus use @T0 and disallow plain @T.  We could
> then also use @T for the implicitely "captured" outermost type we
> refer to as plain 'type' right now.

Would it also be worth trying to push more of the type properties into
the pattern, a bit like md predicates?  (Not the same syntax though,
obviously.)  Was just thinking that postponing things like
"TYPE_UNSIGNED (@T)" until the whole tree has been matched could
be inefficient in some cases.

Might be going over old ground though, sorry.

Richard