[RFC][PR target/39726 P4 regression] match.pd pattern to do type narrowing

[Richard Biener]

On Mon, Feb 9, 2015 at 8:15 AM, Jeff Law <law@redhat.com> wrote:
> On 02/03/15 04:39, Richard Biener wrote:
>>>
>>> I found that explicit types were ignored in some cases.  It was
>>> frustrating to say the least.
>>
>>
>> Huh, that would be a bug.  Do you have a pattern where that happens?
>
> I'll have to recreate them.  In the mean time consider something else I'm
> playing with that causes an odd error from genmatch...
>
> /* If we have a narrowing conversion of an arithmetic or logical
>    operation where both are operands widening conversions from the
>    same type as the outer narrowing conversion.  Then convert the
>    innermost operands to a suitable unsigned type (to avoid introducing
>    undefined behaviour), perform the operation and convert the result to
>    the desired type.  */
>   (simplify
>     (convert (plus (convert@2 @0) (convert @1)))
>     (if (TREE_TYPE (@0) == TREE_TYPE (@1)
>          && TREE_TYPE (@0) == type
>          && INTEGRAL_TYPE_P (type)
>          && TYPE_PRECISION (TREE_TYPE (@2)) >= TYPE_PRECISION (TREE_TYPE
> (@0)))
>       (with { tree utype = unsigned_type_for (TREE_TYPE (@0));}
>         (convert (plus (convert:utype @0) (convert:utype @1)))))))
>
> So given two narrow operands that get widened, added, and the final result
> narrowed back down to the original operand types.  Replace with convert the
> operands to an unsigned type (of same size as the operand), operate on them
> and convert to the final desired type.
>
> This happens to fix 47477 (P2 regression).  Works perfectly for the
> testcase.
>
>
> Of course we'd like to extend that to other operators...  So, adding the
> obvious for iterator...
>
> (for op (plus minus)
>   (simplify
>     (convert (op (convert@2 @0) (convert @1)))
>     (if (TREE_TYPE (@0) == TREE_TYPE (@1)
>          && TREE_TYPE (@0) == type
>          && INTEGRAL_TYPE_P (type)
>          && TYPE_PRECISION (TREE_TYPE (@2)) >= TYPE_PRECISION (TREE_TYPE
> (@0)))
>       (with { tree utype = unsigned_type_for (TREE_TYPE (@0));}
>         (convert (op (convert:utype @0) (convert:utype @1)))))))
>
>
> Which causes genmatch to barf:
>
> build/genmatch --gimple /home/gcc/GIT-2/gcc/gcc/match.pd \
>     > tmp-gimple-match.c
> genmatch: two conversions in a row
>
>
> Not only does genmatch barf, it doesn't give any indication what part of the
> .pd file it found objectionable.

Yeah, I'll have to assign locations to more places at some point.

But the following fixes your testcase, committed to trunk as obvious.

Richard.

2015-02-09  Richard Biener  <rguenther@suse.de>

        * genmatch.c (replace_id): Copy expr_type.

Index: gcc/genmatch.c
===================================================================
--- gcc/genmatch.c      (revision 220540)
+++ gcc/genmatch.c      (working copy)
@@ -982,6 +982,7 @@ replace_id (operand *o, user_id *id, id_
     {
       expr *ne = new expr (e->operation == id ? with : e->operation,
                           e->is_commutative);
+      ne->expr_type = e->expr_type;
       for (unsigned i = 0; i < e->ops.length (); ++i)
        ne->append_op (replace_id (e->ops[i], id, with));
       return ne;


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