Re: [PATCH 1/4]middle-end Vect: Add support for dot-product where the sign for the multiplicant changes.

[Richard Sandiford via Gcc-patches]

Richard Sandiford <richard.sandif...@arm.com> writes:
>> @@ -992,21 +1029,27 @@ vect_recog_dot_prod_pattern (vec_info *vinfo,
>>    /* FORNOW.  Can continue analyzing the def-use chain when this stmt in a 
>> phi
>>       inside the loop (in case we are analyzing an outer-loop).  */
>>    vect_unpromoted_value unprom0[2];
>> +  enum optab_subtype subtype = optab_vector;
>>    if (!vect_widened_op_tree (vinfo, mult_vinfo, MULT_EXPR, WIDEN_MULT_EXPR,
>> -                         false, 2, unprom0, &half_type))
>> +                         false, 2, unprom0, &half_type, &subtype))
>> +    return NULL;
>> +
>> +  if (subtype == optab_vector_mixed_sign
>> +      && TYPE_UNSIGNED (unprom_mult.type)
>> +      && TYPE_PRECISION (half_type) * 4 > TYPE_PRECISION (unprom_mult.type))
>>      return NULL;
>
> Isn't the final condition here instead that TYPE1 is narrower than TYPE2?
> I.e. we need to reject the case in which we multiply a signed and an
> unsigned value to get a (logically) signed result, but then zero-extend
> it (rather than sign-extend it) to the precision of the addition.
>
> That would make the test:
>
>   if (subtype == optab_vector_mixed_sign
>       && TYPE_UNSIGNED (unprom_mult.type)
>       && TYPE_PRECISION (unprom_mult.type) < TYPE_PRECISION (type))
>     return NULL;    
>   
> instead.

And folding that into the existing test gives:

  /* If there are two widening operations, make sure they agree on the sign
     of the extension.  The result of an optab_vector_mixed_sign operation
     is signed; otherwise, the result has the same sign as the operands.  */
  if (TYPE_PRECISION (unprom_mult.type) != TYPE_PRECISION (type)
      && (subtype == optab_vector_mixed_sign
          ? TYPE_UNSIGNED (unprom_mult.type)
          : TYPE_SIGN (unprom_mult.type) != TYPE_SIGN (half_type)))
    return NULL;

Thanks,
Richard