Re: Remove redundant AND from count reduction loop

[Richard Biener <richard.guenther@gmail.com>]

On Fri, Jun 26, 2015 at 12:39 AM, Marc Glisse <marc.glisse@inria.fr> wrote:
> On Wed, 24 Jun 2015, Richard Biener wrote:
>
>> On Wed, Jun 24, 2015 at 11:57 AM, Richard Sandiford
>> <richard.sandiford@arm.com> wrote:
>>>
>>> Richard Biener <richard.guenther@gmail.com> writes:
>>>>
>>>> On Tue, Jun 23, 2015 at 11:27 PM, Marc Glisse <marc.glisse@inria.fr>
>>>> wrote:
>>>>>
>>>>> On Tue, 23 Jun 2015, Richard Sandiford wrote:
>>>>>
>>>>>> +/* Vector comparisons are defined to produce all-one or all-zero
>>>>>> results.
>>>>>> */
>>>>>> +(simplify
>>>>>> + (vec_cond @0 integer_all_onesp@1 integer_zerop@2)
>>>>>> + (if (tree_nop_conversion_p (type, TREE_TYPE (@0)))
>>>>>> +   (convert @0)))
>>>>>
>>>>>
>>>>>
>>>>> I am trying to understand why the test tree_nop_conversion_p is the
>>>>> right
>>>>> one (at least for the transformations not using VIEW_CONVERT_EXPR). By
>>>>> definition of VEC_COND_EXPR, type and TREE_TYPE (@0) are both integer
>>>>> vector
>>>>> types of the same size and number of elements. It thus seems like a
>>>>> conversion is always fine. For vectors, tree_nop_conversion_p
>>>>> apparently
>>>>> only checks that they have the same mode (quite often VOIDmode I
>>>>> guess).
>>>>
>>>>
>>>> The only conversion we seem to allow is changing the signed vector from
>>>> the comparison result to an unsigned vector (same number of elements
>>>> and same mode of the elements).  That is, a check using
>>>> TYPE_MODE (type) == TYPE_MODE (TREE_TYPE (@0)) would probably
>>>> be better (well, technically a TYPE_VECTOR_SUBPARTS && element
>>>> mode compare should be better as generic vectors might not have a vector
>>>> mode).
>>>
>>>
>>> OK.  The reason I was being paranoid was that I couldn't see anywhere
>>> where we enforced that the vector condition in a VEC_COND had to have
>>> the same element width as the values being selected.
>>
>>
>> We don't require that indeed.
>>
>>>  tree-cfg.c
>>> only checks that rhs2 and rhs3 are compatible with the result.
>>> There doesn't seem to be any checking of rhs1 vs. the other types.
>>> So I wasn't sure whether anything stopped us from, e.g., comparing two
>>> V4HIs and using the result to select between two V4SIs.
>>
>>
>> Nothing does (or should).
>
>
> The documentation patch you approved in
> https://gcc.gnu.org/ml/gcc-patches/2012-10/msg01109.html says something
> different. If it is really wrong, could you fix it?

Hmm, that simplifies things.  It would be nice if these constraints
would also be
checked in the gimple verifier...

Richard.

> --
> Marc Glisse