Re: [PATCH PR69848/partial]Propagate comparison into VEC_COND_EXPR if target supports

[Richard Biener <richard dot guenther at gmail dot com>]

On Wed, May 18, 2016 at 5:06 PM, Bin.Cheng <amker.cheng@gmail.com> wrote:
> On Tue, May 17, 2016 at 12:08 PM, Richard Biener
> <richard.guenther@gmail.com> wrote:
>> On Mon, May 16, 2016 at 10:09 AM, Bin.Cheng <amker.cheng@gmail.com> wrote:
>>> On Fri, May 13, 2016 at 5:53 PM, Richard Biener
>>> <richard.guenther@gmail.com> wrote:
>>>> On May 13, 2016 6:02:27 PM GMT+02:00, Bin Cheng <Bin.Cheng@arm.com> wrote:
>>>>>Hi,
>>>>>As PR69848 reported, GCC vectorizer now generates comparison outside of
>>>>>VEC_COND_EXPR for COND_REDUCTION case, as below:
>>>>>
>>>>>  _20 = vect__1.6_8 != { 0, 0, 0, 0 };
>>>>>  vect_c_2.8_16 = VEC_COND_EXPR <_20, { 0, 0, 0, 0 }, vect_c_2.7_13>;
>>>>>  _21 = VEC_COND_EXPR <_20, ivtmp_17, _19>;
>>>>>
>>>>>This results in inefficient expanding.  With IR like:
>>>>>
>>>>>vect_c_2.8_16 = VEC_COND_EXPR <vect__1.6_8 != { 0, 0, 0, 0 }, { 0, 0,
>>>>>0, 0 }, vect_c_2.7_13>;
>>>>>  _21 = VEC_COND_EXPR <vect__1.6_8 != { 0, 0, 0, 0 }, ivtmp_17, _19>;
>>>>>
>>>>>We can do:
>>>>>1) Expanding time optimization, for example, reverting comparison
>>>>>operator by switching VEC_COND_EXPR operands.  This is useful when
>>>>>backend only supports some comparison operators.
>>>>>2) For backend not supporting vcond_mask patterns, saving one LT_EXPR
>>>>>instruction which introduced by expand_vec_cond_expr.
>>>>>
>>>>>This patch fixes this by propagating comparison into VEC_COND_EXPR even
>>>>>if it's used multiple times.  For now, GCC does single_use_only
>>>>>propagation.  Ideally, we may duplicate the comparison before each use
>>>>>statement just before expanding, so that TER can successfully backtrack
>>>>>it from each VEC_COND_EXPR.  Unfortunately I didn't find a good pass to
>>>>>do this.  Tree-vect-generic.c looks like a good candidate, but it's so
>>>>>early that following CSE could undo the transform.  Another possible
>>>>>fix is to generate comparison inside VEC_COND_EXPR directly in function
>>>>>vectorizable_reduction.
>>>>
>>>> I prefer this for now.
>>> Hi Richard, you mean this patch, or the possible fix before your comment?
>>
>> The possible fix before my comment - make the vectorizer generate VEC_COND_EXPRs
>> with embedded comparison.
> Hi,
> Here is updated patch doing that.  It's definitely clearer than the
> original version.
> Bootstrap and test on x86_64.  Also checked the expanding time
> optimization still happens.  Is it OK?

Yes.

Thanks,
Richard.

> Thanks,
> bin
>>
>> Thanks,
>> Richard.
>>