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Re: [PATCH] Improving uniform_vector_p() function.


Actually I will introduce optimizations in the next patch. Currently
the function uniform_vector_p () is rarely used in GCC, but there are
certainly some optimization opportunities with the help of this
function.

For example, when we widen a vector with 8 identical element of short
type to two vectors of int type, GCC emits the following code:

  vect_cst_.15_91 = {_9, _9, _9, _9, _9, _9, _9, _9};
  vect__10.16_92 = [vec_unpack_lo_expr] vect_cst_.15_91;
  vect__10.16_93 = [vec_unpack_hi_expr] vect_cst_.15_91;

When vect_cst_.15_91 is a uniform vector, we know vect__10.16_92 and
vect__10.16_93 are identical so that we can remove the second
[vec_unpack_hi_expr] operation:

  vect_cst_.15_91 = {_9, _9, _9, _9, _9, _9, _9, _9};
  vect__10.16_92 = [vec_unpack_lo_expr] vect_cst_.15_91;
  vect__10.16_93 = vect__10.16_92;


thanks,
Cong


On Tue, Oct 1, 2013 at 2:37 PM, Xinliang David Li <davidxl@google.com> wrote:
> On Tue, Oct 1, 2013 at 10:31 AM, Cong Hou <congh@google.com> wrote:
>> The current uniform_vector_p() function only returns non-NULL when the
>> vector is directly a uniform vector. For example, for the following
>> gimple code:
>>
>> vect_cst_.15_91 = {_9, _9, _9, _9, _9, _9, _9, _9};
>>
>>
>> The current implementation can only detect that {_9, _9, _9, _9, _9,
>> _9, _9, _9} is a uniform vector, but fails to recognize
>> vect_cst_.15_91 is also one. This simple patch searches through
>> assignment chains to find more uniform vectors.
>>
>>
>> thanks,
>> Cong
>>
>>
>>
>> diff --git a/gcc/ChangeLog b/gcc/ChangeLog
>> index 45c1667..b42f8a9 100644
>> --- a/gcc/ChangeLog
>> +++ b/gcc/ChangeLog
>> @@ -1,3 +1,9 @@
>> +2013-10-01  Cong Hou  <congh@google.com>
>> +
>> +       * tree.c: Improve the function uniform_vector_p() so that a
>> +       vector assigned with a uniform vector is also treated as a
>> +       uniform vector.
>> +
>> diff --git a/gcc/tree.c b/gcc/tree.c
>> index 1c881e4..1d6d894 100644
>> --- a/gcc/tree.c
>> +++ b/gcc/tree.c
>> @@ -10297,6 +10297,17 @@ uniform_vector_p (const_tree vec)
>>        return first;
>>      }
>>
>> +  if (TREE_CODE (vec) == SSA_NAME)
>> +    {
>> +      gimple def = SSA_NAME_DEF_STMT (vec);
>> +      if (gimple_code (def) == GIMPLE_ASSIGN)
>
>
> do  this:
>
>              if (is_gimple_assign (def) && gimple_assign_copy_p (def))
>
>> +        {
>> +          tree rhs = gimple_op (def, 1);
>> +          if (VECTOR_TYPE_P (TREE_TYPE (rhs)))
>> +            return uniform_vector_p (rhs);
>> +        }
>> +    }
>> +
>>    return NULL_TREE;
>>  }
>
> Do you have a test case showing what missed optimization this fix can enable ?
>
> David


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