[vect] Support min/max + index pattern

[Joel Hutton via Gcc-patches]

Hi all,

looking for some feedback on this, one thing I would like to draw attention to 
is the fact that this pattern requires 2 separate dependent reductions in the 
epilogue.
The accumulator vector for the maximum/minimum elements can be reduced to a 
scalar result trivially with a min/max, but getting the index from accumulator 
vector of indices is more complex and requires using the position of the 
maximum/minimum scalar result value within the accumulator vector to create a 
mask.

The given solution works but it's slightly messy. 
vect_create_epilogue_for_reduction creates the epilogue for one vectorized 
scalar stmt at a time. This modification makes one
invocation create the epilogue for both related stmts and marks the other as 
'done'. Alternate suggestions are welcome.

Joel

[vect] Support min/max + index pattern

Add the capability to vect-loop to support the following pattern.

for (int i = 0; i < n; i ++)
{
    if (data[i] < best)
    {
        best = data[i];
        best_i = i;
    }
}

gcc/ChangeLog:

        * tree-vect-loop.c (vect_reassociating_reduction_simple_p): New         
                                                                                
 
        function.                                                               
                                                                                
         
        (vect_recog_minmax_index_pattern): New function.                        
                                                                                
     
        (vect_is_simple_reduction): Add multi_use_reduction case.               
                                                                                
     
        (vect_create_epilog_for_reduction): Add minmax+index epilogue handling.

minmax.patch
Description: minmax.patch