[Bug tree-optimization/113326] New: Optimize vector shift with constant delta on shifting-count operand

["fxue at os dot amperecomputing.com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113326

            Bug ID: 113326
           Summary: Optimize vector shift with constant delta on
                    shifting-count operand
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: fxue at os dot amperecomputing.com
  Target Milestone: ---

For shift by induction variable, loop vectorization could generate a series of
vector shifts, whose shifting-count operands are advanced by constant delta. We
could use result of the first vector shift as base, and transform others to be
vector shift on the result by scalar delta, which would eliminate non-uniform
shift vector count constant, and thereby save a load on most architectures.

 int test(int array[16]);

 int foo(int value)
 {
   int array[16];

   for (int i = 0; i < 16; i++)
      array[i] = value >> i;

   return test(array);
 }

 // Current
 vect_value = { value, value, value, value };
 vect_shift_0 = vect_value >> { 0, 1, 2, 3 };
 vect_shift_1 = vect_value >> { 4, 5, 6, 7 };
 vect_shift_2 = vect_value >> { 8, 9, 10, 11 };
 vect_shift_3 = vect_value >> { 12, 13, 14, 15 };

 // To be optimized to
 vect_value = { value, value, value, value };
 vect_shift_0 = vect_value >> { 0, 1, 2, 3 };
 vect_shift_1 = vect_shift_0 >> { 4, 4, 4, 4 };
 vect_shift_2 = vect_shift_0 >> { 8, 8, 8, 8 };
 vect_shift_3 = vect_shift_0 >> { 12, 12, 12, 12 };