guideline

When measured performance indicates, perform packed Boolean array shift operations by using slice assignments rather than repeated bit-wise assignment.

   subtype Word_Range is Integer range 0 .. 15;
   type Flag_Word is array (Word_Range) of Boolean;
   pragma Pack (Flag_Word);
   Word : Flag_Word;
   ...

   -- Loop to shift by one bit
   for Index in 0 .. 14 loop
      Word (Index) := Word (Index + 1);
   end loop;
   Word (15) := False;

   -- Use slice assignment to shift by one bit
   Word (0 .. 14) := Word (1 .. 15);
   Word (15) := False;

rationale