[Bug tree-optimization/115060] New: Probable an issue around usage of vect_look_through_possible_promotion in tree-vect-patterns.cc

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115060

            Bug ID: 115060
           Summary: Probable an issue around usage of
                    vect_look_through_possible_promotion in
                    tree-vect-patterns.cc
           Product: gcc
           Version: 15.0
            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: ---

The purpose of the function is to peel off type casts to find out root
definition for a given value. If patterns are involved, intermediate
pattern-defined SSAs would be traversed instead of the originals. A subtlety
here is that the root SSA (as the return value) might be the original one, even
it has been recognized as a pattern. For example,

   a = (T1) patt_b;
   patt_b = (T2) c;        // b = ...
   patt_c = not-a-cast;    // c = old_seq

Given 'a', the function will return 'c', instead of 'patt_c'. If a caller only
does something based on type information of returned SSA, there is no problem.
But if caller's pattern recog analysis is to parse definition statement of the
SSA, the new pattern statement is bypassed. This tends to be inconsistent with
processing logic of the pattern formation pass, which is not quite rational,
and seems to be an issue, though does not cause any mistake. Anything that I
missed here?

Take one code snippet as example:

vect_recog_mulhs_pattern ()
{
  ...

  vect_unpromoted_value unprom_rshift_input;
  tree rshift_input = vect_look_through_possible_promotion
    (vinfo, gimple_assign_rhs1 (last_stmt), &unprom_rshift_input);

  ...

  /* Get the definition of the shift input.  */
  stmt_vec_info rshift_input_stmt_info
    = vect_get_internal_def (vinfo, rshift_input);
  if (!rshift_input_stmt_info)
    return NULL;
  gassign *rshift_input_stmt
    = dyn_cast <gassign *> (rshift_input_stmt_info->stmt);
  if (!rshift_input_stmt)
    return NULL;

   ...

  if (gimple_assign_rhs_code (rshift_input_stmt) == PLUS_EXPR)  // How about if
rshift_input_stmt has a pattern replacement?
    {
      ...
    }