[Bug tree-optimization/68576] scev failed for loop auto parallelize

[amker.cheng at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68576

--- Comment #5 from bin.cheng <amker.cheng at gmail dot com> ---
The type conversion comses from lim, below code:
      /* In case this is a stmt that is not unconditionally executed
         when the target loop header is executed and the stmt may
         invoke undefined integer or pointer overflow rewrite it to
         unsigned arithmetic.  */
      if (is_gimple_assign (stmt)
          && INTEGRAL_TYPE_P (TREE_TYPE (gimple_assign_lhs (stmt)))
          && TYPE_OVERFLOW_UNDEFINED (TREE_TYPE (gimple_assign_lhs (stmt)))
          && arith_code_with_undefined_signed_overflow
               (gimple_assign_rhs_code (stmt))
          && (!ALWAYS_EXECUTED_IN (bb)
              || !(ALWAYS_EXECUTED_IN (bb) == level
                   || flow_loop_nested_p (ALWAYS_EXECUTED_IN (bb), level))))
        gsi_insert_seq_on_edge (e, rewrite_to_defined_overflow (stmt));

Once it's introduced, SCEV can't resolve it but assume possible overflow.

By mannually disabling lim pass, SCEV works as expected and now outer loop can
be parallelized.