vrp: Handle VCE in vrp_simplify_cond_using_ranges [PR80635]

vrp: Handle VCE in vrp_simplify_cond_using_ranges [PR80635]

> So I wonder what other optimizations are prevented here?

> Why does uninit warn with VCE but not with NOP_EXPR?  Or does the
> warning disappear because of those other optimizations you mention?

The optimization that it prevents is in this particular case in tree-vrp.c
(vrp_simplify_cond_using_ranges):

      if (!is_gimple_assign (def_stmt)
          || !CONVERT_EXPR_CODE_P (gimple_assign_rhs_code (def_stmt)))
        return;
so it punts on VIEW_CONVERT_EXPR, with NOP_EXPR it optimizes that:
  _9 = (bool) maybe_a$4_7;
  if (_9 != 0)
into:
  _9 = (bool) maybe_a$4_7;
  if (maybe_a$4_7 != 0)

Now, if I apply my patch but manually disable this
vrp_simplify_cond_using_ranges optimization, then the uninit warning is
back, so on the uninit side it is not about VIEW_CONVERT_EXPR vs. NOP_EXPR,
both are bad there, uninit wants the guarding condition to be
that SSA_NAME and not some demotion cast thereof.
We have:
  # maybe_a$m_6 = PHI <_5(4), maybe_a$m_4(D)(6)>
  # maybe_a$4_7 = PHI <1(4), 0(6)>
...
One of:
  _9 = VIEW_CONVERT_EXPR<bool>(maybe_a$4_7);
  if (_9 != 0)
or:
  _9 = (bool) maybe_a$4_7;
  if (_9 != 0)
or:
  if (maybe_a$4_7 != 0)
followed by:
    goto <bb 11>; [0.00%]
  else
    goto <bb 14>; [0.00%]
...
  <bb 11> [count: 0]:
  set (maybe_a$m_6);
and uninit wants to see that maybe_a$m_4(D) is not used if
bb 11 is encountered.

This patch fixes it by teaching vrp_simplify_cond_using_ranges
to handle VCE (when from an integral type) in addition to
NOP_EXPR/CONVERT_EXPR, of course as long as the VCE or demotion
doesn't change any values, i.e. when the range of the VCE or
conversion operand fits into the target type.

2021-02-25  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/80635
	* tree-vrp.c (vrp_simplify_cond_using_ranges): Also handle
	VIEW_CONVERT_EXPR if modes are the same, innerop is integral and
	has mode precision.

	* g++.dg/warn/pr80635-1.C: New test.
	* g++.dg/warn/pr80635-2.C: New test.