[Bug tree-optimization/124909] gcc.dg/match-shift-cmp-3 fails with -fshort-enums

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=124909

--- Comment #10 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
The pattern is e.g.
/* (y << x) == x -> false and (y << x) != x -> true when y != 0.  */
(for cmp (eq ne)
 (simplify
  (cmp:c (nop_convert1? (lshift @0 @1)) (convert2? @1))
  (if (INTEGRAL_TYPE_P (TREE_TYPE (@0))
       && tree_expr_nonzero_p (@0))
      { constant_boolean_node (cmp != EQ_EXPR, type); })))
so obviously it matches fine for
  _1 = 4294967295 << x;
  x.0_2 = (unsigned int) x;
  D.3011 = _1 == x.0_2;
where x is 32-bit test_enum, but doesn't match for
  _1 = (int) x;
  _2 = 4294967295 << _1;
  _3 = (unsigned int) x;
  D.3011 = _2 == _3;
when x is 16-bit or 8-bit test_enum because the lshift doesn't use x but (int)
x.
So, you'd need to use (convert3? @1) instead of @1 in the second argument of
lshift,
but then (but maybe even when not doing that) one needs to be careful about
which conversions to allow and which to disallow.  I'd be worried about
narrowing conversions, or when @1 isn't integral at all (say floating point).