https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96929
Jakub Jelinek <jakub at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Ever confirmed|0 |1
Status|UNCONFIRMED |NEW
Last reconfirmed| |2020-09-04
CC| |jakub at gcc dot gnu.org
--- Comment #1 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
We already have a rule for this:
/* Optimize -1 >> x for arithmetic right shifts. */
(simplify
(rshift integer_all_onesp@0 @1)
(if (!TYPE_UNSIGNED (type)
&& tree_expr_nonnegative_p (@1))
@0))
but the rule requires that the shift count is non-negative.
That was added in PR38359 fix.
Even current wide_int_binop has:
case RSHIFT_EXPR:
case LSHIFT_EXPR:
if (wi::neg_p (arg2))
{
tmp = -arg2;
if (code == RSHIFT_EXPR)
code = LSHIFT_EXPR;
else
code = RSHIFT_EXPR;
}
else
tmp = arg2;
and so it treats rshift shifts by negative values as left shifts.
So, if we wanted to fix this PR, we'd need to remove the
tree_expr_nonnegative_p and change wide_int_binop to perhaps best punt on
negative arg2 instead of trying to handle it. Wonder what it will break.
