https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97979
--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
/* Fold (X {&,^,|} C2) << C1 into (X << C1) {&,^,|} (C2 << C1)
(X {&,^,|} C2) >> C1 into (X >> C1) & (C2 >> C1). */
(for shift (lshift rshift)
(for bit_op (bit_and bit_xor bit_ior)
(simplify
(shift (convert?:s (bit_op:s @0 INTEGER_CST@2)) INTEGER_CST@1)
(if (tree_nop_conversion_p (type, TREE_TYPE (@0)))
(with { tree mask = int_const_binop (shift, fold_convert (type, @2), @1); }
(bit_op (shift (convert @0) @1) { mask; }))))))
and mask ends up NULL.
tree mask = int_const_binop (shift, fold_convert (type, captures[3]),
captures[4]);
(gdb) p debug_tree (captures[3])
<integer_cst 0x7ffff6953138 type <integer_type 0x7ffff68255e8 int> constant
65535>
(gdb) p debug_tree (captures[4])
<integer_cst 0x7ffff682a3a8 type <integer_type 0x7ffff6825690 unsigned int>
constant 4294967294>
(gdb) p debug_tree (type)
<integer_type 0x7ffff68255e8 int public SI
so we push in mismatched signs and folding interprets the unsigned shift
amout as signed?
