https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87415
--- Comment #3 from Aldy Hernandez <aldyh at gcc dot gnu.org> --- Created attachment 44753 --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=44753&action=edit untested patch 1-bit signed fields are weird in that 0 - (-MIN) is still -MIN. In any other world, it is an impossible result. For instance, in an 8-bit signed world, 0 - (-128) is invalid. One option would be to treat signed bit fields as TYPE_OVERFLOW wraps, since they seem to have wrapping properties, but I'm afraid such a heavy handed approach would yield latent bugs across the compiler. What VRP seems to currently be doing in set_and_canonicalize_value_range(), is just special casing the wrapping of 1-bit fields: /* Wrong order for min and max, to swap them and the VR type we need to adjust them. */ if (tree_int_cst_lt (max, min)) { tree one, tmp; /* For one bit precision if max < min, then the swapped range covers all values, so for VR_RANGE it is varying and for VR_ANTI_RANGE empty range, so drop to varying as well. */ if (TYPE_PRECISION (TREE_TYPE (min)) == 1) { set_value_range_to_varying (vr); return; } ... } I've done the same in set_value_range_with_overflow(). The attached untested patch fixes the problem on my end.
