https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78546
--- Comment #7 from Jakub Jelinek <jakub at gcc dot gnu.org> --- Preliminary analysis shows 2 issues. 1) r217349 regressed an optimization, before that change we have been able to b.0_2 = b; x_3 = 0xffffffffffffffff8000000000000000 - b.0_2; y_4 = 1 - x_3; if (y_4 != 9223372036854775809) gimple_simplified to y_4 = b.0_2 + 9223372036854775809; Replaced 'y_4 != 9223372036854775809' with 'b.0_2 != 0' into: b.0_2 = b; x_3 = 0xffffffffffffffff8000000000000000 - b.0_2; if (b.0_2 != 0) but we no longer are able to do that. 2) simplify-rtx.c opts during combine attempt to do the similar transformation, but do it incorrectly, 1 - (0xffffffffffffffff8000000000000000 - b) is turned into (b + 0xffffffffffffffff8000000000000001) rather than (b + 0x8000000000000001). I'll debug 2) now.
