https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112723
--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> --- It's combine on RTL that is able to eliminate the addition/subtraction and postreload the redundant shift. Nothing on GIMPLE optimizes c.0_2 = (unsigned int) c_5(D); _3 = c.0_2 + 2147483648; c_8 = (int) _3; _4 = c_8 * 2; to _4 = c_5(D) * 2; note you need to write c += -2147483647 - 1; to avoid using 'long' (but I guess it's necessary to avoid undefined behavior). So what's missing is a simplification pattern that ignores ops that only alter bits that are dont-care in a later operation. That fits the backprop pass best, not special simplification patterns.
