In an expression like OP((x | C), K), if the inner constant (C) and the effective mask (M) corresponding to OP(_, K) are disjoint ((C & M) == 0), then the OR with the constant is unneeded and can be optimized away since the constant will be 'cleared' by the mask, giving: OP(x, K) For example, code like: int foo(int x) { return (x | 0xfffff000) & 0xfff; } is now optimized into: foo: and.32 %r3 <- %arg1, $0xfff ret.32 %r3 while previously the OR mask was not optimized away: foo: or.32 %r2 <- %arg1, $0xfffff000 and.32 %r3 <- %r2, $0xfff ret.32 %r3 Signed-off-by: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>