`(Constant <OP> a) & b & ~c` should compile to `(Constant <OP> a) & (b & ~c)`

[Validark]

This code: (C Godbolt Zig Godbolt, LLVM Godbolt)

const C: u64 = 1234;

fn foo(a: u64, b: u64, c: u64) u64 {
    return (C ^ a) & b & ~c;
}

Compiles to:

foo:
        xor     rdi, 1234
        and     rdi, rsi
        andn    rax, rdx, rdi
        ret

Should be:

foo:
        xor     rdi, 1234
        andn    rax, rdx, rsi ; Can be done in parallel to xor!
        and     rax, rdi
        ret

[pinskia]

Isn't this just reassociation? That is how GCC gets it. Interesting If I remove the ~ from ~c, LLVM is able to do the reassociation and produce good code. Maybe there is some non-balancing of the tree here going on and not takeing into account ~ can be cheap.

[manik-muk]

I'd like to take a stab at doing this optimization!

[jayfoad]

Yes it is reassociation, and should be done consistently for any associative operation, not just for this very specific case. Also this transformation is not always beneficial since it can increase register pressure if both b and c have further uses.