Optimize `if` conditions with integer equality to matches on the integer

[oli-obk]

If we generate the MIR for

fn black_box<T>(t: T) -> T { t }

fn main() {
    let x = black_box(42);
    
    let y = if x == 43 {
        "foo"
    } else {
        "bar"
    };
}

we get something like

        _3 = Eq(move _4, const 43i32);
        StorageDead(_4);
        switchInt(_3) -> [false: bb2, otherwise: bb3];

which we could also write as

        switchInt(_4) -> [43i32: bb3, otherwise: bb2];

(plus move the StorageDead to the beginning of bb2 and bb3). The same goes for != (Ne).

cc @rust-lang/wg-mir-opt

[simonvandel]

I would like to work on this
@rustbot claim

[simonvandel]

Hi @oli-obk
While working on this I see that SwitchInt values are of type u128. This would mean that this optimization could only be done if the integer value is unsigned.

Is it an oversight that SwitchInt does not allow to switch on signed values?

[bjorn3]

The u128 is just a zero-extended version of the value to switch on. For example if i == -1i8 would be something like Switch(i) -> [0xff: bb1, otherwise: bb2].

[tmiasko]

FYI: Codegen does the this transformation in other direction:

rust/src/librustc_codegen_ssa/mir/block.rs

Lines 220 to 224 in 30f0a07

	let switch_llty = bx.immediate_backend_type(bx.layout_of(switch_ty));
	let llval = bx.const_uint_big(switch_llty, values[0]);
	let cmp = bx.icmp(IntPredicate::IntEQ, discr.immediate(), llval);
	helper.maybe_sideeffect(self.mir, &mut bx, targets.as_slice());
	bx.cond_br(cmp, lltrue, llfalse);

[oli-obk]

Hmm... that's good to know. Maybe we should additionally have the opposite optimization as a cleanup before we go into codegen, then this codegen arm can just become an assert.