1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58

use arret_runtime::abitype;
use arret_runtime::boxed;

use crate::mir::value::Value;

pub fn plan_phi_abi_type(lhs: &Value, rhs: &Value) -> abitype::AbiType {
    use crate::mir::specific_abi_type::*;

    match (lhs, rhs) {
        (Value::Reg(lhs_reg_value), Value::Reg(rhs_reg_value))
            if lhs_reg_value.abi_type == rhs_reg_value.abi_type =>
        {
            // We have identical ABI types; this is easy
            rhs_reg_value.abi_type.clone()
        }
        (lhs, rhs) => {
            use std::iter;

            // We prefer working with unboxed values whenever possible. However, having to box a
            // value is much worse than working with a boxed temporary. Boxing requires calling in
            // to the allocator which is one of the most expensive things we can do.
            //
            // If both values are boxed then create an boxed phi. This prevents us from "wasting"
            // a box from prematurely unboxing it and then having to allocate to re-box it later.
            let both_boxed_non_bools = [lhs, rhs].iter().all(|value| {
                match value {
                    Value::Const(any_ref) => {
                        match any_ref.as_subtype() {
                            boxed::AnySubtype::True(_) | boxed::AnySubtype::False(_) => {
                                // LLVM has trouble following bool values through boxing and
                                // unboxing. Also, boxing bools is relatively cheap because we just
                                // need to return a pointer to the correct singleton value.
                                false
                            }
                            _ => {
                                // `Const`s can be either boxed or unboxed
                                // This effectively means "whatever the other value wants"
                                true
                            }
                        }
                    }
                    Value::Reg(reg_value) => {
                        matches!(reg_value.abi_type, abitype::AbiType::Boxed(_))
                    }
                    _ => true,
                }
            });

            let values_iter = iter::once(lhs).chain(iter::once(rhs));

            if both_boxed_non_bools {
                specific_boxed_abi_type_for_values(values_iter).into()
            } else {
                specific_abi_type_for_values(values_iter)
            }
        }
    }
}