Rematerialization

首先追踪 issue 中描述的 Function trait 和 Edit enum，以及现有的 VReg、PReg、spill 和 reload 处理。确定是否可以在不支持复杂 rematerialization 的情况下设计简单常量 rematerialization 的范围；完成内容应包括元数据接口、rematerialization 编辑，以及对 spill 和 reload 行为的覆盖。

As an alternative to spilling and reloading, a vreg value can sometimes be recomputed from other available values. There are two ways this can be supported in regalloc2:

Simple rematerialization

Supporting rematerialization for constants, including runtime constants which only depend on pinned registers (e.g. VMContext), is relatively straightforward. The client needs to track rematerialization metadata for each VReg, after which we can elide spills for this vreg (unless there is a stack use) and replace reloads with rematerializations.

trait Function {
    fn can_rematerialize(&self, vreg: VReg) -> bool;
}

enum Edit {
    /// The code sequence for rematerialization is only allowed to
    /// use `dest` and the dedicated scratch register.
    Rematerialize {
        vreg: VReg,
        // This is not an Allocation because rematerializing
        // into a stack slot doesn't make sense.
        dest: PReg,
    }
}

Complex rematerialization

Rematerialization involving values in other vregs is more complex and probably not worth the effort of implementing. It has been done before as part of a research project on V8 but AFAIK this has not made it into the main V8 codebase.