[perf] Constructors where `this` escapes are slow

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还没有人认领这个 Issue。

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难度
4/5
预计耗时
3-5 天
新手友好度
35/100
Issue 类型
重构
描述清晰度
基本清楚
活跃度
活跃
技术栈
cpp, rust
领域
compilers, performance

调研方向

The issue describes how the translator emits constructors for C++ types whose this escapes, so start in the code that generates the Rust new function for constructors. Read the godbolt example linked in the issue to compare the inline and escaping cases. The proposed change is to allocate the storage before the constructor call and pass it in; done means escaping-this output no longer copies the struct and the existing transpiler tests still pass. The Rc/RefCell case is a separate, related cost.

由索引模型根据 Issue 内容生成。

描述

In unsafe constructors are translated as:

fn new() -> Self {
  let this = Self { ... };
  ... body of the constructor ...
  this
}

When this escapes in the body of the constructor, for example init(&mut this), the translated new becomes slower than necessary: this is constructed inside the body then memcpy'ed in the caller.

When this does not escape in the body of the constructor, Rust returns the this inline, without a memcpy. See this example.

C++ does no memcpy. The semantics of the constructor is: a storage, i.e. this, is passed to the constructor, the constructor initialises this, then the storage is returned.

I think that the solution for this performance problem is to follow what C++ does: create this before calling the constructor and pass the storage to the constructor. This would also fix #248.

In performance terms, refcount is worse because it does 3 memcpy's even when this does not escape:

fn new() -> Self {
  let __this = Rc::new(RefCell::new(Self { ... }));
  let this = __this.as_pointer();
  Rc::try_unwrap(this) // memcpy 1: transforms Rc<RefCell<S>> into Result<RefCell<S>, Rc<RefCell<S>>>
    .ok().unwrap() // memory 2: transforms Result<RefCell<S>, Rc<RefCell<S>>> into RefCell<S>
    .into_inner() // memcp 3: transforms RefCell<S> into S
}

The solution is the same as unsafe: create this before calling the constructor and pass the storage to the constructor.

主要语言
Rust
星标
310
派生
18
平均合并
10 小时 18 分钟
30 天内合并 PR
125

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