Will the compiler optimize initialization of a buffer in a loop?

help
1

If I write something like this:

fn foo<R: Read>(mut source: R) {
    loop {
        let mut buf = Vec::with_capacity(1024);
        let _ = source.read(&mut buf);
        // ... do something with `buf` ...
    }
}

Will the compiler recognize that it can use the same chunk of memory for each loop iteration or will this allocate new memory each time?

2

The compiler will most likely allocate each time, since allocation/deallocation is AFAIK a visible side-effect. Allocator, however, will most likely reuse the same block, so it should be fast enough for you to not worry.

1 Like
3

No: Could the compiler automatically re-use `Box` allocations as an optimization? · Issue #93707 · rust-lang/rust · GitHub

If "do something" is simple enough that the allocator call matters, then you should write

fn foo<R: Read>(mut source: R) {
    let mut buf = Vec::with_capacity(1024);
    loop {
        buf.clear();
        let _ = source.read(&mut buf);
        // ... do something with `buf` ...
    }
}

instead to manually re-use the allocation.

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4

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