Will data be copied byte by byte when moving a variable?

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I'm new to rust, and I am wondering what exactly happens when moving a variable.

struct Point {
    x: i32,
    y: i32,
}

fn main() {
    let p = Point { x: 1, y: 1 };
    let q = p;
}

When let q = p;, will the data (size of which is 8 bytes) be copied from a memory address to another? Since p is moved here thus it cannot be used anymore, I think it is good to make q's underlying memory address equal to p's. In another word, I think it is OK that nothing is copied in the machine code.

So my question is: will data be copied byte by byte when moving a variable? If it will, why?

rust
1 Answers

[W]ill data be copied byte by byte when moving a variable?

In general, yes. To move a value, Rust simply performs a bitwise copy. If the value is not Copy, the source won't be used anymore after the move. If the value is Copy, both the source and the destination can be used.

However, there are many cases when the compiler backend can eliminate the copy by proving that the code beahves identical without the copy. This optimization happens completely in LLVM. In your example, the LLVM IR still contains the instructions to move the data, but the generated code does not contain the move even in debug mode.

If it will, why?

There are many reasons why the compiler can be unable to use the same memory for source and destination. In your example, with two variables in the same stack frame, it's easy to see that the move is not needed, but the code is a bit pointless anyway (though sometimes people do move values inside a function to make a variable immutable).

Here are just a few illustrations why the compiler may be unable to reuse the source memory for the destination:

  • The source value may be on the stack, while the destination is on the heap or vice versa. The statement let b = Box::new(3); will move the value 3 from the stack to the heap'; let i = *b; will move it from the heap back to the stack. It's still possible that the compiler can eliminate these moves, e.g. by writing the constant 3 to the heap immediately, without writing it to the stack first.

  • Source and destination may be on different stack frames, when moving values across functions – e.g. when passing a value into a function, or when returning a value from a function.

  • Source an destination values may be stored in struct fields, so they need to have the right offset inside the struct.

These are just a few examples. The takeaway is that in general, a move may result in a bitwise copy. Keep in mind that a bitwise copy is very cheap, though, and that the optimizer usually does a good job, so you should only worry about this if you actually have a proven performance bottleneck.

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