CODEWITHSUNDEEP
rustcalling-convention
Łukasz Lew
Łukasz Lew

Reputation: 50328

Why does Rust have both call by value and call by reference?

Some languages, like Haskell, make no distinction between pass-by-value and pass-by-reference. The compiler can then approximately choose the most efficient calling convention with a heuristic. One example heuristic would be for the Linux x64 ABI: if the size of parameter is greater than 16 bytes, pass a pointer to the stack otherwise pass the value in registers.

What is the advantage of keeping both notions of pass-by-value and pass-by-reference (non-mutable of course) in Rust and forcing the user to choose?

Could it be the case that pass-by-value is syntactic sugar for pass-by-reference + copy if the value is seen to be modified?

Upvotes: 3

Views: 2603

Answers (2)

rp123
rp123

Reputation: 3643

[Edited: changed exampled to work in release mode]

It isn't syntactic sugar, as one can see by looking at the generated code.

Given these functions:

fn by_value(v: (u64, u64)) -> u64 {
  v.0 + v.1
}

fn by_ref(v: &(u64, u64)) -> u64 {
  v.0 + v.1
}

then if one was syntactic sugar for another, we'd expect them to generate identical assembly code, or at least identical calling conventions. But actually, we find that by_ref passes v in the rdi and rsi registers, whereas by_value passes a pointer to v in the rdi register and has to follow that pointer to get the value: (see details, use release mode):

by_value:
  movq  8(%rdi), %rax
  addq  (%rdi), %rax
  retq

by_ref:
  leaq  (%rdi,%rsi), %rax
  retq

Upvotes: 0

Steve Klabnik
Steve Klabnik

Reputation: 15559

Two things:

  1. Rust will transform certain pass-by-value calls into pass-by-reference, based on a similar heuristic.
  2. Pass-by-value indicates ownership transfer, while pass-by-reference indicates borrowing. These are very different, and totally orthogonal from the asm-level concern you're asking about.

In other words, in Rust, these two forms have different semantics. That doesn't preclude also doing the optimization, though.

Upvotes: 12

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