this post was submitted on 08 Mar 2025
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Rust Programming

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Which is faster? (self.rust)
submitted 2 months ago* (last edited 2 months ago) by commander to c/rust@lemmy.ml
14 comments fedilink hide all child comments
 
let mut variable: Type;
loop {
    variable = value;
}

or

loop {
    let variable: Type = value;
}
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[–] anhkagi@jlai.lu 8 points 2 months ago* (last edited 2 months ago) (2 children)

I would say that they are equivalent. If I'm not mistaken, let statements only reserves space on the stack, and this only increments the stack register.

And on the latter snippet, the compiler would certainly not bother to modify the stack pointer as the type doesn't change.

[–] anhkagi@jlai.lu 17 points 2 months ago* (last edited 2 months ago) (1 children)

according to godbolt: https://rust.godbolt.org/z/hP5Y3qMPW

use rand::random;

pub fn main1() {
    let mut var : u128;
    loop {
        var = random();
    }
}

pub fn main2() {
    loop {
        let var : u128 = random();
    }
}

compiles to:

example::main1::h45edf333d7832d08:
.Lfunc_begin8:
        sub     rsp, 24
.LBB8_1:
.Ltmp80:
        mov     rax, qword ptr [rip + rand::random::he3c23ceb967a3e28@GOTPCREL]
        call    rax
        mov     qword ptr [rsp + 8], rdx
        mov     qword ptr [rsp], rax
        jmp     .LBB8_1
.Ltmp81:
.Lfunc_end8:

example::main2::h1a899b25b96d66db:
.Lfunc_begin9:
        sub     rsp, 24
.LBB9_1:
.Ltmp82:
        mov     rax, qword ptr [rip + rand::random::he3c23ceb967a3e28@GOTPCREL]
        call    rax
        mov     qword ptr [rsp + 8], rdx
        mov     qword ptr [rsp], rax
        jmp     .LBB9_1
.Ltmp83:
.Lfunc_end9:
        jmp     .LBB9_1

So yeah, exactly the same thing.

[–] commander 5 points 2 months ago

Fascinating! Thank you for sharing this.

[–] Giooschi@lemmy.world 3 points 2 months ago* (last edited 2 months ago) (1 children)

let statements only reserves space on the stack

It is not guaranteed to do that. It could also use a register or be optimized out completly (like in the example you posted in the other comment).

The stack pointer is also not changed for each local variable, but instead for each function call, so it wouldn't make a difference anyway.

[–] anhkagi@jlai.lu 1 points 2 months ago

Ok! Thanks for the clarification !