submitted 2 months ago* (last edited 2 months ago) by [email protected] to c/[email protected]

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let mut variable: Type;
loop {
    variable = value;
}

loop {
    let variable: Type = value;
}

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[-] [email protected] 8 points 2 months ago* (last edited 2 months ago)

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.

[-] [email protected] 17 points 2 months ago* (last edited 2 months ago)

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.

[-] [email protected] 5 points 2 months ago

Fascinating! Thank you for sharing this.

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this post was submitted on 08 Mar 2025

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