Possibility of in-place constructing of static mut struct for embedded applications

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1

I have the following (simplified) code as part of an embedded application:

use rand_hc::Hc128Rng;
use rand_core::{RngCore, SeedableRng};

use crate::board_hal::{EEPROMRead, EEPROMProgram};

// Don't use AtomicRefCell because init_rng stack becomes too big
pub(crate) static mut RNG: Option<Hc128Rng> = None;

const RNG_SEED_U32_LEN: usize = 32/core::mem::size_of::<u32>();
const RNG_SEED_LOC: u32 = 0x80;

// WARNING: one temp RNG instace is created on the stack
// Using AtomicRefCell leads to *two*
pub fn init_rng() {
    let mut seed: [u32; RNG_SEED_U32_LEN] = [0; RNG_SEED_U32_LEN];

    unsafe {
        EEPROMRead(seed.as_mut_ptr(), RNG_SEED_LOC, RNG_SEED_U32_LEN.try_into().unwrap());
        let seed_u8_slice = bytemuck::bytes_of(&seed);
        RNG = Some(Hc128Rng::from_seed(seed_u8_slice.try_into().unwrap()));
    }
}

Which shows up as having the following stack when compiled in release mode:

image
image698×259 14.3 KB

The disassembly also shows that an instance of Hc128Rng (~4KB in size when the target device has 32KB of RAM) is constructed on the stack and then memcpyed into the static variable's location, instead of being constructed in-place at the static variable's location. Is there a way to do an in-place construction of large structs like this (possibly using core::ptr::write or similar)?

In this case, the immediate solution is to switch to an RNG with a smaller state (which I'll probably end up doing), but the general question still stands.

2

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