Add a static immutable zero aligned type

[junderw]

The zeroed fn can not be used in static assignments.

In environments where it is no_std and no allocator are present, the only way to get a slice of AlignedTypes is dynamically, so preallocated_gen_new can't be used.

By offering this as a static, it can be used in static assignments as such:

#[cfg(target_pointer_width = "32")]
static mut CONTEXT_BUFFER: [AlignedType; 69645] = [ZERO_ALIGNED; 69645];
#[cfg(target_pointer_width = "64")]
static mut CONTEXT_BUFFER: [AlignedType; 69646] = [ZERO_ALIGNED; 69646];
static mut SECP256K1: Option<Secp256k1<AllPreallocated>> = None;

pub fn get_context(seed: Option<&[u8; 32]>) -> &'static Secp256k1<AllPreallocated<'static>> {
    unsafe {
        if SECP256K1.is_none() {
            SECP256K1 = Some(
                Secp256k1::preallocated_gen_new(&mut CONTEXT_BUFFER)
                    .expect("CONTEXT_BUFFER size is wrong"),
            );
        }
        if let Some(seed) = seed {
            SECP256K1.as_mut().unwrap().seeded_randomize(seed);
        }
        SECP256K1.as_ref().unwrap()
    }
}

[junderw]

Alternatively, adding more options to the global context so that it can be obtained with no_std and without an allocator by offering CONTEXT_BUFFER sizes for each alignment like I've shown above.

But seeing as the handling of context in this library looks to change in the near future, this is a minimal change that would simplify things a bit.

[elichai]

After #331 we can just make zeroed() a const fn and then you could use it in a static initialization

[junderw]

For now, this isn't very pretty, but it seems to be working.

I am looking forward to #331

// These would be marked for various pointer widths. removed for brevity
const ALIGN_SIZE: usize = 69_646;
static mut CONTEXT_BUFFER: [u8; ALIGN_SIZE * 16] = [0_u8; ALIGN_SIZE * 16];

let aligned = CONTEXT_BUFFER
    .as_mut_ptr()
    .cast::<[AlignedType; ALIGN_SIZE]>()
    .as_mut()
    .unwrap();

[elichai]

For now, this isn't very pretty, but it seems to be working.

I am looking forward to #331

// These would be marked for various pointer widths. removed for brevity
const ALIGN_SIZE: usize = 69_646;
static mut CONTEXT_BUFFER: [u8; ALIGN_SIZE * 16] = [0_u8; ALIGN_SIZE * 16];

let aligned = CONTEXT_BUFFER
    .as_mut_ptr()
    .cast::<[AlignedType; ALIGN_SIZE]>()
    .as_mut()
    .unwrap();

That's actually wrong AFAICT, as a pointer to CONTEXT_BUFFER doesn't have to be as aligned as AlignedType and casting it to a mutable reference to AlignedType is UB.

there are a few things you can do, assuming you need this to be static memory and you use a recent compiler version then this should be safe:

const ALIGN_SIZE: usize = 69_646;
static mut CONTEXT_BUFFER: [AlignedType; ALIGN_SIZE] = unsafe {std::mem::transmute([0u8; ALIGN_SIZE*16])};

the transmute is a "copy" so it doesn't have alignment requirements.

If you're using a somewhat older compiler then you can do this:

const ALIGN_SIZE: usize = 69_646;
static mut CONTEXT_BUFFER: [u8; ALIGN_SIZE * 16 + 16] = [0_u8; ALIGN_SIZE * 16 + 16];

let (_, aligned, _) = CONTEXT_BUFFER.align_to::<AlignedType>();
assert!(aligned.len() >= ALIGN_SIZE);

and if even older you might have to check alignment yourself by casting to usize etc.

[junderw]

const ALIGN_SIZE: usize = 69_646;
static mut CONTEXT_BUFFER: [AlignedType; ALIGN_SIZE] = unsafe {std::mem::transmute([0u8; ALIGN_SIZE*16])};

This worked fine. Thanks!

[apoelstra]

Where do we stand with this?

[junderw]

@elichai 's comment about making zeroed a const fn is probably the best course of action.

Though depending on the timeframe for the updates required to allow that, I would appreciate this being added, as it would mean it's available quicker.

[apoelstra]

Ok, perfect. I think a new MSRV is probably a month or more away since it's all blocked on getting the Taproot stuff out first. So let's move forward with this.

@elichai what do you think? Good to merge this?

[apoelstra]

ACK 9cf552e

[elichai]

@junderw can you make this an associated constant? ( so you could do AlignedType::ZERO )

[junderw]

done

[apoelstra]

ACK 5e6d0f1

[elichai]

Missed this notification, post-merge ACK 5e6d0f1