When Optimization Backfires: Unaligned Memory Access in Solana

More storage means more lamports in Solana. And although every byte is important, there's always a debate between optimization vs simplicity. In this post, we'll walk through an example of how keeping things simple by design can help avoid potentially dangerous situations.

May 23, 2025

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7

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Catalin Neagu

Solana Account Storage and Structure Packing

Account storage in Solana is contiguous. So when you're storing structures on-chain, one of the first optimizations that comes to mind is to remove any padding after serialization and include as little metadata as possible, so the rent-exempt balance is minimized.

One approach we've seen in Solana programs is defining a structure with zero padding by default, like so:

#[repr(C, packed)]
#[derive(Clone, Copy, Debug)]
pub struct Metadata {
    pub id: u8,
    pub amount: u64,
    pub len: u8,
}

The packed attribute tells the compiler to lay out this struct contiguously, with no padding between fields.

So its in-memory layout looks like this:

| id (1 byte) | amount (8 bytes) | len (1 byte) |

Without the packed attribute, the compiler inserts padding to ensure proper alignment. The structure would instead look like this:

| id (1) | padding (7) | amount (8) | len (1) | padding (7) |

Your first thought might be: "Packed is more efficient!" Less space, no padding, fewer lamports required, sounds great, right?

But Wait, Alignment Matters

The problem is that the packed representation ignores alignment, which is a subtle but important issue that can lead to undefined behavior or even runtime crashes.

Why?

On a 64-bit processor, loading the amount field (a u64, i.e. 8 bytes) expects it to be aligned to an 8-byte boundary. If it's not aligned, reading from it might:

  • Crash the program (in some languages/platforms)
  • Cause performance penalties
  • Or even worse, lead to silent bugs

Rust is strict about this. It won’t even compile if you try to access an unaligned field directly, the compiler catches it and throws an error.

A Safer Approach: Use Byte Arrays

To avoid these alignment pitfalls, especially when accessing data off-chain, or through CPI (Cross-Program Invocation),  a simpler, safer pattern is to treat the account data as a raw byte array.

Instead of accessing amount directly, you’d do something like:

let amount_bytes = &account_data[1..9];
let amount = u64::from_le_bytes(amount_bytes.try_into().unwrap());

This method:

  • Preserves your efficient, packed layout
  • Avoids alignment issues
  • Works seamlessly across program boundaries

Yes, it adds a bit of overhead in terms of code and logic, but the peace of mind and portability it brings is worth it.

Final Thoughts

When optimizing for storage on Solana, it's tempting to reach for every byte you can save, but it’s important not to trade safety for size. Alignment issues are easy to overlook but can cause real problems in low-level systems programming.

Sometimes, the simplest solution, a good old byte slice, is the safest one.