Direct Account Pointers in Program Input

Summary

Serialize pointers for instruction accounts to allow direct account access without requiring parsing of the account section.

Motivation

Currently, sBPF programs in ABIv1 must parse the accounts section of the serialized input region to extract account boundary information in order to provide the program with a slice of accounts. This parsing represents most of the entrypoint's cost.

Since account boundaries are already known to the VM when preparing the program input, they can instead be serialized directly into the input. This allows the entrypoint to derive the accounts slice without iterating over or parsing the accounts section.

As a result, the entrypoint consumes a constant number of compute units, independent of the number of accounts.

New Terminology

• accounts slice pointer: A slice of 64-bit (8-byte) pointers, with one pointer per instruction account, stored at the end of the program input parameters.

Detailed Design

When the feature is activated, the VM serializes the slice of account pointers after the program ID in the program input parameters for any ABIv1 program (i.e., those that do not use Loader v1). All other parameters remain unchanged:

- 8 bytes: number of accounts (little-endian)
- <variable>: accounts section
- 8 bytes: length of instruction data (little-endian)
- <variable>: instruction data bytes
- 32 bytes: program ID
- 0-7 bytes: padding bytes to align offset to 8-bytes
- [u64; <number of accounts>]: slice of account pointers

Each account pointer is the address (u64 little-endian) of the first byte of the account information. They all must be valid addresses within the input section. Duplicated accounts will have the same pointer value, i.e., they will point to the same account. These are the same addresses that entrypoints previously had to compute when parsing the accounts section, but it is now provided directly as an input parameter.

The slice of account pointers is made available to all ABIv1 programs, under all loaders, regardless of whether it is read or not. Since this data is appended after the standard input parameters, existing programs are unaffected.

Given that, after SIMD-0321 is activated, programs will receive a direct pointer to the instruction data, programs can derive the location of the slice of account pointers without parsing the accounts section:

• read the number of accounts from r1.
• read the length of the instruction data from r2 - 8.
• pass the following three references to the program entrypoint:
  1. program_id: a &[u8] slice created from r2 + <length of the instruction data> with length equal to 32 bytes.
  2. accounts: a &[AccountView] slice created from r2 + <length of the instruction data> + 32 + padding with length equal to <number of accounts>.
  3. instruction_data: a &[u8] slice created from r2 with length equal to <length of instruction data>.

Alternatives Considered

While ABIv2 proposes a new serialization layout that provides a similar slice of accounts and likewise avoids parsing the accounts section, the approach proposed in this SIMD is comparatively simpler and fully backward compatible with current ABIv1.

Impact

On-chain programs are positively impacted by this change. The program entrypoint complexity is significantly reduced, along with the number of compute units consumed. The implementation is relatively simple, as it relies on information that is already available during program input serialization.

• Current entrypoint (pinocchio):

• Estimated entrypoint (after the changes proposed):

The benchmark above represents the cost of parsing accounts in the entrypoint of a program with an empty instruction processor. The scaffold code for it can be found here.

Security Considerations

Since programs currently do not read data from the input parameters beyond the program ID, this change does not introduce any security concerns.

Backwards Compatibility

This feature is fully backwards compatible with current ABIv1 since no program entrypoint reads data from the input parameters after the program ID.