Macro hopper_fast_entrypoint 

macro_rules! hopper_fast_entrypoint {
    ( $process_instruction:expr ) => { ... };
    ( $process_instruction:expr, $maximum:expr ) => { ... };
}

Without the simd-0321 feature the “fast” entrypoint is an alias for the standard scanning entrypoint. The SIMD-0321 gate is live on every public cluster (mainnet-beta 2026-04-01); the r2 form is sound to build and stays opt-in only because it measured CU-neutral against the fused scanning walk for ~368 bytes of extra .text. The two-argument r2 form also null-checks the register and falls back to scanning, so it is safe even where the gate is somehow inactive.