blockifier: avoid redundant memory reads in read_felt_array

[gkaempfer]

Summary

Follow-up perf optimization on #15008 ("blockifier: handle null empty spans"), which added an empty-span check to read_felt_array in crates/blockifier/src/execution/syscalls/hint_processor.rs.

That check reads the array's start and end pointer cells via vm.get_maybe(...) to detect the null-empty-span case. In the non-empty (common) path, the function then re-read the same two memory cells via vm.get_relocatable(...) — a redundant Memory::get() lookup for addresses already fetched a few lines earlier.

read_felt_array runs on every array-typed syscall argument (calldata, emit_event keys/data, etc.), i.e. roughly once per syscall across a block, so the extra lookup is pure overhead in a hot path.

This PR reuses the already-fetched Option<MaybeRelocatable> values instead of re-reading memory, via a small helper (relocatable_from_memory_value) that reproduces VirtualMachine::get_relocatable's exact error semantics (MemoryError::UnknownMemoryCell vs MemoryError::ExpectedRelocatable) so behavior is unchanged in every case — empty span (null or real), non-empty span, missing memory cell, wrong-type memory cell.

Also strengthens test coverage: the existing "rejects mixed null/pointer span" test now asserts the exact error variant and address (previously just is_err()), and two new tests cover the felt-typed-end-pointer and unwritten-cell error paths, since preserving get_relocatable's error semantics is the main correctness risk of this kind of refactor.

Expected impact

Cuts the memory-lookup work of read_felt_array roughly in half for the non-empty-array case (2 lookups instead of 4), on a function invoked for essentially every array-typed syscall argument in every transaction.

Test plan

• SEED=0 cargo test -p blockifier read_felt_array — 8 passed, 0 failed
• SEED=0 cargo test -p blockifier --lib — 985 passed, 0 failed
• cargo clippy -p blockifier --lib — clean
• scripts/rust_fmt.sh — no changes needed
• Reviewed by an Opus subagent for correctness (verified error-semantics parity against cairo-vm's Memory::get/get_relocatable) and style; findings applied.

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Generated by Claude Code

[cursor]

PR Summary

Low Risk
Hot-path refactor of syscall array parsing with intended identical behavior; risk is limited to error-variant/address mismatches, which the new tests cover.

Overview
Avoids a second VM memory lookup in read_felt_array on the common non-empty path. Start/end cells already fetched with get_maybe for the empty-span check are converted in-place instead of being re-read via get_relocatable.

A small helper, relocatable_from_memory_value, maps those cached values to the same MemoryError variants (ExpectedRelocatable vs UnknownMemoryCell) that get_relocatable would have produced.

Tests now assert exact error type and address, and cover felt-typed end pointers and unwritten cells.

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• Started at 2026-08-24T13:44:23Z