Optimize redundant MPI EQ polynomial evaluations

[shirin-shahabi]

Summary

Eliminate redundant MPI EQ polynomial evaluations

Status

Halted at gate2.

Reason

["Out-of-scope change: the hunk in compiler/circuit-std-rs/src/logup.rs (improved error message for query_id out of bounds) is not mentioned in the approved proposal 'sumcheck-mpi-eq-redundancy', which is strictly scoped to sumcheck_gkr_vanilla.rs and sumcheck_gkr_square.rs. This line must be reverted or submitted under a separate proposal.", 'DRY violation: the unsafe byte-serialization + root_broadcast_bytes pattern is duplicated verbatim in both sumcheck_gkr_square.rs and sumcheck_gkr_vanilla.rs. This should be extracted to a helper — either a new method on MPIEngine (e.g. broadcast_challenge_field_slice) or a free function in the sumcheck prover_helper module — and called from both sites.', 'API asymmetry: sumcheck_gkr_square::prepare_mpi accesses mpi_config via self.mpi_config, while sumcheck_gkr_vanilla::prepare_mpi now takes mpi_config as an explicit &impl MPIEngine parameter. The approach should be consistent across both helpers. Either both should take the parameter explicitly or both should use self.mpi_config.']

Methodology

In sumcheck_gkr_vanilla.rs:233, EqPolynomial::eq_eval_at is called for ALL MPI world ranks redundantly. Only the root process needs the full evaluation. Implement rank-selective evaluation: compute on root, broadcast result to others. Same fix needed in sumcheck_gkr_square.rs:175.

Acceptance (unmet)

cargo test --release -p sumcheck passes. MPI rank 0 computes eq_eval, others receive broadcast.

---

DSPy-trace: f285aa53-5f1a-4157-91cc-b6197a260081

[shirin-shahabi]

Closed after gate2 failure: ["Out-of-scope change: the hunk in compiler/circuit-std-rs/src/logup.rs (improved error message for query_id out of bounds) is not mentioned in the approved proposal 'sumcheck-mpi-eq-redundancy', which is strictly scoped to sumcheck_gkr_vanilla.rs and sumcheck_gkr_square.rs. This line must be reverted or submitted under a separate proposal.", 'DRY violation: the unsafe byte-serialization + root_broadcast_bytes pattern is duplicated verbatim in both sumcheck_gkr_square.rs and sumcheck_gkr_vanilla

[shirin-shahabi]

Dry run violation without explicit benchmark result, need to improve DSPy-trace: f285aa53-5f1a-4157-91cc-b6197a260081; suggested formating.