perf(mpcs): copy jagged q from committed column MLEs#62
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## Problem Dynamic RAM init could write non-zero values into committed witness padding rows. That violates the expected zero-padding invariant for RMM witness commitments and causes CPU shard verification to fail with `InvalidPcsOpen`. ## Design Rationale Keep the existing padded CPU proving flow and add the missing dynamic-init guard so only real instances populate committed witness rows. This preserves the RMM zero-padding contract without changing verifier logic. Related: scroll-tech/gkr-backend#62 ## Change Highlights - `ceno_zkvm`: guard dynamic RAM init witness writes with `i < num_instances` so padded witness rows remain zero. - No verifier changes. ## Benchmark / Performance Impact ### Operation CPU benchmark: block `23587691`, shard `0`, `CENO_MAX_CELL_PER_SHARD=805306368`. | Operation | baseline | this PR (s) | Improve (before -> this PR) | |-----------|-----------------------------------|-------------|-----------------------------| | reth-block | 135 | 124 | 8.1% | | app.prove | 134 | 123 | 8.2% | | app.verify | 0.300 | 0.288 | 4.0% | ### Layer | Layer | before: no guard + remote gkr (s) | this PR (s) | Improve (before -> this PR) | |-------|-----------------------------------|-------------|-----------------------------| | create_proof_of_shard | 130 | 119 | 8.5% | | commit_traces | 17.2 | 10.3 | 40.1% | | prove_batched_main_constraints | 32.4 | 32.0 | 1.2% | | pcs_opening | 36.8 | 33.9 | 7.9% | Benchmark command(s): ```sh CENO_MAX_CELL_PER_SHARD=805306368 \ OUTPUT_PATH=metrics_23587691_shard0_cpu_dynamic_guard_no_gkrzero_maxcell805306368_20260608.json \ RUST_LOG=info \ target/release/ceno-reth-benchmark-bin \ --block-number 23587691 \ --chain-id 1 \ --cache-dir block_data \ --mode prove-app \ --app-proofs ./app_proof.bitcode \ --shard-id 0 ``` Baseline used the same benchmark with Ceno patched to `902b3e3c^` (`7d8086c2`) and remote `gkr-backend` tag `v1.0.0-alpha.31`. Environment (CPU/GPU, core count, rust toolchain, commit hash): - CPU shard run, `rustc 1.93.0-nightly (07bdbaedc 2025-11-19)` - before: Ceno `7d8086c2`, remote `gkr-backend v1.0.0-alpha.31` - this PR: Ceno `902b3e3c` raw data: - before: `sanity_23587691_shard0_cpu_fresh_baseline_remote_gkr_no_guard_maxcell805306368_20260608.log`, failed verification with `InvalidPcsOpen` - this PR: `sanity_23587691_shard0_cpu_dynamic_guard_no_gkrzero_maxcell805306368_20260608.log`, passed verification ## Testing ```sh cargo check --package ceno_zkvm CENO_MAX_CELL_PER_SHARD=805306368 target/release/ceno-reth-benchmark-bin --block-number 23587691 --chain-id 1 --cache-dir block_data --mode prove-app --app-proofs ./app_proof.bitcode --shard-id 0 ``` ## Risks and Rollout Low risk: the change only skips dynamic RAM init writes for rows outside `num_instances`. Rollback is reverting this guard. ## Follow-ups (optional) None. ## Copilot Reviewer Directive (keep this section) When Copilot reviews this PR, apply `.github/copilot-instructions.md` strictly.
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Problem
Jagged q materialization transposes from the row-major RMM backing. For Ceno's current compact-q flow, the committed column MLEs are already available in the required order, so rebuilding q from the row-major layout does extra strided work.
Approach
copy_from_slicefrom the committed column MLE prefix.Performance
Reth CPU shard-0, block
23587691,CENO_MAX_CELL_PER_SHARD=805306368:v1.0.0-alpha.31Testing
cargo fmt --checkcargo test -p mpcs jagged --lib(26 passed)Risk / Rollout
Low scope: only changes q materialization source in jagged commit. Rollback is reverting this PR.
Copilot Reviewer Directive (keep this section)
When Copilot reviews this PR, apply
.github/copilot-instructions.mdstrictly.[severity] path:line (symbol)format.PR metadata: descriptionas a finding.