pad the ppt program columns to allow lengths that arent powers of 2

[ohad-nir-starkware]

Note

Medium Risk
Changes the verify_program builtin AIR and witness generation to add a conditional/padded program column and to gate memory lookups, which can affect proof soundness/compatibility if any of the new padding/cond wiring is wrong. Scope is contained to program preprocessing and verify_program paths but touches core proving/verification constraints.

Overview
Supports non-power-of-two program lengths in the preprocessed trace by padding the program table to a fixed 1 << PROGRAM_LOG_LEN_BOUND and adding an extra program column (curr_program_28) that acts as an is-active/cond flag.

Updates the verify_program builtin to use a new MemVerifyCond subroutine so memory address/id and id/value relations are enforced only when cond=1, and adjusts witness generation to (a) compute cond in fast deduction, (b) avoid out-of-bounds memory reads on padding rows via a safe address, and (c) only feed memory subcomponent inputs for active lanes. Also updates adapter segment bounds, registry metadata, and the slow test to use the non-padded opcode program.

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This change is 

[cursor]

Missing assertion that program fits within bound

High Severity

The verify_program segment size is hardcoded to 1 << PROGRAM_LOG_LEN_BOUND (4096), but there is no assertion that the actual program length (initial_ap - 2 - initial_pc) fits within this bound. If a program exceeds 4096 entries, the segment silently covers only 4096 addresses, entries beyond that are never memory-verified, and ProgramColumn::new truncates the preprocessed data. This is a potential soundness issue where program instructions go unverified.

Additional Locations (1)

• stwo_cairo_prover/crates/common/src/preprocessed_columns/program.rs#L60-L61

 

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Cursor Bugbot has reviewed your changes and found 1 potential issue.

There are 2 total unresolved issues (including 1 from previous review).

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[cursor]

Missing assertion that program fits within bound

Medium Severity

set_program_table accepts programs of any length, but ProgramColumn::new truncates to 1 << PROGRAM_LOG_LEN_BOUND (4096) entries. Previously, padded_len was data.len().next_power_of_two(), which accommodated any program size. Now, if program.len() > 4096, the preprocessed columns silently omit entries beyond the bound, and the cond column incorrectly marks all 4096 rows as active (since i < data.len() is always true). This produces an incorrect proof without any error.

Additional Locations (1)

• stwo_cairo_prover/crates/common/src/preprocessed_columns/program.rs#L60-L61