spec: HINT chip

[nicole-graus]

Description:

Specifies the hint ECALL implemented in #876, as ECALL number -31.

The chip constrains where the hinted value lands and that it is 32 bytes, but never the value itself — a prover may return anything. Soundness therefore rests on an obligation on the calling program: verify the hint, and recompute in software when the check fails. That obligation is declared as HNT-A1 and discussed in the chapter.

[github-actions]

Kimi Code Review

⚠️ Review failed: Kimi API request failed with status 401

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Automated review by Kimi (Moonshot AI)

[github-actions]

Codex Code Review

• High — spec/about_ecalls.typ:40: The PR declares ECALL -31 supported, but adds no executor syscall variant/handler or prover HINT table. The executor therefore returns UnknownSyscall, and no trace can satisfy the new specification. Implement both VM and prover support before advertising this ECALL, or remove it from the supported list.

[github-actions]

Kimi Code Review

⚠️ Review failed: Kimi API request failed with status 401

---

Automated review by Kimi (Moonshot AI)

[github-actions]

Codex Code Review

No issues found in the PR changes. The new constraint specification also passes the repository’s structural validation.

[erik-3milabs]

Quick review:

1. the constraint suite and spec text are not presenting a unified picture of what this chip is supposed to do. Case and point: both addr_in and selector are read from memory, but never actually used. Also, the text talks about constraining the input, while this PR description explicitly mentions not doing so.
   • the textual description is also rather bloated; please trim the excess text.
2. we always imagined the prover providing hints to the guest program through memory: the prover would write hints in a memory section and the guest would read them from there. In other words, I don't think including this HINT chip allows the prover to do something he was not yet able to do. What is the reason for introducing this chip?

[erik-3milabs]

see above