bug: Unknown Hint: Core(Core(EvalCircuit

[notlesh]

Current behavior:

Hint fails with:

Unknown Hint: Core(Core(EvalCircuit { n_add_mods: Deref(CellRef { register: AP, offset: -6 }), add_mod_builtin: Deref(CellRef { register: FP, offset: -6 }), n_mul_mods: Deref(CellRef { register: AP, offset: -4 }), mul_mod_builtin: Deref(CellRef { register: FP, offset: -5 }) }))

This happens presumably any time mul_mod is called. The problem is that this is not handled in cairo-vm.

Expected behavior:

Hint should succeed.

Steps to reproduce:

Run prove_block against affected blocks such as 160035, 160045, 160068, 160074, 160084, or 160093

[notlesh]

I started by handling the hint in cairo-vm: Moonsong-Labs/cairo-vm#44

But this led to an error shortly later: Unknown memory cell at address 13:6

It turns out there is some logic missing from cairo-vm's fill_memory as compared to cairo-lang's. This comment (although unclear no me) suggests what is wrong:

# Note that we can't read 'n' here because sierra expects this function to compute it.

https://github.com/starkware-libs/cairo-lang/blob/4ea4fe8e167845a3402ae2ea0a8b6004aad18dd5/src/starkware/cairo/lang/builtins/modulo/mod_builtin_runner.py#L373

[notlesh]

A naive attempt to avoid reading n, as suggested in the comment mentioned above, isn't sufficient. It turns out there is a lot of extra code to calculate this value instead of reading it.

I began implementing this, but it turned into quite a rabbit hole, as there seems to be a lot more code omitted in the cairo-vm implementation. Even worse, a lot of the code that is omitted is actually dead code, so it's not obvious what exactly actually needed among the missing code.

[notlesh]

The implementation from cairo seems to work 🎉 We're now passing the following blocks:

160035, 160045, 160068, 160074, 160084