Cairo 1.0 Q&A

[AbdelStark]

Ask all your questions about Cairo 1.0

[AlejandroLabourdette]

Hi, Is it possible to execute bitwise operations on felts?.
Something like:

    let a: felt = 1;
    let b: felt = 3;
    let c = a & b;

[drcapybara]

I also have this question. I do not see any bitwise operations for felt yet. What is the effort to implement this?

[AlejandroLabourdette]

I also have this question. I do not see any bitwise operations for felt yet. What is the effort to implement this?

Hi @drcapybara I think the reason is to motivate developers to use uN instead of felt252. You can also see that other basic operations (like comparisons >, <, ...) have been removed. As I remember one of the policies of this new Cairo1 is to move away from the use of felts so that could be it.

[Ljiacheng]

I try to declare the official hello_starknet contract with command:
starknet declare --contract hello_starknet.sierra.json --account jc2

And I get error:

Sending the transaction with max_fee: 0.000001 ETH (1378300000000 WEI).
Got BadRequest while trying to access https://alpha4.starknet.io/gateway/add_transaction. Status code: 500; text: {"code": "StarknetErrorCode.COMPILATION_FAILED", "message": "Compilation failed. Error: Invalid Sierra program.\n"}.
Traceback (most recent call last):
  File "/home/ljc/cairo_venv/lib/python3.9/site-packages/services/external_api/client.py", line 187, in _send_request
    return await self._parse_response(
  File "/home/ljc/cairo_venv/lib/python3.9/site-packages/services/external_api/client.py", line 231, in _parse_response
    raise BadRequest(status_code=response.status, text=text)
services.external_api.client.BadRequest: HTTP error ocurred. Status: 500. Text: {"code": "StarknetErrorCode.COMPILATION_FAILED", "message": "Compilation failed. Error: Invalid Sierra program.\n"}
Error: BadRequest: HTTP error ocurred. Status: 500. Text: {"code": "StarknetErrorCode.COMPILATION_FAILED", "message": "Compilation failed. Error: Invalid Sierra program.\n"}

Maybe my starknet version is wrong? Or just the official contract does't work?

starknet version:
starknet 0.11.0.2

[AlejandroLabourdette]

When I update a mutable variable inside a break condition of a loop it throw an error. This is the code:

let mut a = 0
loop {
  if condition {
    a = 1;
    break();
  }
  some_code
}

and this is the error:

error: Unsupported match. Currently, matches require one arm per variant, in the order of variant definition.
 --> fixed_bytes.cairo:110:9
        break();
        ^******^

This error is kind of odd, shouldn't this logic valid in Cairo1?

[ColinWttt]

How to write code in Cairo that is similar to Rust's format! ?

format!("Hello, {}!", "world"); // => "Hello, world!"

use case:

let token_0 = 'ETH';
let token_1 = 'USDC';
let name = format!("{} / {} LP",token_0,token_1 );   // => "ETH / USDC LP"

[temyurchenko]

In crates/cairo-lang-starknet/src/plugin/storage.rs, function handle_legacy_mapping_storage_var generates Cairo logic for computing addresses:

/// Generate getters and setters skeleton for a non-mapping member in the storage struct.
fn handle_legacy_mapping_storage_var(address: &str) -> String {
    format!(
        "
    mod $storage_var_name$ {{$extra_uses$
        use starknet::SyscallResultTrait;
        use starknet::SyscallResultTraitImpl;

        fn address(key: $key_type$) -> starknet::StorageBaseAddress {{
            starknet::storage_base_address_from_felt252(
                hash::LegacyHash::<$key_type$>::hash({address}, key))
        }}
        fn read(key: $key_type$) -> $value_type$ {{
            // Only address_domain 0 is currently supported.
            let address_domain = 0_u32;
            starknet::StorageAccess::<$value_type$>::read(
                address_domain,
                address(key),
            ).unwrap_syscall()
        }}
        fn write(key: $key_type$, value: $value_type$) {{
            // Only address_domain 0 is currently supported.
            let address_domain = 0_u32;
            starknet::StorageAccess::<$value_type$>::write(
                address_domain,
                address(key),
                value,
            ).unwrap_syscall()
        }}
    }}"
    )
}

Note the line

            starknet::storage_base_address_from_felt252(
                hash::LegacyHash::<$key_type$>::hash({address}, key))

The result of the inner hash is in the [0, PRIME) range. However, the base address should be in the [0, 2^251 - 256) range. Thus, there is an (incredibly small) probability of getting a panic. One solution is to take the result modulo 2^251 - 256, as described here, although I imagine it is quite expensive.
Maybe the small probability of getting such a situation is not worth safe-guards, please tell if so and feel free to close.

[jeluard]

How can I generate from a cairo file the memory and trace data as can be found in the old playground debug panel?

[zhouop0]

how can i get current block number in cairo contract? Just like block.number in solidity

[kwkr]

Hello! I'm currently utilizing the following crate:
https://docs.rs/cairo-lang-sierra-gas/latest/cairo_lang_sierra_gas
as a part of a package that will help the users analyze their Cairo code when it comes to how much gas it consumes. So far I was able to use the package in order to get gas costs for specific functions in the generated Sierra code as well for a specific libfuncs. I still have a couple of questions regarding the process of calculating gas that I wasn't been able to answer by myself by only browsing the code. If you could answer them or point me to a documentation (that I unfortunately wasn't able to find myself), I would be very gratefyul.

Here are my questions:

1. Why there is a differentiation between pre and post cost? From what I saw, precosts are also taken into accoun when calculating post costs and I'm not sure why are they even separated?
2. https://docs.rs/cairo-lang-sierra-gas/latest/cairo_lang_sierra_gas/gas_info/struct.GasInfo.html
   the gas info object contains the costs info for a specific function within Sierra but it also contains the field variable_values. What exactly are the variables and why are they used?
3. I'm using the following function:
   https://docs.rs/cairo-lang-sierra-gas/latest/cairo_lang_sierra_gas/core_libfunc_cost/fn.core_libfunc_cost.html
   to calculate a cost for a specific libfunction. Why does it return a vector? Can there be mutliple costs for a specific libfunc? If so, what are the situations when it might happen?

Thanks in advance for any clarification here!

[orizi]

First of all - you can probably consider just stating the value of the gas-builtin counter - it would indicate the actual amount of used gas in the counter - if you are actually trying to consider the gas mechanism, instead of the actual used resources.
if you want the actual used resources - you should count it specifically - using the vm itself.

You need the data you requested only for the process of emulating the actual used resources using the gas mechanism.

to answer the questions specifically:

1. Pre cost is the cost of builtins that the amount of usage of them is not affected by ap-changes or gas fetching (which are all builtins but the RangeCheck builtin), Post cost are the steps, holes and RangeChecks costs. The separation is to solve the required gas taken and ap changes in stages: the pre-cost (which is independent), the ap-changes (which only depends on the pre-costs), and the post costs (dependent on both).
2. This are values of gas redeposits and withdraws and burns (happens at branch_align and withdraw_gas{_,all}).
3. This the cost per branch of the libfunc.

[kwkr]

Thanks for the detailed answer @orizi. I'm currently building a solution that is based on the gas mechanism (if I understand it correctly) because I use Sierra and it's metadata in order to get the costs for specific functions/libfuncs and then while executing the program, to illustrate where and how much gas is consumed. If this is an acceptable approach, it could be really informative when it comes to illustrating how it works in Cairo in general.

Some new questions that came to my mind after thinking about your answer:

• from what I understand there are actually two ways to get the gas count. The "real" one is to use the resources consumed by the VM, which is the most accurate since the sequencers use it to actually charge the users. The Sierra-based approach is kind of an approximation (since you used the word emulation?) or is it supposed to be exactly the same? If I understand correctly, Sierra itself should already give the sequencer some info about the future gas consumption before running the program to be able to tell whether the user has enough gas to execute specific code?
• what is the definition of builtins exactly? Is this a name for some base operations (like Poseidon application etc.)?

[orizi]

• The gas mechanism is an approximation of the worst case of the actual consumed resources, so it would charge different costs of different actually taken paths, (e.g. adding two u128s and overflowing has a different actual code being run then when not overflowing). Currently, when different branches are merged - the given gas consumption is merged as well using the branch_align command which burns the extra unused gas from the cheap branch. in general, there could've been more usage of the redeposit_gas command which would make this approximation be more tight with the actual usage of resources, currently it is a very non-tight upper bound.
• Builtins are extensions of the Cairo CPU that enables more complex actions such as Pedersen, Poseidon, bitwise operations and so on.