How to handle calls to mapping of big structs

Question

Good day y'all, i'm tying to write a contract which is supposed to interact with another one that has a big struct that i need to retrieve to do some calculation, the struct have 14 properties and it look like this:

mapping(uint256 => Round) public rounds;

struct Round {
    uint256 epoch;
    uint256 startTimestamp;
    uint256 lockTimestamp;
    uint256 closeTimestamp;
    int256 lockPrice;
    int256 closePrice;
    uint256 lockOracleId;
    uint256 closeOracleId;
    uint256 totalAmount;
    uint256 bullAmount;
    uint256 bearAmount;
    uint256 rewardBaseCalAmount;
    uint256 rewardAmount;
    bool oracleCalled;
}

Trying to simply get this struct and return the struct throws stack too deep error:

function getRound(address contractAddress, uint256 epoch) private view returns (Round memory round) {
 (
      ,,,,,
      round.closePrice,
      ,,
      round.totalAmount,
      round.bullAmount,
      round.bearAmount,
      ,
      round.rewardAmount,
      round.oracleCalled
  ) = IContractInterface(contractAddress).rounds(epoch);

I'm not understanding the problem here, i know about the 16 variable limit and i tried to hardcode "contractAddress" and "epoch" to try only retrieving the 14 variables of the struct + 1 variable for the Round struct but still i'm receiving this error

CompilerError: Stack too deep. Try compiling with --via-ir (cli) or the equivalent viaIR: true (standard JSON) while enabling the optimizer. Otherwise, try removing local variables. When compiling inline assembly: Variable dataEnd is 2 slot(s) too deep inside the stack. Stack too deep. Try compiling with --via-ir (cli) or the equivalent viaIR: true (standard JSON) while enabling the optimizer. Otherwise, try removing local variables.

Can you help me out understanding what i'm doing wrong? Thanks!

Answer 1

Since you have a mapping, a struct and 14 elements in your struct it is not surprising that you have the stack too deep error.

The EVM callstack can only handle properly the 16/17 elements. For instance the DUP opcode can only duplicate the 16 first elements of the stack while the SWAP opcode can swap from the first through the 17th element.

Possible solutions would be to:

• lighten your variables in your struct.

• split your variables in several structs.

• pack your variables (in your case having 4 epoch times, you could use four uint64 that would result in using only 1 memory slot).

• Use internal functions to split the input & output.

There are couples of other solutions.

Here are two useful links that will provide you explanation and possible fixes.

https://soliditydeveloper.com/stacktoodeep

https://medium.com/aventus/stack-too-deep-error-in-solidity-5b8861891bae

(this last one was already provided by ChefAharoni in his comment and is an in depth explanation of the error).