How to return a constant variable in huff

Question

Let's say I have a constant variable in huff:

#define constant myAddress = 0x0000000000000000000000000000000000000000

How would I make a macro to read it? Since it doesn't have a storage slot, I wouldn't use sload...

#define macro GET_ADDRESS() = takes (0) returns (0) {
    // No storage slot
    [VALUE_LOCATION]   
    sload                

    0x00 mstore
    0x20 0x00 return
}

Answer 1

When you want to push a constant to the stack, you use bracket syntax [MY_CONST] & the compiler will insert PUSH<N> value, where N = the number of bytes in the constant’s value.

After you've pushed the constant to the stack, you can return the 32 bytes with 0x20 0x00 return

#define macro GET_ADDRESS() = takes (0) returns (0) {
    [PRICE_FEED_ADDRESS] 0x00 mstore
    0x20 0x00 return
}

Full example code for just returning a constant as a view function:

// SPDX-License-Identifier: MIT

/* Interface */
#define function getAddress() view returns (address)

/* Constants */
#define constant MY_CONST_ADDRESS = 0x0000000000000000000000000000000000000001 


/* Macrocs (Functions) */
#define macro GET_ADDRESS() = takes (0) returns (0) {
    [MY_CONST_ADDRESS] 0x00 mstore
    0x20 0x00 return
}


#define macro MAIN() = takes (0) returns (0) {
    // Identify which function is being called.
    0x00 calldataload 0xE0 shr

    // We tell our code where to go based on the function selector
    // 0x38cc4831 is the function selector of `getAddress`
    // Here is some info on quickly getting a func selector: https://ethereum.stackexchange.com/questions/131693/fastest-way-to-find-the-function-in-a-contract-from-its-selector
    dup1 0x38cc4831 eq getAddress jumpi 

    getAddress:
        GET_ADDRESS()
}