`bytes public data` getter loops forever for equivalence checking

[msooseth]

For a trivial contract:

contract A {
    bytes public data;
    function stuff(uint a, uint b) public pure returns (uint) {
       unchecked {
         uint c = a + b/2;
         return c;
       }
    }
}

we loop forever when we want to check equivalence:

$EXPSOLC --bin-runtime --via-ir --optimize --ssa-cfg-codegen $FILE | tail -n1 > with.s
$EXPSOLC --bin-runtime --via-ir --optimize $FILE | tail -n1 > without.s
cabal run exe:hevm -- equivalence --code-a-file with.s --code-b-file without.s

The two contracts are slightly different, so they don't compare byte-for-byte, and the getter for bytes public data loops forever for copying.

To get to the reason, let's compile a much simpler code:

contract A {
    bytes public data;
}

Which , when compiled with: $EXPSOLC --bin-runtime --via-ir --ir --ir-optimized $FILE we get:

            function abi_encode_t_bytes_storage_to_t_bytes_memory_ptr(value, pos) -> ret {
                let slotValue := sload(value)
                let length := extract_byte_array_length(slotValue)
                pos := array_storeLengthForEncoding_t_bytes_memory_ptr(pos, length)
                switch and(slotValue, 1)
                case 0 {
                    // short byte array
                    mstore(pos, and(slotValue, not(0xff)))
                    ret := add(pos, mul(0x20, iszero(iszero(length))))
                }
                case 1 {
                    // long byte array
                    let dataPos := array_dataslot_t_bytes_storage(value)
                    let i := 0
                    for { } lt(i, length) { i := add(i, 0x20) } {
                        mstore(add(pos, i), sload(dataPos))
                        dataPos := add(dataPos, 1)
                    }
                    ret := add(pos, i)
                }
            }

In the yul, which copies the data when data() is called. The actual bytecode is:

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

which decompiles to these interesting parts:

[106]  JUMPDEST  
[107]  DUP4  
[108]  DUP3  
[109]  LT  
[10a]  PUSH2  011b
[10d]  JUMPI

[11b]  JUMPDEST  
[11c]  DUP1  
[11d]  SLOAD  
[11e]  DUP3  
[11f]  DUP7  
[120]  ADD  
[121]  PUSH1  20
[123]  ADD  
[124]  MSTORE  
[125]  PUSH1  01
[127]  ADD  
[128]  SWAP1  
[129]  PUSH1  20
[12b]  ADD  
[12c]  SWAP1  
[12d]  PUSH2  0106
[130]  JUMP

where the LT checks the loop condition, and the SLOAD ... MSTORE does the copy.