Providing an empty string to --yul-optimizations without enabling yul optimization triggers uncaught exception

[danceratopz]

Description

This is a edge case in command-line arg parsing related to assembly optimization flags: Providing the empty string or a string containing only whitespace to --yul-optimizations triggers an uncaught exception.

Environment

• Compiler version: 0.8.23 - 0.8.26-develop.2024.3.15+commit.33e33241.mod.Linux.g++
• Target EVM version (as per compiler settings): Default.
• Framework/IDE (e.g. Truffle or Remix): None, triggered on the command-line
• EVM execution environment / backend / blockchain client: None.
• Operating system: Linux

Not present in 0.8.22, most likely due to the change in command-line parsing introduced in #14657.

Steps to Reproduce

Executing either (example Yul provided below):

solc --strict-assembly --yul-optimizations " " simple.yul

or

solc --strict-assembly --yul-optimizations "" simple.yul

Triggers the exception:

Uncaught exception:
/solidity/libyul/YulStack.cpp(179): Throw in function solidity::yul::YulStack::optimize(solidity::yul::Object&, bool)::<lambda()>
Dynamic exception type: boost::wrapexcept<solidity::yul::YulAssertion>
std::exception::what: Yul assertion failed
[solidity::util::tag_comment*] = Yul assertion failed

Contrarily:

solc --strict-assembly --yul-optimizations ":" simple.yul

outputs unoptimized bytecode, as expected:

======= simple.yul (EVM) =======

Pretty printed source:
object "object" {
    code {
        {
            sstore(0, f(1, 2))
            return(0, 32)
        }
        function f(a, b) -> c
        { c := add(a, b) }
    }
}


Binary representation:
6009600260016010565b5f5560205ff35b019056

Text representation:
    /* "simple.yul":145:152   */
  tag_2
    /* "simple.yul":150:151   */
  0x02
    /* "simple.yul":147:148   */
  0x01
    /* "simple.yul":145:152   */
  tag_1
  jump	// in
tag_2:
    /* "simple.yul":142:143   */
  0x00
    /* "simple.yul":135:153   */
  sstore
    /* "simple.yul":182:184   */
  0x20
    /* "simple.yul":179:180   */
  0x00
    /* "simple.yul":172:185   */
  return
    /* "simple.yul":35:115   */
tag_1:
    /* "simple.yul":86:95   */
  add
    /* "simple.yul":35:115   */
  swap1
  jump	// out

If the --optimize flag is added, the behavior is also as expected:

solc --strict-assembly --optimize --yul-optimizations "" simple.yul

which outputs the optimized bytecode.

Here's the contents of simple.yul used above, but this issue should be triggered regardless of input:

{
    function f(a, b) -> c {
        c := add(a, b)
    }

    sstore(0, f(1, 2))
    return(0, 32)
}

[nikola-matic]

Thanks for the report @danceratopz, this one is legit. Technically, we should have been more explicit as to what an empty optimizer sequence is (especially when the Yul optimizer is disabled) - i.e. ":" or just :, since this will set both the optimizer and the clean up sequence to empty.

In any case, this should fail in the CLI/standard JSON portion with a sensible error message, and not cause an ICE in the YulStack. Thankfully, it should be a trivial fix, and available in the next release.

Thanks again!