EVM codegen in solc optimizations prevents contract builds

[xermicus]

Even though we request the optimized Yul IR (and no EVM bytecode) from solc, due to some optimization passes, EVM codegen is still run. This can lead to errors like Could not create stack layout after 1000 iterations. when compiling contracts. But PVM is in no way connected to the EVM stack, and Yul has no stack layout.

Setting yulDetails.stackAllocation: false does not help in all cases, found a reproducer that just OOMs (my 64gb machine) in that case.

solc is notorious for such bugs. Because even --optimize --no-optimize-yul forcefully enables stack allocation, we have no other choice left than to request the unoptimized Yul. This should be fine; any remaining solc optimizer gains which are not covered yet by either the NY opt or LLVM passes can just be re-implemented in NY.

[0xOmarA]

Thanks @xermicus for opening this issue.

We discovered this when trying to compile the contracts here and more specifically the OuterCount5.sol contract in ageVerifier/external/zkpassport-circuits/src/solidity/src/ultra-honk-verifiers.

Solc fails to generate Yul for that particular contract, failing with the following error when ran:

> solc --via-ir --optimize --optimize-runs 200 --ir-optimized OuterCount5.sol
Error: Yul assertion failed:
/solidity/libyul/backends/evm/StackHelpers.h(122): Throw in function static void solidity::yul::Shuffler<ShuffleOperations>::shuffle(Args &&...) [ShuffleOperations = ShuffleOperations, Args = <std::vector<std::variant<PreviousSlot, std::variant<solidity::yul::FunctionCallReturnLabelSlot, solidity::yul::FunctionReturnLabelSlot, solidity::yul::VariableSlot, solidity::yul::LiteralSlot, solidity::yul::TemporarySlot, solidity::yul::JunkSlot>>> &, const std::vector<std::variant<solidity::yul::FunctionCallReturnLabelSlot, solidity::yul::FunctionReturnLabelSlot, solidity::yul::VariableSlot, solidity::yul::LiteralSlot, solidity::yul::TemporarySlot, solidity::yul::JunkSlot>> &, (lambda at /Users/distiller/project/libyul/backends/evm/StackLayoutGenerator.cpp:303:30) &, unsigned long &>]
Dynamic exception type: boost::wrapexcept<solidity::yul::YulAssertion>
std::exception::what: Could not create stack layout after 1000 iterations.
[solidity::util::tag_comment*] = Could not create stack layout after 1000 iterations.

Since Yul generation fails for this contract then it means that we can't compile it to PolkaVM.

Steps to reproduce this are as follows:

git clone --recurse-submodules https://github.com/paritytech/ageVerifier.git
cd ageVerifier/external/zkpassport-circuits/src/solidity/src/ultra-honk-verifiers
resolc OuterCount5.sol --solc "$(which solc)" -Oz --metadata-hash none --bin

Using standard JSON input with solc to prevent stack allocation through optimizer.details.yulDetails.stackAllocation: false circumvents the above error, but leads to an OOM error.

[xermicus]

Thanks a lot @0xOmarA for bringing it up and digging all the details out! We will see what we can do about it in the follow weeks.