SSA CFG Codegen: make function return label slot handling explicit

[clonker]

The function return label (the address the callee jumps back to) is now tracked as an explicit FunctionReturnLabel slot on the virtual stack throughout a function's CFG, rather than being treated as an implicit/invisible slot below the tracked stack.

[blishko]

Can you maybe explain in more detail what is the benefit of this explicit tracking? Or what is the problem with the current implicit approach?

I am having trouble understanding why is this even needed.

[clonker]

Can you maybe explain in more detail what is the benefit of this explicit tracking? Or what is the problem with the current implicit approach?

The implicit approach required the code transform to lie about stack heights and do manual shuffling at return sites, bypassing the layout generator. Making it explicit lets the existing layout/shuffle infrastructure handle it uniformly, which is simpler and eliminates a TODO:

solidity/libyul/backends/evm/ssa/CodeTransform.cpp

Lines 345 to 365 in 6313134

	m_assembly.setStackHeight(m_assembly.stackHeight() + 1);
	// Build target: [rv1, rv2, ..., rvN-1, rv0] (rv0 at top).
	// After shuffle: EVM = [label, rv1, ..., rvN-1, rv0].
	// After SWAP(N): EVM = [rv0, rv1, ..., rvN-1, label].
	// After JUMP: EVM = [rv0, rv1, ..., rvN-1].
	// TODO: it would be better to account for function return labels from the entry of the function to the end symbolically
	StackData returnTarget;
	if (!_functionReturn.returnValues.empty())
	{
	returnTarget.reserve(_functionReturn.returnValues.size());
	for (std::size_t i = 1; i < _functionReturn.returnValues.size(); ++i)
	returnTarget.push_back(StackSlot::makeValueID(_functionReturn.returnValues[i]));
	returnTarget.push_back(StackSlot::makeValueID(_functionReturn.returnValues.front()));
	StackShuffler<AssemblyCallbacks>::shuffle(m_stack, returnTarget, {}, returnTarget.size());
	m_assembly.appendInstruction(evmasm::swapInstruction(
	static_cast<unsigned>(_functionReturn.returnValues.size())
	));
	}
	else
	StackShuffler<AssemblyCallbacks>::shuffle(m_stack, returnTarget, {}, 0);
	m_assembly.appendJump(0, AbstractAssembly::JumpType::OutOfFunction);

Beyond that it also

• makes things more explicit, which at least I like stylistically; and
• for shuffling it can be advantageous, too: right now the return label slot is invisible and pinned beneath the function call stack. After this change it is visible to the shuffler as regular slot and can be placed intermittently elsewhere, so we gain flexibility.

[blishko]

Seems good to me! I have some questions about the changes in the tests.

[blishko]

LGTM!