Experimental EIP-7979 subroutines for internal calls in the IR pipeline.

[gcolvin]

Experimental EIP-7979 subroutines for internal calls in the IR pipeline

Draft. A demonstration for the EIP-7979 discussion, not a merge candidate.

EIP-7979 (proposed for Hegotá) gives the EVM a return stack and three instructions: CALLSUB, CALLDEST, RETURNSUB. This PR makes the optimized IR code transform use them for internal function calls when --evm-version @future is selected, so that the question "what does it cost a compiler to target this?" has a concrete answer: 159 lines, 21 files.

solc --experimental --evm-version @future --via-ir --optimize --asm Calls.sol

What changes in the generated code

	today	with @future
call	PUSH ret; <args>; PUSH f; JUMP / ret: JUMPDEST	<args>; PUSH f; CALLSUB
function entry	f: JUMPDEST with the return label on the stack	f: CALLDEST, arguments only
function return	shuffle return label to top; JUMP	RETURNSUB
call to a function that never returns	PUSH f; JUMP	unchanged: a JUMP may land on a CALLDEST

The return-label slot disappears from every function's stack layout, which also relieves stack-too-deep pressure by one slot per active call.

Results on a small contract

Calls.sol in the description below has a plain internal call, a nested one, and a recursive one. Runtime code under @future: 3 functions as CALLDEST, 7 CALLSUB, 4 RETURNSUB, and no plain JUMP at all; the only remaining control flow is JUMPI.

Executed on the EIP-7979 reference implementation in execution-specs (ethereum/execution-specs#3575) with three inputs, the results match the legacy build and the gas per external call is lower:

compute(a, b)	legacy (osaka)	@future
(0, 0)	3530	3480
(3, 4)	4034	3915
(10, 5)	4202	4060

The runtime is 31 bytes larger (317 vs 286): the evmasm inliner recognises JUMP-based calls but not CALLSUB, so fewer small helpers are inlined. That is a follow-up in the inliner, not a cost of the instructions.

The output also validates under EIP-8337 (fully static control flow, no underflow), checked with its reference validator (ethereum/execution-specs#3576). Getting there surfaced one general rule for optimizers targeting these EIPs, implemented here: code shared across subroutines must be entered as a subroutine. The block deduplicator had merged the identical overflow-panic tails of checked_add and checked_mul into one JUMPDEST block reached from two subroutines; with subroutines enabled it now marks every block it merges as a CALLDEST.

Changes

• libevmasm: the three instructions with gas tiers; RETURNSUB terminates control flow, CALLSUB alters it (so the peephole optimizer and CSE treat them correctly); a Tag item can be marked as a subroutine entry — assembled as CALLDEST, printed as tag_N: (calldest), exported and imported as calldest in assembly JSON; CALLDEST costs what JUMPDEST costs; the block deduplicator never merges a CALLDEST block with a JUMPDEST block (a CALLSUB must never be redirected onto a JUMPDEST) and marks merged blocks as entries.
• liblangutil: EVMVersion::hasSubroutines(), true for @future only; the opcode gate knows the three instructions.
• libyul: appendSubroutineLabel, appendCallSubTo, appendReturnSub on AbstractAssembly, implemented for EthAssemblyAdapter and NoOutputAssembly; ControlFlowGraphBuilder stops pushing a return-label slot for calls; StackLayoutGenerator stops expecting one at returns; OptimizedEVMCodeTransform emits the new instructions.
• libevmasm, post-pass: SubroutineEntryMarker runs at the end of optimisation and marks any block reached from more than one subroutine entry as a CALLDEST. On 1,756 semantic-test contracts, with EIP-8337's proposed framing rule, 98.9% of the optimized output validates.

Deliberately out of scope

Legacy (non-IR) code generation; the unoptimized Yul code transform; teaching the evmasm inliner about CALLSUB; documentation. The change is gated behind an experimental EVM version and does not affect any other version's output.

The demo contract

contract Calls {
    function square(uint256 x) internal pure returns (uint256) { return x * x; }
    function sumOfSquares(uint256 a, uint256 b) internal pure returns (uint256) {
        return square(a) + square(b);
    }
    function factorial(uint256 n) internal pure returns (uint256) {
        if (n < 2) return 1;
        return n * factorial(n - 1);
    }
    function compute(uint256 a, uint256 b) external pure returns (uint256) {
        return sumOfSquares(a, b) + factorial(b);
    }
}

CI status

The semantic-test jobs for @future fail because they execute compiled code on evmone, which does not implement EIP-7979: every test whose IR-pipeline code makes an internal call halts at its first CALLSUB ("the test passed without Yul"). The same tests pass when the bytecode is run on the EIP-7979 reference implementation in execution-specs (checked for selfdestruct_post_cancun.sol, the full test_create2_and_terminate scenario). All other jobs pass. An evmone implementation is at ipsilon/evmone#1706; on it, the same output runs correctly at 10–12% less execution gas.