refactor: centralize memory layout constants in helpers.py

Co-Authored-By: Claude claude-opus-4-6

Author: pdobacz

tests/monad_eight/staking_precompile/helpers.py

@@ -3,6 +3,16 @@
 from execution_testing import Bytecode, Op
 from execution_testing.forks.helpers import Fork
 
+# Memory layout for precompile call tests.
+# MSTORE writes a 32-byte word; the 4-byte selector sits at the
+# rightmost bytes of that word.
+WRONG_SEL_MSTORE_OFFSET = 0
+CORRECT_SEL_MSTORE_OFFSET = 32
+
+# Where the 4-byte selectors actually start in memory:
+WRONG_SEL_ARGS_OFFSET = WRONG_SEL_MSTORE_OFFSET + 28
+CORRECT_SEL_ARGS_OFFSET = CORRECT_SEL_MSTORE_OFFSET + 28
+
 
 def generous_gas(fork: Fork) -> int:
     """Return generous parametrized gas to always be enough."""
@@ -27,13 +37,32 @@ def tx_calldata(selector: int, calldata_size: int) -> bytes:
     return sel_bytes + b"\x00" * max(0, calldata_size - 4)
 
 
+def calldata_mem_end(calldata_size: int) -> int:
+    """
+    Return safe first memory offset past all written regions.
+
+    Accounts for EXTRA_CALLDATA (+1 byte beyond calldata_size)
+    and both MSTORE regions.
+    """
+    return (
+        max(
+            CORRECT_SEL_ARGS_OFFSET + calldata_size,
+            CORRECT_SEL_MSTORE_OFFSET + 32,
+        )
+        + 1
+    )
+
+
 def build_calldata(selector: int, calldata_size: int) -> Bytecode:
     """
-    Build bytecode that stores a selector and padding in memory.
+    Build bytecode that stores a selector in memory.
 
-    Place the 4-byte selector at mem[60:64] and assume mem[64:..] has
-    rest of calldata
+    Place the 4-byte selector at
+    mem[CORRECT_SEL_ARGS_OFFSET:CORRECT_SEL_ARGS_OFFSET+4]
+    and leave the rest zero-initialized for calldata args.
     """
-    selector_calldata_offset = 32
-    code = Op.MSTORE(selector_calldata_offset, selector)
+    # Not used b/c input is zero beyond the selector;
+    # left here only for clarity
+    del calldata_size
+    code = Op.MSTORE(CORRECT_SEL_MSTORE_OFFSET, selector)
     return code

tests/monad_eight/staking_precompile/test_precompile_call.py

@@ -32,7 +32,15 @@
 from execution_testing.forks.helpers import Fork
 from execution_testing.test_types.receipt_types import TransactionReceipt
 
-from .helpers import build_calldata, generous_gas, tx_calldata
+from .helpers import (
+    CORRECT_SEL_ARGS_OFFSET,
+    WRONG_SEL_ARGS_OFFSET,
+    WRONG_SEL_MSTORE_OFFSET,
+    build_calldata,
+    calldata_mem_end,
+    generous_gas,
+    tx_calldata,
+)
 from .spec import (
     ALL_FUNCTIONS,
     ERROR_INPUT_TOO_SHORT,
@@ -61,11 +69,6 @@
 slot_all_gas_consumed = 0x6
 
 
-def _calldata_mem_end(calldata_size: int) -> int:
-    """Return the first memory offset after build_calldata's writes."""
-    return 60 + calldata_size + 1
-
-
 def _mload_of(msg: bytes) -> int:
     """
     Compute the MLOAD uint256 value after a raw error message is
@@ -350,32 +353,24 @@ def scenario_call_code(
     """
     scenario_set = set(scenarios)
 
-    # Memory layout: non-overlapping buffers
-    #   MSTORE(0, 0xDEADBEEF) -> wrong selector at mem[28:32]
-    #   build_calldata(selector, size) -> selector at mem[60:64]
-    wrong_sel_args_offset = 28
-    correct_sel_args_offset = 60
-
-    # NOTE: in case of wrong selector scenario, we do not include any
-    # extra calldata for (non-existent) function arguments
     setup: Bytecode = build_calldata(
         func.selector, func.calldata_size
-    ) + Op.MSTORE(0, 0xDEADBEEF)
+    ) + Op.MSTORE(WRONG_SEL_MSTORE_OFFSET, 0xDEADBEEF)
 
     if CallScenario.WRONG_SELECTOR in scenario_set:
-        args_offset = wrong_sel_args_offset
+        args_offset = WRONG_SEL_ARGS_OFFSET
         args_size = 4
     elif CallScenario.TRUNCATED_SELECTOR in scenario_set:
-        args_offset = correct_sel_args_offset
+        args_offset = CORRECT_SEL_ARGS_OFFSET
         args_size = 3
     elif CallScenario.SHORT_CALLDATA in scenario_set:
-        args_offset = correct_sel_args_offset
+        args_offset = CORRECT_SEL_ARGS_OFFSET
         args_size = func.calldata_size - 1
     elif CallScenario.EXTRA_CALLDATA in scenario_set:
-        args_offset = correct_sel_args_offset
+        args_offset = CORRECT_SEL_ARGS_OFFSET
         args_size = func.calldata_size + 1
     else:
-        args_offset = correct_sel_args_offset
+        args_offset = CORRECT_SEL_ARGS_OFFSET
         args_size = func.calldata_size
 
     if ret_size > 0:
@@ -475,7 +470,7 @@ def test_input_size(
             Op.CALL(
                 gas=gas,
                 address=STAKING_PRECOMPILE,
-                args_offset=60,
+                args_offset=CORRECT_SEL_ARGS_OFFSET,
                 args_size=input_size,
                 ret_offset=0,
                 ret_size=32,
@@ -548,7 +543,7 @@ def test_selector(
             Op.CALL(
                 gas=gas,
                 address=STAKING_PRECOMPILE,
-                args_offset=60,
+                args_offset=CORRECT_SEL_ARGS_OFFSET,
                 args_size=args_size,
                 ret_offset=0,
                 ret_size=32,
@@ -606,7 +601,7 @@ def test_gas(
             Op.CALL(
                 gas=gas,
                 address=STAKING_PRECOMPILE,
-                args_offset=60,
+                args_offset=CORRECT_SEL_ARGS_OFFSET,
                 args_size=func.calldata_size,
                 ret_offset=0,
                 ret_size=32,
@@ -666,7 +661,7 @@ def test_call_opcodes(
             call_opcode(
                 gas=func.gas_cost + 10000,
                 address=STAKING_PRECOMPILE,
-                args_offset=60,
+                args_offset=CORRECT_SEL_ARGS_OFFSET,
                 args_size=func.calldata_size,
                 ret_offset=0,
                 ret_size=32,
@@ -718,8 +713,7 @@ def test_revert_returns(
     """
     Test return data on success and on each revert reason.
     """
-    mem_end = _calldata_mem_end(func.calldata_size)
-    ret_offset = max(mem_end, 96)
+    ret_offset = calldata_mem_end(func.calldata_size)
     rdc_offset = ret_offset + 32
 
     delegating_eoa: Address | None = None
@@ -875,8 +869,7 @@ def test_call_with_value(
     Payable functions accept value; non-payable functions revert with
     "value is nonzero".
     """
-    mem_end = _calldata_mem_end(func.calldata_size)
-    ret_offset = max(mem_end, 96)
+    ret_offset = calldata_mem_end(func.calldata_size)
     rdc_offset = ret_offset + 32
 
     contract = (
@@ -887,7 +880,7 @@ def test_call_with_value(
                 gas=func.gas_cost + 10000,
                 address=STAKING_PRECOMPILE,
                 value=value,
-                args_offset=60,
+                args_offset=CORRECT_SEL_ARGS_OFFSET,
                 args_size=func.calldata_size,
                 ret_offset=ret_offset,
                 ret_size=32,
@@ -969,8 +962,7 @@ def test_check_order(
     Each combination triggers exactly two failure causes. The test
     derives the expected outcome from the higher-priority failure.
     """
-    mem_end = _calldata_mem_end(func.calldata_size)
-    ret_offset = max(mem_end, 96)
+    ret_offset = calldata_mem_end(func.calldata_size)
     rdc_offset = ret_offset + 32
 
     delegating_eoa: Address | None = None
@@ -1083,8 +1075,7 @@ def test_check_order_triple(
     if not _pairwise_compatible(tuple(normalized)):
         pytest.skip("normalized scenarios are incompatible")
 
-    mem_end = _calldata_mem_end(func.calldata_size)
-    ret_offset = max(mem_end, 96)
+    ret_offset = calldata_mem_end(func.calldata_size)
     rdc_offset = ret_offset + 32
 
     scenarios = {scenario1, scenario2, scenario3}
@@ -1439,7 +1430,7 @@ def test_syscall_rejected(
             Op.CALL(
                 gas=100000,
                 address=STAKING_PRECOMPILE,
-                args_offset=60,
+                args_offset=CORRECT_SEL_ARGS_OFFSET,
                 args_size=36,
                 ret_offset=0,
                 ret_size=32,