tail call recursion with continuation passing

nimbus/vm/computation.nim

@@ -34,6 +34,9 @@ proc newBaseComputation*(vmState: BaseVMState, blockNumber: UInt256, message: Me
     else:
       blockNumber.toFork.forkToSchedule
   result.forkOverride = forkOverride
+  # a dummy/terminus continuation proc
+  result.nextProc = proc() =
+    discard
 
 proc isOriginComputation*(c: BaseComputation): bool =
   # Is this computation the computation initiated by a transaction
@@ -168,6 +171,8 @@ proc transferBalance(computation: BaseComputation, opCode: static[Op]) =
       db.addBalance(computation.msg.storageAddress, computation.msg.value)
 
 template continuation*(comp: BaseComputation, body: untyped) =
+  # this is a helper template to implement continuation
+  # passing and convert all recursion into tail call
   var tmpNext = comp.nextProc
   comp.nextProc = proc() =
     body
@@ -189,20 +194,26 @@ proc executeOpcodes*(computation: BaseComputation) {.gcsafe.}
 
 proc applyMessage*(computation: BaseComputation, opCode: static[Op]) =
   computation.snapshot()
-  defer: computation.dispose()
+  defer:
+    computation.dispose()
 
   when opCode in {CallCode, Call, Create}:
     computation.transferBalance(opCode)
     if computation.isError():
       computation.rollback()
+      computation.nextProc()
       return
 
   if computation.gasMeter.gasRemaining < 0:
     computation.commit()
+    computation.nextProc()
     return
 
+  continuation(computation):
+    postExecuteVM(computation)
+
   executeOpcodes(computation)
-  postExecuteVM(computation)
+  computation.nextProc()
   
 proc addChildComputation*(computation: BaseComputation, child: BaseComputation) =
   if child.isError:

nimbus/vm/interpreter/opcodes_impl.nim

@@ -593,13 +593,15 @@ op create, inline = false, value, startPosition, size:
   var childComp = setupCreate(computation, memPos, len, value)
   if childComp.isNil: return
 
-  childComp.applyMessage(Create)
-  computation.addChildComputation(childComp)
+  continuation(childComp):
+    computation.addChildComputation(childComp)
 
-  if childComp.isError:
-    push: 0
-  else:
-    push: childComp.msg.storageAddress
+    if childComp.isError:
+      push: 0
+    else:
+      push: childComp.msg.storageAddress
+
+  childComp.applyMessage(Create)
 
 proc callParams(computation: BaseComputation): (UInt256, UInt256, EthAddress, EthAddress, EthAddress, UInt256, UInt256, UInt256, UInt256, MsgFlags) =
   let gas = computation.stack.popInt()
@@ -683,7 +685,7 @@ proc staticCallParams(computation: BaseComputation): (UInt256, UInt256, EthAddre
     emvcStatic) # is_static
 
 template genCall(callName: untyped, opCode: Op): untyped =
-  proc `callName Setup`(computation: BaseComputation, callNameStr: string): (BaseComputation, int, int) =
+  proc `callName Setup`(computation: BaseComputation, callNameStr: string): BaseComputation =
     let (gas, value, to, sender,
           codeAddress,
           memoryInputStartPosition, memoryInputSize,
@@ -750,28 +752,32 @@ template genCall(callName: untyped, opCode: Op): untyped =
       childMsg,
       some(computation.getFork))
 
-    result = (childComp, memOutPos, memOutLen)
+    computation.memOutPos = memOutPos
+    computation.memOutLen = memOutLen
+    result = childComp
 
   op callName, inline = false:
     ## CALL, 0xf1, Message-Call into an account
     ## CALLCODE, 0xf2, Message-call into this account with an alternative account's code.
     ## DELEGATECALL, 0xf4, Message-call into this account with an alternative account's code, but persisting the current values for sender and value.
     ## STATICCALL, 0xfa, Static message-call into an account.
-    var (childComp, memOutPos, memOutLen) = `callName Setup`(computation, callName.astToStr)
+    var childComp = `callName Setup`(computation, callName.astToStr)
+
+    continuation(childComp):
+      addChildComputation(computation, childComp)
+
+      if childComp.isError:
+        push: 0
+      else:
+        push: 1
+
+      if not childComp.shouldEraseReturnData:
+        let actualOutputSize = min(computation.memOutLen, childComp.output.len)
+        computation.memory.write(
+          computation.memOutPos,
+          childComp.output.toOpenArray(0, actualOutputSize - 1))
 
     childComp.applyMessage(opCode)
-    addChildComputation(computation, childComp)
-
-    if childComp.isError:
-      push: 0
-    else:
-      push: 1
-
-    if not childComp.shouldEraseReturnData:
-      let actualOutputSize = min(memOutLen, childComp.output.len)
-      computation.memory.write(
-        memOutPos,
-        childComp.output.toOpenArray(0, actualOutputSize - 1))
 
 genCall(call, Call)
 genCall(callCode, CallCode)

nimbus/vm_types.nim

@@ -67,7 +67,10 @@ type
     dbsnapshot*:            Snapshot
     instr*:                 Op
     opIndex*:               int
+    # continuation helpers
     nextProc*:              proc()
+    memOutLen*:             int
+    memOutPos*:             int
 
   Error* = ref object
     info*:                  string