EVM: Eliminate recursion entirely

This patch eliminates recursion entirely from the EVM when ENABLE_EVMC=0.

Signed-off-by: Jamie Lokier <[email protected]>

nimbus/vm/computation.nim

@@ -309,36 +309,28 @@ template chainTo*(c, toChild: Computation, after: untyped) =
  c.continuation = proc() =
  after
 
-proc execCallOrCreateAux(c: var Computation) {.noinline.} =
- # Perform recursion with minimum-size stack per level. The exception
- # handling is very subtle. Each call to `snapshot` must have a corresponding
- # `dispose` on exception. To minimise this proc's stackframe, `defer` is
- # moved to the outermost proc only. `{.noinline.}` is also used to make
- # extra sure they stay separate. `c` is a `var` parameter at every level of
- # recursion, so the outermost proc sees every change to `c`, which is why `c`
- # is updated instead of using `let`. On exception, the outermost `defer`
- # walks the `c.parent` chain to call `dispose` on each `c`.
- if c.beforeExec():
- return
- c.executeOpcodes()
- while not c.continuation.isNil:
- # Parent and child refs are updated and cleared so as to avoid circular
- # refs (like a double-linked list) or dangling refs (to finished child).
- (c.child, c, c.parent) = (nil.Computation, c.child, c)
- execCallOrCreateAux(c)
- c.dispose()
- (c.parent, c) = (nil.Computation, c.parent)
- (c.continuation)()
- c.executeOpcodes()
- c.afterExec()
-
 proc execCallOrCreate*(cParam: Computation) =
- var c = cParam
+ var (c, before) = (cParam, true)
  defer:
  while not c.isNil:
  c.dispose()
  c = c.parent
- execCallOrCreateAux(c)
+
+ # No actual recursion, but simulate recursion including before/after/dispose.
+ while true:
+ while true:
+ if before and c.beforeExec():
+ break
+ c.executeOpcodes()
+ if c.continuation.isNil:
+ c.afterExec()
+ break
+ (before, c.child, c, c.parent) = (true, nil.Computation, c.child, c)
+ if c.parent.isNil:
+ break
+ c.dispose()
+ (before, c.parent, c) = (false, nil.Computation, c.parent)
+ (c.continuation)()
 
 proc merge*(c, child: Computation) =
  c.logEntries.add child.logEntries