Optimizing dynamic dispatch

opt_dispatch.nim

if a of Z:
  echo "Z"
elif a of Y:
  echo "Y"
elif a of X:
  echo "X"
  
'of' is expensive and will become MORE expensive for proper DLL support
(string comparisons!). Thus we reserve a slot for the exact matched object
type and short-cut the code:


if getClass(a) == slot:
  goto jmpTarget
  
if a of Z:
  jmpTarget = thisBranch
  echo "Z"
elif a of Y:
  jmpTarget = thisBranch
  echo "Y"
elif a of X:
  jmpTarget = thisBranch
  echo "X"
slot = getClass(a) # not valid because of exceptions
--> move to 'jmpTarget=' :-(


--> without computed gotos:

if getClass(a) == slot:
  case jmpTarget
  of 0: L0:
    echo "Z"
  
  of 1: L1:
    echo "Y"
  
  of 2: L2:
    echo "X"
  return

slot = getClass(a)
if a of Z:
  jmpTarget = 0
  goto L0
elif a of Y:
  jmpTarget = 1
  goto L1
elif a of X:
  jmpTarget = 2
  goto L2
  
Note: It is NOT possible to make 'slot' part of the object type descriptor!
'slot' depends on the concrete dispatcher and there can be more than 1 method
for an object type. ;-)

However we can do:

FastDispatch:
  case jmpTarget
  of 0:
    if getClass(a) == slot:
    L0:
      echo "Z"
      
    else:
      goto SlowDispatch
  of 1: L1:
    echo "Y"
  
  of 2: L2:
    echo "X"
  return

SlowDispatch:
  slot = getClass(a)
  if a of Z:
    jmpTarget = 0
    goto L0
  elif a of Y:
    jmpTarget = 1
    goto L1
  elif a of X:
    jmpTarget = 2
    goto L2

--> First version is simpler and produces smaller code.



--> Generalization to a multi method:

if getClass(a) == slotA and getClass(b) == slotB:
  case jmpTarget
  of 0: L0:
    echo "Z"
  
  of 1: L1:
    echo "Y"
  
  of 2: L2:
    echo "X"
  return

slotA = getClass(a)
slotB = getClass(b)
if a of Z and b of Z:
  jmpTarget = 0
  goto L0
elif a of Y and b of X:
  jmpTarget = 1
  goto L1
elif a of X and b of Y:
  jmpTarget = 2
  goto L2