[Python-Dev] Switch statement

[Phillip J. Eby]

At 12:10 AM 6/20/2006 +1000, Nick Coghlan wrote:
>Caching on first use would be the easiest to explain I think. Something like:
>
>      if jump_dict is NULL:
>          jump_dict = {FIRST_CASE  : JUMP_TARGET_1,
>                       SECOND_CASE : JUMP_TARGET_2,
>                       THIRD_CASE  : JUMP_TARGET_3}
>      jump_to_case(value, jump_dict)
>      ELSE_CLAUSE
>      jump_to_end()

Sadly, it's not *quite* that simple, due to the fact that co_lnotab must be 
increase in line numbers as bytecode offsets increase.  It would actually 
look more like:

      LOAD_DEREF jumpdictN
      JUMP_IF_FALSE  initfirstcase

do_switch:
      ...

initfirstcase:
      DUP_TOP
      # compute case value
      LOAD_CONST firstcaseoffset
      ROT_THREE
      STORE_SUBSCR
      JUMP_FORWARD initsecondcase

firstcaseoffset:
      first case goes here
      ...

initsecondcase:
      DUP_TOP
      # compute case value
      LOAD_CONST secondcaseoffset
      ROT_THREE
      STORE_SUBSCR
      JUMP_FORWARD initthirdcase

secondcaseoffset:
      second case goes here
      ...

...

initlastcase:
      DUP_TOP
      # compute case value
      LOAD_CONST lastcaseoffset
      ROT_THREE
      STORE_SUBSCR
      JUMP_ABSOLUTE doswitch

lastcaseoffset:
      last case goes here



The above shenanigans are necessary because the line numbers of the code 
for computing the case expressions have to be interleaved with the line 
numbers for the code for the case suites.

Of course, we could always change how co_lnotab works, which might be a 
good idea anyway.  As our compilation techniques become more sophisticated, 
it starts to get less and less likely that we will always want bytecode and 
source code to be in exactly the same sequence within a given code object.