[Python-Dev] Switch statement

Mon Jun 19 16:29:49 CEST 2006

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.