[fpc-devel] Case code pattern

[Martok]

Am 14.08.2018 um 11:27 schrieb Marco Borsari via fpc-devel:
>  From what I can read from Wikipedia, every compound of the case is 
> enclosed in
> a procedure (or in a function, as you said the table is typed), declared 
> with
> nostackframe, and called by an array of index, right?

The blocks are just regular labels. What I meant was the jump array index is considered typed, which allows some optimizations.
This is noteworthy because FPC is the only compiler I could find that does that.


Roughly:
----------------------------------------------------------------------------------------
case x of
  0: code;
  1: Morecode;
  2: EvenMoreCode;
  //... assume we have enough case labels to get a jumptable
end;
----------------------------------------------------------------------------------------

Gets translated as if one had written:
----------------------------------------------------------------------------------------
label label0,label1,label2,{...,}afterend;
const table: array [lowestcaselabel..highestcaselabel] of CodePointer = (@label0, @label1, @label2{,...});
if (x<lowestcaselabel) or (x>highestcaselabel) then
  goto @afterend;
goto table[x];
label0:
  code;
  goto afterend;
label1:
  Morecode;
  goto afterend;
label2:
  EvenMoreCode;
{...}
afterend:
----------------------------------------------------------------------------------------

Iff it follows from the strong typing of the array index that the "if" at the start is always false for all defined values of
the type of x, it is omitted.

-- 
Regards,
Martok

Ceterum censeo b32079 esse sanandam.