Confused about PAP object layout

[Ömer Sinan Ağacan]

Right, I think that's the problem. We then pass the same "size" to
scavenge_large_bitmap as the size of the bitmap. So we assume size of the bitmap
is pap->n_args.

So the call stack is

- scavenge_PAP, calls scavenge_PAP_payload with pap->n_args as "size"
- scavenge_PAP_payload, calls scavenge_large_bitmap with "size" (== pap->n_args)
  as the bitmap's size

Is this expected?

Ömer

Simon Marlow <marlowsd at gmail.com>, 14 Şub 2020 Cum, 18:08 tarihinde şunu yazdı:
>
> On Fri, 14 Feb 2020 at 11:49, Ömer Sinan Ağacan <omeragacan at gmail.com> wrote:
>>
>> Hi Simon,
>>
>> In this code: (slightly simplified)
>>
>>     StgPtr
>>     scavenge_PAP (StgPAP *pap)
>>     {
>>         evacuate(&pap->fun);
>>         return scavenge_PAP_payload (pap->fun, pap->payload, pap->n_args);
>>     }
>>
>>     StgPtr
>>     scavenge_PAP_payload (StgClosure *fun, StgClosure **payload, StgWord size)
>>     {
>>         const StgFunInfoTable *fun_info =
>> get_fun_itbl(UNTAG_CONST_CLOSURE(fun));
>>         StgPtr p = (StgPtr)payload;
>>
>>         switch (fun_info->f.fun_type) {
>>         case ARG_GEN_BIG:
>>             scavenge_large_bitmap(p, GET_FUN_LARGE_BITMAP(fun_info), size);
>>             p += size;
>>             break;
>>         ...
>>         }
>>         return p;
>>     }
>>
>> Here the `size` argument in `scavenge_PAP_payload` is the number of arguments
>> applied to the function in `pap->fun`. But when scavenging the function's bitmap
>> we're using this number as the size of the bitmap which doesn't make sense to
>> me, because I think size of the function's bitmap and size of the PAP's payload
>> may be different.
>
>
> "size" is an argument to scavenge_PAP_payload(), and when we call it we pass pap->n_args as the value, not the bitmap's size.
>
> Does that help?
>
> Cheers
> Simon
>
>
>>
>>
>> Or in other words I may have the same function used in many PAPs with different
>> n_args, but that'd be buggy if this code is correct.
>>
>> I haven't checked every single place where we build a PAP but for example the
>> `NEW_PAP` macro uses the argument's (another PAP) function directly, without
>> making any bitmap-related changes, but bumps n_args by one. If the code above is
>> right, then this new PAP will be scavenged incorrectly.
>>
>> Am I missing anything?
>>
>> Thanks,
>>
>> Ömer