Min closure payload size?

[Gabor Greif]

Just guessing here, maybe this thunk type lives in (read-only?) static
sections, and as such it will never be overwritten with forwarding
pointers?

    Gabor

On 2/5/19, Ömer Sinan Ağacan <omeragacan at gmail.com> wrote:
> I just came across a closure that is according to this code is not valid:
>
>     >>> print *get_itbl(0x7b2870)
>     $8 = {
>       layout = {
>         payload = {
>           ptrs = 0,
>           nptrs = 0
>         },
>         bitmap = 0,
>         large_bitmap_offset = 0,
>         __pad_large_bitmap_offset = 0,
>         selector_offset = 0
>       },
>       type = 21,
>       srt = 3856568,
>       code = 0x404ef0 <r1Al_info>
> "H\215E\360L9\370rDH\203\354\bL\211\350H\211\336H\211\307\061\300\350|\034\062"
>     }
>
> This is a THUNK_STATIC with 0 ptrs and nptrs in the payload.
>
> Ömer
>
> Ömer Sinan Ağacan <omeragacan at gmail.com>, 4 Şub 2019 Pzt, 16:23
> tarihinde şunu yazdı:
>>
>> Hi,
>>
>> I was trying to understand why some info tables that have no ptrs and
>> nptrs like
>> GCD_CAF end up with 1 nptrs in the generated info table and found this
>> code in
>> Constants.h:
>>
>>     /*
>> -----------------------------------------------------------------------------
>>        Minimum closure sizes
>>
>>        This is the minimum number of words in the payload of a
>>        heap-allocated closure, so that the closure has enough room to be
>>        overwritten with a forwarding pointer during garbage collection.
>>
>> --------------------------------------------------------------------------
>> */
>>
>>     #define MIN_PAYLOAD_SIZE 1
>>
>> We use this in a few places in the compiler and add at least one word
>> space in
>> the payload. However the comment is actually wrong, forwarding pointers
>> are made
>> by tagging the info ptr field so we don't need a word in the payload for
>> forwarding pointers. I tried updating this as 0 but that caused a lot of
>> test
>> failures (mostly in GHCi). I'm wondering if I'm missing anything or is it
>> just
>> some code assuming min payload size 1 without using this macro.
>>
>> Any ideas?
>>
>> Ömer
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