Re: Min closure payload size?

6 Feb 2019


      I don't think so, for two reasons:

- We update static thunks just fine so I don't think they're in a read-only
  section.

- Forwarding pointers are needed when moving objects, and we don't move static
  objects, so we don't need to make them forwarding pointers (I think you
  confused forwarding pointers with indirections generated by thunk updates?).

Ömer

Gabor Greif , 6 Şub 2019 Çar, 03:47 tarihinde şunu yazdı:
...
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  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 
"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 , 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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