Re: Min closure payload size?

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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