Min closure payload size?

[Simon Peyton Jones]

I'm relying on Simon M here.  I'm out of my depth!

Simon

| -----Original Message-----
| From: ghc-devs <ghc-devs-bounces at haskell.org> On Behalf Of Ömer Sinan
| Agacan
| Sent: 05 February 2019 13:38
| To: ghc-devs <ghc-devs at haskell.org>
| Subject: Re: Min closure payload size?
| 
| 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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