Min closure payload size?

Ömer Sinan Ağacan omeragacan at gmail.com
Tue Sep 10 12:14:31 UTC 2019

It took 7 months to figure this one out. The compacting GC (which is enabled by
default when heap residency is above 30%) needs two bits per closure: one for
the actual "liveness" mark bit, another one for whether the object's new
location (computed during two scans in compacting GC) is in the next block in
the heap chain, instead of the current block (which the "free" pointer points

This bit is important becuase in the second pass we get info pointer of an
object and update any references to it (so that they point to its new location)
in one pass (the "unthreading" business), and we can't compute the size of the
object being moved (to see whether we can move the object to the current free
location or we have to move to the next block in the heap) before we get the
info table. So if we want to avoid this second mark bit we'd need to do one pass
to get the info table, compute the size, see if it fits in the current block
etc. and then do another pass to actually "unthread" (i.e. update references to
the object with the new location).

However I was able to lift this restriction by just bumping the bitmap size, to
allocate two bits per word in the heap (instead of one). It seems to work fine
(passes the test suite), I'll test this a little bit more and then submit a MR.
This reduces closure sizes of objects with no payload, e.g.:

    {-# LANGUAGE MagicHash #-}

    import GHC.Int
    import GHC.Prim

    main = print (I# (closureSize# True))

This now prints 1 instead of 2 as before.


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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