Re: Min closure payload size?

https://gitlab.haskell.org/ghc/ghc/merge_requests/1701 Ömer Ömer Sinan Ağacan , 10 Eyl 2019 Sal, 15:14 tarihinde şunu yazdı:

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 to).

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

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