Storing lifted and unlifted pointer types in the same Array#

[Simon Peyton Jones]

. I'd like to exploit this to put unlifted and lifted pointers into the same array

I think this’ll be ok provided both are pointers. You cannot put an Int# in place of a pointer (whether the latter is lifted or unlifted) or you’ll crash the GC.

So your type for coerceToUnlifted looks too general to me.

But I have not devoted enough time to be certain.

Hope this helps a little

Simon


From: Libraries [mailto:libraries-bounces at haskell.org] On Behalf Of Reiner Pope
Sent: 17 November 2017 05:38
To: libraries at haskell.org
Subject: Storing lifted and unlifted pointer types in the same Array#

Hi libraries@,

In GHC.Prim, it is documented that Array# and ArrayArray# have the same runtime representation. I'd like to exploit this to put unlifted and lifted pointers into the same array (for example, unlifted pointers at even indices and lifted pointers at odd indices). The reason I want to do this is to save some array header words, and also to improve the cache locality of my data, so that a read of a (lifted, unlifted) pointer pair only needs to touch a single cache line rather than two.

I believe it is safe to do this (put unlifted and lifted pointers in the same array), as long as I always read at the same liftedness and type that I ran the write at for that index. Can you confirm?

Specifically, I'm expecting to use a code pattern like the following, to read an unlifted value from an Array#, by coercing the Array# to ArrayArray#, then doing the unlifted read there:

coerceToArrayArray :: Array# a -> ArrayArray#
coerceToArrayArray = unsafeCoerce#

coerceToUnlifted :: forall (b :: TYPE 'UnliftedRep). ArrayArray# -> b
coerceToUnlifted = unsafeCoerce#

indexUnliftedFromArray ::
  forall a (b :: TYPE 'UnliftedRep). Array# a -> Int# -> b
indexUnliftedFromArray arr i =
  coerceToUnlifted (indexArrayArrayArray# (coerceToArrayArray arr) i)

Similarly, I'm expecting to do writes by coercing the array to a MutableArrayArray# and doing the writes to that.

Reiner
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