How to find out GHC's memory representations of data types?
ghc, haskell
Solution
My first idea was to use this neat litte function, due to Simon Marlow:
{-# LANGUAGE MagicHash,UnboxedTuples #-}
module Size where
import GHC.Exts
import Foreign
unsafeSizeof :: a -> Int
unsafeSizeof a =
case unpackClosure# a of
(# x, ptrs, nptrs #) ->
sizeOf (undefined::Int) + -- one word for the header
I# (sizeofByteArray# (unsafeCoerce# ptrs)
+# sizeofByteArray# nptrs)
Using it:
Prelude> :!ghc -c Size.hs
Size.hs:15:18:
Warning: Ignoring unusable UNPACK pragma on the
third argument of `BitVec257'
In the definition of data constructor `BitVec257'
In the data type declaration for `BitVec257'
Prelude Size> unsafeSizeof $! BitVec514 (BitVec257 1 2 True 3 4) (BitVec257 1 2 True 3 4)
74
(Note that GHC is telling you that it cannot unbox `Bool` since it's a sum type.)
The above function claims that your data type uses 74 bytes on a 64-bit machine. I find that hard to believe. I'd expect the data type to use 11 words = 88 bytes, one word per field. Even `Bool`s take one word, as they are pointer to (statically allocated) constructors. I'm not quite sure what's going on here.
As for alignment I believe every field should be word aligned.
Problem
Recently, blog entries such as Computing the Size of a Hashmap explained how to reason about space complexities of commonly used container types. Now I'm facing the question of how to actually "see" which memory layout my GHC version chooses (depending on compile flags and target architecture) for weird data types (constructors) such as ``` data BitVec257 = BitVec257 {-# UNPACK #-} !Word64 {-# UNPACK #-} !Word64 {-# UNPACK #-} !Bool {-# UNPACK #-} !Word64 {-# UNPACK #-} !Word64 data BitVec514 = BitVec514 {-# UNPACK #-} !BitVec257 {-# UNPACK #-} !BitVec257 ``` In C there's the `sizeof` and `offsetof` operator, which allows me to "see" what size and alignment was chosen for the fields of C `struct`. I've tried to look at GHC Core in the hope to find some hint there, but I didn't know what to look for. Can somebody point me in the right direction?