Inaccurate GHCStats size for 32 bit arch

[mitchellwrosen]

I see some hard-coded 8s in Weigh.GHCStats that assume a 64 bit architecture. I can think of two ways to fix this:

• Replace 8 with SIZEOF_HSWORD
• Replace the generic traversal with something like:

-- Get the size of an arbitrary closure
sizeOf :: a -> Int64                                                                                                       
sizeOf x =                                                                                                                 
  case unpackClosure# x of                                                                                                 
    (# _, xs, ys #) ->                                                                                                    
      case SIZEOF_HSWORD of                                                                                                
        I# wordSize ->                                                                                                     
          I64# (wordSize +# (wordSize *# (sizeofArray# xs)) +# sizeofByteArray# ys)

ghcStatsSizeInBytes :: Int64
ghcStatsSizeInBytes = $([| sizeOf $! GCStats 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 |])

[mitchellwrosen]

Random aside: is there any way to get GHC to calculate a top-level thing like ghcStatsSizeInBytes at compile time w/o $([| x |])? Would inline + -O2 work?

[chrisdone]

What's the reliability of sizeOf?

Replace 8 with SIZEOF_HSWORD

Sounds good.

[mitchellwrosen]

I'm not sure what the reliability of sizeOf is, I modified it from an old Simon Marlow snippet: https://ghcmutterings.wordpress.com/2009/02/12/53/

I do understand how it works but I don't know how often the semantics of e.g. sizeofByteArray will change, or if unpackClosure will be around forever, etc.

But if you meant how likely is it to work on Haskell things, I assume it is accurate for all closures (data constructors, functions, and thunks).