GHC Handbook

Note [Worker/wrapper for CPR]

GHC/Core/Opt/WorkWrap/Utils.hs:1725 compiler 

'mkWWcpr_entry' is the entry-point to the worker/wrapper transformation that
exploits CPR info. Here's an example:
```
  f :: ... -> (Int, Int)
  f ... = <body>
```
Let's assume the CPR info `body_cpr` for the body of `f` says
"unbox the pair and its components" and `body_ty` is the type of the function
body `body` (i.e., `(Int, Int)`). Then `mkWWcpr_entry body_ty body_cpr` returns

  * A result-unpacking expression for the worker, with a hole for the fun body:
    ```
      unpack body = ( case <body> of r __DEFAULT -> )    -- (1)
                    ( case r of (i, j) ->           )    -- (2)
                    ( case i of I# a ->             )    -- (2)
                    ( case j of I# b ->             )    -- (2)
                    ( (# a, b #)                    )    -- (3)
    ```
  * A result-building expression for the wrapper, with a hole for the worker call:
    ```
      build wkr_call = ( case <wkr_call> of (# a, b #) -> )    -- (3)
                       ( (I# a, I# b)                     )    -- (2)
    ```
  * The result type of the worker, e.g., `(# Int#, Int# #)` above.

To achieve said transformation, 'mkWWcpr_entry'

  1. First allocates a fresh result binder `r`, giving a name to the `body`
     expression and contributing part (1) of the unpacker and builder.
  2. Then it delegates to 'mkWWcpr_one', which recurses into all result fields
     to unbox, contributing the parts marked with (2). Crucially, it knows
     what belongs in the case scrutinee of the unpacker through the communicated
     Id `r`: The unpacking expression will be free in that variable.
     (This is a similar contract as that of 'mkWWstr_one' for strict args.)
  3. 'mkWWstr_one' produces a bunch of *transit vars*: Those result variables
     that have to be transferred from the worker to the wrapper, where the
     constructed result can be rebuilt, `a` and `b` above. Part (3) is
     responsible for tupling them up in the worker and taking the tuple apart
     in the wrapper. This is implemented in 'move_transit_vars'.

References 0

This Note does not link to any other.

Referenced by 7