Unboxed sum types
This page explains the motivation and implementation of unpacking for sum types.
XUnboxedSums
have been available since GHC 8.2.1.
See also UnliftedDataTypes
Current status
See the UnboxedSums label.
Motivation
GHC does a good job of unpacking product types. Given a declaration like
data T1 a b = C1 a b
data T2 a b = C2 {# UNPACK #} !(T1 a b)
C2
will have a representation where all the overhead of the C1
constructor,
both the pointer to it in the C2
constructor and the info table pointer in
the C1
constructor, has been removed. This saves two words and one
indirection compared to a packed representation, which uses five words.
Unfortunately, a similar example using sum types cannot be unpacked today:
data T1 a = Some a  None
data T2 a = C !(T1 a)  Cannot UNPACK here
Here the representation of the C
constructor will contain a pointer to e.g.
the Some
constructor. The Some
constructor will be a separate heap object
and thus needs one word to store its info table pointer.
In this example there is an alternative, unpacked representation that is more
memory efficient and has fewer indirections. We could store a constructor tag
together with the union of the fields of T1
inside C
. Conceptually the
memory layout would look like this (in practice we group pointer and
nonpointer fields together):
T2 info table pointer  T1 constructor tag  Fields of Some  Fields of None 

(In this case None
has no fields.)
This representation saves one word and one indirection compared to the packed representation, which uses four words.
Source language design
We add new builtin types for anonymous unboxed sums. These are directly analogous to the existing anonymous unboxed tuples. Specifically:

A new language extension
UnboxedSums
. 
We add a family of new builtin type constructors for unboxed sums:
(##), (##), (##), (##), etc
A sum of n ""s is a n+1 ary sum. (Just like tuples
(,)
,(,,)
, etc.) 
Each narysum type constructor comes with n data constructors, with systematicallyderived names, thus:
data (##) a b c = (# _   #) a  (#  _  #) b  (#   _ #) c
The
_
indicates which disjunct of the sum we mean. 
You use the type constructor in a distfix way, like so:
(# Int  Bool #) means (##) Int Bool (# Int  Bool  Int #) means (##) Int Bool Int (# Int  Bool #) means (##) Int Bool
And similarly the data constructors:
(#  True #) means (#  _ #) True (#  'c'  #) means (#  _  #) 'c'

You can use the data constructors both in terms (to construct) and in patterns (to decompose).
case x of (# x    #) > ... (#  y   #) > ... ...two more disjuncts needed to be exhaustive

Unboxed sums are first class values. They can be passed as an argument to a function, returned as its result, be the type of a data constructor field, and so on. Of course, unboxed sums are unlifted (cannot be bottom), and should be represented efficiently (more on that below).

Just as for unboxed tuples: The components of an unboxed sum type may be of kind
*
or#
. So(# Int#  Bool #)
is fine. And you can nest unboxed sums and tuple arbitrarily, e.g.(# (# Int,Bool #)  Char# #)
(# (# Int#  Char # #)  Int #)
All of these rules follow the same pattern as the rules for boxed/unboxed tuples.
Design questions
For largearity anonymous sums, the data constructor syntax requires counting
vertical bars. This is annoying. Might we consider switching to a new syntax
where (# 0 of 3  x #)
means (# x   #)
and (# 2 of 6  y #)
means `(# 
 y    #)`? I (Richard) saw this syntax in an email and thought it might be
an improvement.
Implementation
Wiredin types
Unboxed sums get implemented very like boxed and unboxed tuples; see compiler/GHC/Builtin/Types.hs.
The Core language
No changes in Core.
Core to STG
When going to STG we need to eliminate the unboxed sums. This can be done in compiler/GHC/Stg/Unarise.hs, just like for tuples.
Given the Core function
f :: (# t_1  ...  t_n #) > ...
we convert it to a call to STG which includes the tag and the maximum number of pointer and nonpointer arguments we might need. Example:
Core  STG 

f :: (# Int#  Bool #) > ...  f :: Word > Word > Pointer > ... 
f :: (# Int#  Word# #) > ...  f :: Word > Word > ... 
See notes in compiler/GHC/Stg/Unarise.hs for more details.
Code generation
New StgArg
constructor StgRubbishArg
and new CmmArg
are added for
efficient compilation of sums. See StgRubbishArg
in
compiler/GHC/Stg/Syntax.hs.
Unpacking
(NOTE (osa): This part is not yet implemented, the patch is trivial and I'm going to submit it soon)
Given
data T1 a = Some a  None
data T2 a = C {# UNPACK #} !(T1 a)
we generate a "worker" constructor
C (# a  (# #) #)
((# #)
playing the role of void.)
We then translate the construction of C
as follows:
C x
===> (translates to)
case x of
Some y > C (# y  #)
None > C (#  (# #) #)
We then translate the elimination of C
as follows:
case e of
C x > ... x ...
===> (translates to)
case e of
C x' >
let x = case x' of
(# y  #) > Some y
(#  _ #) > None
in ... x ...
This above reboxing will go away, using caseofcase and
caseofknownconstructor, if we scrutinize x
again.
Exploiting nullary constructors
Joachim writes: The current proposed layout for a
data D a = D a {# UNPACK #} !(Maybe a) would be
[D’s pointer] [a] [tag (0 or 1)] [Just’s a]
So the representation of
D foo (Just bar) is [D_info] [&foo] [1] [&bar]
and of D foo Nothing is [D_info] [&foo] [0] [&dummy]
where dummy
is something that makes the GC happy.
But assuming this dummy object is something that is never a valid heap objects of its own, then this should be sufficient to distinguish the two cases, and we could actually have that the representation of
D foo (Just bar) is [D_info] [&foo] [&bar]
and of D foo Nothing is [D_info] [&foo] [&dummy]
and an case analysis on D would compare the pointer in the third word with the wellknown address of dummy to determine if we have Nothing or Just. This saves one word.
If we generate a number of such static dummy objects, we can generalize this tagfield avoiding trick to other data types than Maybe. It seems that it is worth doing that if
 the number of constructors is no more than the number of static dummy objects, and
 there is one constructor which has more pointer fields than all other constructors.
Also, this trick cannot be applied repeatedly: If we have
data D = D {# UNPACK #} !(Maybe a)  D'Nothing
data E = E {# UNPACK #} !(D a)
then it cannot be applied when unpacking D
into E
. (Or maybe it can, but care has to be taken that D
’s Nothing
is represented by a different dummy object than Maybe
’s Nothing
.)
Anyways, this is an optimization that can be implemented once unboxed sum type are finished and working reliably.