[GHC] #10652: Better cache performance in Array#

[GHC]

#10652: Better cache performance in Array#
-------------------------------------+-------------------------------------
        Reporter:  MikeIzbicki       |                   Owner:
            Type:  feature request   |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler          |                 Version:  7.10.1
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  Runtime           |  Unknown/Multiple
  performance bug                    |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:
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Comment (by rwbarton):

 One idea I had was to create a new family of array types along with boxed
 and unboxed arrays, called unpacked arrays. (Note this is the opposite of
 UnpackingArrays.)

 The idea is that whenever `T` is a type that can be unpacked with the `{-#
 UNPACK #-}` pragma, we can form values of type `UnpackedArray# T`. The
 heap representation of such a value is of the form
 {{{
 UnpackedArray#_info T_con_info n a1 b1 c1 a2 b2 c2 ... an bn cn
 }}}
 representing an array containing the values
 {{{
 T_con_info a1 b1 c1
 T_con_info a2 b2 c2
 ...
 T_con_info an bn cn
 }}}
 (Since `T` can be unpacked, it has just one constructor.)

 Compared to an `Array#` this costs one extra word `T_con_info`. It might
 be possible to save this word with a new type of info table layout, then
 the heap representation of `Array# a` would be identical (modulo info
 pointer value) to that of `UnpackedArray# (Box a)` where `data Box a = Box
 a`.

 The GC can use `T_con_info` to understand the layout of the rest of the
 heap object, while mutator operations like indexing would use operations
 from a magically-generated `Unpack` type class. This last part might be
 tricky.

 Then `IntArrayTree` could hold an `UnpackedArray# IntArrayTree` to avoid
 one of the layers of indirection. It won't apply to tibbe's unordered-
 containers though, since `IntMap` has multiple constructors. (I guess it
 could be used inside `Collision !Hash !(A.Array (Leaf k v))`, but that's
 rather pointless.)

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10652#comment:14>
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