[GHC] #9476: Implement late lambda-lifting

[GHC]

#9476: Implement late lambda-lifting
-------------------------------------+-------------------------------------
        Reporter:  simonpj           |                Owner:  nfrisby
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.8.2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Runtime           |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #8763             |  Differential Rev(s):
       Wiki Page:  LateLamLift       |
-------------------------------------+-------------------------------------

Comment (by sgraf):

 I'm currently thinking about Pros and Cons involved with (re-)implementing
 this as an optimization on STG rather than Core.

 Pros:
 - Much less involved analysis that doesn't need to stay in sync with
 CorePrep
 - LLF only enables intra-module opportunities in the simplifier anyway
 - Runtime arguments are immediately visible
 - Lifting to top-level only is orthogonal enough to full laziness that it
 might be worthwhile to split anyway
 - Integrating more low-level information might be more feasible (like
 calling convention, e.g. number of argument registers, or `stg_ap_*`
 patterns)

 Cons:
 - The Simplifier might not be able to inline as much (which probably had
 limited effect anyway)
 - It's not enough to look at Core alone to gauge allocation (quite a
 bummer)
 - No Core Lint (but STG Lint, still)
 - Potentially we need to re-invent some utility functions we already have
 for Core
 - Perform the lifting by hand, rather than let `FloatOut` do the work

 I'm really interested to hear opinions/objections about this!

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9476#comment:15>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler