More notes (Aug 11)
~~~~~~~~~~~~~~~~~~
* CmmInfo.cmmToRawCmm expands info tables to their representations
  (needed for .cmm files as well as the code generators)

* Why is FCode a lazy monad?  That makes it inefficient.
  We want laziness to get code out one procedure at a time,
  but not at the instruction level.
  UPDATE (31/5/2016): FCode is strict since 09afcc9b.

Things we did
  * Remove CmmCvt.graphToZgraph (Conversion from old to new Cmm reps)
  * Remove HscMain.optionallyConvertAndOrCPS (converted old Cmm to
    new, ran pipeline, and converted back)
  * Remove CmmDecl. Put its types in Cmm.  Import Cmm into OldCmm
    so it can get those types.


More notes (June 11)
~~~~~~~~~~~~~~~~~~~~

* In CmmContFlowOpts.branchChainElim, can a single block be the
  successor of two calls?

* Check in ClosureInfo:
     -- NB: Results here should line up with the results of SMRep.rtsClosureType

More notes (May 11)
~~~~~~~~~~~~~~~~~~~
In CmmNode, consider spliting CmmCall into two: call and jump

Notes on new codegen (Aug 10)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Things to do:
 - Proc points pass all arguments on the stack, adding more code and
   slowing down things a lot. We either need to fix this or even better
   would be to get rid of proc points.

 - Sort out Label, LabelMap, LabelSet versus BlockId, BlockEnv, BlockSet
   dichotomy. Mostly this means global replace, but we also need to make
   Label an instance of Outputable (probably in the Outputable module).

   EZY: We should use Label, since that's the terminology Hoopl uses.

 - AsmCodeGen has a generic Cmm optimiser; move this into new pipeline
   EZY (2011-04-16): The mini-inliner has been generalized and ported,
   but the constant folding and other optimizations need to still be
   ported.

 - AsmCodeGen has post-native-cg branch eliminator (shortCutBranches);
   we ultimately want to share this with the Cmm branch eliminator.

 - At the moment, references to global registers like Hp are "lowered" 
   late (in CgUtils.fixStgRegisters). We should do this early, in the
	new native codegen, much in the way that we lower calling conventions.
	Might need to be a bit sophisticated about aliasing.

 - Move to new Cmm rep:
     * Make native CG consume New Cmm; 
     * Convert Old Cmm->New Cmm to keep old path alive
     * Produce New Cmm when reading in .cmm files

 - Top-level SRT threading is a bit ugly

 - See "CAFs" below; we want to totally refactor the way SRTs are calculated

 - Garbage-collect https://gitlab.haskell.org/ghc/ghc/wikis/commentary/compiler/cps
   moving good stuff into 
   https://gitlab.haskell.org/ghc/ghc/wikis/commentary/compiler/new-code-gen-pipeline

 - Currently AsmCodeGen top level calls AsmCodeGen.cmmToCmm, which is a small
   C-- optimiser.  It has quite a lot of boilerplate folding code in AsmCodeGen
   (cmmBlockConFold, cmmStmtConFold, cmmExprConFold), before calling out to
   CmmOpt.  ToDo: see what optimisations are being done; and do them before
   AsmCodeGen.

 - If we stick CAF and stack liveness info on a LastCall node (not LastRet/Jump)
   then all CAF and stack liveness stuff be completed before we split
   into separate C procedures.

   Short term:
     compute and attach liveness into LastCall
     right at end, split, cvt to old rep
     [must split before cvt, because old rep is not expressive enough]

   Longer term: 
     when old rep disappears, 
     move the whole splitting game into the C back end *only*
	 (guided by the procpoint set)

----------------------------------------------------
	Proc-points
----------------------------------------------------

Consider this program, which has a diamond control flow, 
with a call on one branch
 fn(p,x) {
        h()
	if b then { ... f(x) ...; q=5; goto J }
             else { ...; q=7; goto J }
     J: ..p...q...
  }
then the join point J is a "proc-point".  So, is 'p' passed to J
as a parameter?  Or, if 'p' was saved on the stack anyway, perhaps
to keep it alive across the call to h(), maybe 'p' gets communicated
to J that way. This is an awkward choice.  (We think that we currently
never pass variables to join points via arguments.)

Furthermore, there is *no way* to pass q to J in a register (other
than a parameter register).

What we want is to do register allocation across the whole caboodle.
Then we could drop all the code that deals with the above awkward
decisions about spilling variables across proc-points.

Note that J doesn't need an info table.

What we really want is for each LastCall (not LastJump/Ret) 
to have an info table.   Note that ProcPoints that are not successors
of calls don't need an info table.

Figuring out proc-points
~~~~~~~~~~~~~~~~~~~~~~~~
Proc-points are identified by
CmmProcPoint.minimalProcPointSet/extendPPSet Although there isn't
that much code, JD thinks that it could be done much more nicely using
a dominator analysis, using the Dataflow Engine.

----------------------------------------------------
		CAFs
----------------------------------------------------

* The code for a procedure f may refer to either the *closure* 
  or the *entry point* of another top-level procedure g.  
  If f is live, then so is g.  f's SRT must include g's closure.

* The CLabel for the entry-point/closure reveals whether g is 
  a CAF (or refers to CAFs).  See the IdLabel constructor of CLabel.

* The CAF-ness of the original top-level definitions is figured out
  (by TidyPgm) before we generate C--.  This CafInfo is only set for
  top-level Ids; nested bindings stay with MayHaveCafRefs.

* Currently an SRT contains (only) pointers to (top-level) closures.

* Consider this Core code
	f = \x -> let g = \y -> ...x...y...h1...
                  in ...h2...g...
  and suppose that h1, h2 have IdInfo of MayHaveCafRefs.
  Therefore, so will f,  But g will not (since it's nested).

  This generates C-- roughly like this:
     f_closure: .word f_entry
     f_entry() [info-tbl-for-f] { ...jump g_entry...jump h2... }
     g_entry() [info-tbl-for-g] { ...jump h1... }

  Note that there is no top-level closure for g (only an info table).
  This fact (whether or not there is a top-level closure) is recorded
  in the InfoTable attached to the CmmProc for f, g
  INVARIANT: 
     Any out-of-Group references to an IdLabel goes to
     a Proc whose InfoTable says "I have a top-level closure".
  Equivalently: 
     A CmmProc whose InfoTable says "I do not have a top-level
     closure" is referred to only from its own Group.

* So:   info-tbl-for-f must have an SRT that keeps h1,h2 alive
        info-tbl-for-g must have an SRT that keeps h1 (only) alive

  But if we just look for the free CAF refs, we get:
	f   h2 (only)
        g   h1

  So we need to do a transitive closure thing to flesh out 
  f's keep-alive refs to include h1.

* The SRT info is the C_SRT field of Cmm.ClosureTypeInfo in a
  CmmInfoTable attached to each CmmProc.  CmmPipeline.toTops actually does
  the attaching, right at the end of the pipeline.  The C_SRT part
  gives offsets within a single, shared table of closure pointers.

* DECIDED: we can generate SRTs based on the final Cmm program
  without knowledge of how it is generated.

