path: root/compiler/cmm/cmm-notes

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.

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

* "Remove redundant reloads" in CmmSpillReload should be redundant; since
  insertLateReloads is now gone, every reload is reloading a live variable.
  Test and nuke.

* When stubbing dead slots be careful not to write into an area that
  overlaps with an area that's in use.  So stubbing needs to *follow* 
  stack layout.


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

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

Things to do:
 - We insert spills for variables before the stack check! This is the reason for
   some fishy code in StgCmmHeap.entryHeapCheck where we are doing some strange
	things to fix up the stack pointer before GC calls/jumps.

	The reason spills are inserted before the sp check is that at the entry to a
	function we always store the parameters passed in registers to local variables.
	The spill pass simply inserts spills at variable definitions. We instead should
	sink the spills so that we can avoid spilling them on branches that never
	reload them.

	This will fix the spill before stack check problem but only really as a side
	effect. A 'real fix' probably requires making the spiller know about sp checks.

   EZY: I don't understand this comment. David Terei, can you clarify?

 - 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.

 - CmmInfo.cmmToRawCmm uses Old.Cmm, so it is called after converting Cmm.Cmm to
   Old.Cmm. We should abstract it to work on both representations, it needs only to
   convert a CmmInfoTable to [CmmStatic].

 - Improve performance of Hoopl.

   A nofib comparison of -fasm vs -fnewcodegen nofib compilation parameters
   (using the same ghc-cmm branch +libraries compiled by the old codegenerator)
   is at http://fox.auryn.cz/msrc/0517_hoopl/32bit.oldghcoldgen.oldghchoopl.txt
   - the code produced is 10.9% slower, the compilation is +118% slower!

   The same comparison with ghc-head with zip representation is at
   http://fox.auryn.cz/msrc/0517_hoopl/32bit.oldghcoldgen.oldghczip.txt
   - the code produced is 11.7% slower, the compilation is +78% slower.

   When compiling nofib, ghc-cmm + libraries compiled with -fnew-codegen
   is 23.7% slower (http://fox.auryn.cz/msrc/0517_hoopl/32bit.oldghcoldgen.hooplghcoldgen.txt).
   When compiling nofib, ghc-head + libraries compiled with -fnew-codegen
   is 31.4% slower (http://fox.auryn.cz/msrc/0517_hoopl/32bit.oldghcoldgen.zipghcoldgen.txt).

   EZY: Also importantly, Hoopl uses dramatically more memory than the
   old code generator.

 - Are all blockToNodeList and blockOfNodeList really needed? Maybe we could
   splice blocks instead?

 - lowerSafeForeignCall seems too lowlevel. Just use Dataflow. After that
   delete splitEntrySeq from HooplUtils.

 - manifestSP seems to touch a lot of the graph representation. It is
   also slow for CmmSwitch nodes O(block_nodes * switch_statements).
   Maybe rewrite manifestSP to use Dataflow?

 - 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.

 - NB that CmmProcPoint line 283 has a hack that works around a GADT-related
   bug in 6.10.

 - 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.

 - Question: currently we lift procpoints to become separate
   CmmProcs.  Do we still want to do this?
    
   NB: and advantage of continuing to do this is that
   we can do common-proc elimination!

 - 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

 - Consider module names

 - Top-level SRT threading is a bit ugly

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

 - Pull out Areas into its own module
   Parameterise AreaMap (note there are type synonyms in CmmStackLayout!)
   Add ByteWidth = Int
   type SubArea    = (Area, ByteOff, ByteWidth) 
   ByteOff should not be defined in SMRep -- that is too high up the hierarchy
   
 - SMRep should not be imported by any module in cmm/!  Make it so.
	-- ByteOff etc   ==>  CmmExpr
        -- rET_SMALL etc ==> CmmInfo
   Check that there are no other imports from codeGen in cmm/

 - If you eliminate a label by branch chain elimination,
   what happens if there's an Area associated with that label?

 - Think about a non-flattened representation?

 - LastCall: 
    * Use record fields for LastCall!
    * cml_ret_off should be a ByteOff
    * Split into 
         LastCall (which has a successor) and
         LastJump (which does not, includes return?)
	   - does not have cml_cont, cml_ret_args, cml_ret_off
	 LastForeignCall 
           - safe! 
           - expands into save/MidForeignCall/restore/goto
	   - like any LastCall, target of the call gets an info table

 - JD: remind self of what goes wrong if you turn off the 
   liveness of the update frame

 - Garbage-collect http://hackage.haskell.org/trac/ghc/wiki/Commentary/Compiler/CPS
   moving good stuff into 
   http://hackage.haskell.org/trac/ghc/wiki/Commentary/Compiler/NewCodeGenPipeline


 - We believe that all of CmmProcPoint.addProcPointProtocols is dead.  What
   goes wrong if we simply never call it?

 - Something fishy in CmmStackLayout.hs
   * In particular, 'getAreaSize' returns an AreaMap, but we *know* the width of
	LocalRegs, so it'd be better to return FiniteMap AreaId ByteWidth
   * setSuccSPs looks fishy.  Rather than lookin in procPoints, it could
	just lookup the block in areaSize which, after all, has a binding
	for precisely successors of calls.  All other blocks (including proc
        points that are not successors of a call, we think) can be treated
        uniformly: zero-size Area, and use inSP.


 - 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.

 - Modularise the CPS pipeline; instead of ...; A;B;C; ...
                                use  ..; ABC; ....

 - Most of HscMain.tryNewCodeGen does not belong in HscMain.  Instead
	if new_cg then
             StgCmm.codeGen
             processCmm  [including generating "raw" cmm]
        else
             CodeGen.codeGen
             cmmToRawCmm


 - 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 to 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)

----------------------------------------------------
	Modules in codeGen/
----------------------------------------------------


------- Shared ---------
Bitmap.hs
SMRep.lhs

CmmParse.y
CgExtCode.hs   used in CmmParse.y

------- New codegen ---------

StgCmm.hs
StgCmmBind.hs
StgCmmClosure.hs     (corresponds to old ClosureInfo)
StgCmmCon.hs
StgCmmEnv.hs
StgCmmExpr.hs
StgCmmForeign.hs
StgCmmGran.hs
StgCmmHeap.hs
StgCmmHpc.hs
StgCmmLayout.hs
StgCmmMonad.hs
StgCmmPrim.hs
StgCmmProf.hs
StgCmmTicky.hs
StgCmmUtils.hs

------- Old codegen (moribund) ---------
CodeGen.lhs
CgBindery.lhs
CgCallConv.hs
CgCase.lhs
CgClosure.lhs
CgCon.lhs
CgExpr.lhs
CgLetNoEscape.lhs
CgForeignCall.hs
CgHeapery.lhs
CgHpc.hs
CgInfoTbls.hs
CgMonad.lhs
CgParallel.hs
CgPrimOp.hs
CgProf.hs
CgStackery.lhs
CgTailCall.lhs
CgTicky.hs
CgUtils.hs
ClosureInfo.lhs

----------------------------------------------------
	Modules in cmm/
----------------------------------------------------

-------- Testing stuff ------------
DynFlags: -frun-cpsz

-------- Moribund stuff ------------
OldCmm.hs      Definition of flowgraph of old representation
               Imports some data types from (new) Cmm
OldCmmUtil.hs  Utilites that operates mostly on on CmmStmt
OldPprCmm.hs   Pretty print for CmmStmt, GenBasicBlock and ListGraph
CmmOpt.hs      Hopefully-redundant optimiser

-------- Stuff to keep ------------
CmmPipeline.hs            Driver for new pipeline

CmmLive.hs                Liveness analysis, dead code elim
CmmProcPoint.hs           Identifying and splitting out proc-points

CmmSpillReload.hs         Save and restore across calls

CmmCommonBlockElim.hs     Common block elim
CmmContFlowOpt.hs         Other optimisations (branch-chain, merging)

CmmBuildInfoTables.hs     New info-table 
CmmStackLayout.hs         and stack layout 
CmmCallConv.hs
CmmInfo.hs                Defn of InfoTables, and conversion to exact byte layout

---------- Cmm data types --------------
Cmm.hs              Cmm instantiations of dataflow graph framework
  CmmExpr.hs        Type of Cmm expression
  CmmType.hs        Type of Cmm types and their widths
  CmmMachOp.hs      MachOp type and accompanying utilities

PprCmm.hs	    Pretty printer for Cmm
  PprCmmExpr.hs     Pretty printer for CmmExpr

MkGraph.hs          Interface for building Cmm for codeGen/Stg*.hs modules

CmmUtils.hs
CmmLint.hs

PprC.hs	            Pretty print Cmm in C syntax

CLabel.hs           CLabel
BlockId.hs          BlockId, BlockEnv, BlockSet

----------------------------------------------------
      Top-level structure
----------------------------------------------------

* New codgen called in HscMain.hscGenHardCode, by calling HscMain.tryNewCodeGen, 
  enabled by -fnew-codegen (Opt_TryNewCodeGen)

  THEN it calls CmmInfo.cmmToRawCmm to lay out the details of info tables
      type Cmm    = GenCmm CmmStatic CmmInfo     (ListGraph CmmStmt)
      type RawCmm = GenCmm CmmStatic [CmmStatic] (ListGraph CmmStmt)

* HscMain.tryNewCodeGen
    - STG->Cmm:         StgCmm.codeGen (new codegen)
    - Optimize and CPS: CmmPipeline.cmmPipeline
    - Convert:          CmmCvt.cmmOfZgraph (convert to old rep) very self contained

* StgCmm.hs  The new STG -> Cmm conversion code generator
  Lots of modules StgCmmXXX


----------------------------------------------------
      CmmPipeline.cmmPipeline   The new pipeline
----------------------------------------------------

CmmPipeline.cmmPipeline:
   1. Do control flow optimization
   2. Do cpsTop for each procedures separately
   3. Build SRT representation; this spans multiple procedures
        (unless split-objs)
   4. Do control flow optimization on all resulting procedures

cpsTop:
  * CmmCommonBlockElim.elimCommonBlocks:
	eliminate common blocks 

  * CmmProcPoint.minimalProcPointSet
	identify proc-points
        no change to graph

  * CmmProcPoint.addProcPointProtocols
	something to do with the MA optimisation
        probably entirely unnecessary

  * Spill and reload:
     - CmmSpillReload.dualLivenessWithInsertion
       insert spills/reloads across 
	   LastCalls, and 
	   Branches to proc-points
     Now sink those reloads (and other instructions):
     - CmmSpillReload.rewriteAssignments
     - CmmSpillReload.removeDeadAssignmentsAndReloads

  * CmmStackLayout.stubSlotsOnDeath
	debug only: zero out dead slots when they die

  * Stack layout
     - CmmStackLayout.lifeSlotAnal: 
       find which sub-areas are live on entry to each block

     - CmmStackLayout.layout
       Lay out the stack, returning an AreaMap
         type AreaMap = FiniteMap Area ByteOff
          -- Byte offset of the oldest byte of the Area, 
          -- relative to the oldest byte of the Old Area

     - CmmStackLayout.manifestSP
       Manifest the stack pointer

   * Split into separate procedures

      - CmmProcPoint.procPointAnalysis
        Given set of proc points (computed earlier by
           CmmProcPoint.minimalProcPointSet) find which blocks 
           are reachable from each
	Each block should be reachable from *one* proc point, so 
	  the blocks reachable from P are the internal nodes of 
	  the final procedure P
        NB: if we the earlier analysis had produced too few proc-points 
            we should nevertheless be fine by code duplication; but
	    that is not implemented

      - CmmProcPoint.splitAtProcPoints
	Using this info, split into separate procedures

      - CmmBuildInfoTables.setInfoTableStackMap
	Attach stack maps to each info table


----------------------------------------------------
	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 defininions 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.

----------------------------------------------------
		Foreign calls
----------------------------------------------------

See Note [Foreign calls] in CmmNode!  This explains that a safe
foreign call must do this:
  save thread state
  push info table (on thread stack) to describe frame
  make call (via C stack)
  pop info table
  restore thread state
and explains why this expansion must be done late in the day.

Hence, 
  - Every foreign call is represented as a middle node

  - *Unsafe* foreign calls are simply "fat machine instructions"
      and are passed along to the native code generator

  - *Safe* foreign calls are "lowered" to unsafe calls by wrapping
      them in the above save/restore sequence. This step is done
      very late in the pipeline, just before handing to the native
      code gen.   
  
      This lowering is done by BuildInfoTables.lowerSafeForeignCalls


NEW PLAN for foreign calls:
  - Unsafe foreign calls remain as a middle node (fat machine instruction)
    Even the parameter passing is not lowered (just as machine instrs
    get arguments).

  - Initially, safe foreign calls appear as LastCalls with 
	

----------------------------------------------------
		Cmm representations
----------------------------------------------------

* CmmDecl.hs
     The type [GenCmm d h g] represents a whole module, 
	** one list element per .o file **
	Without SplitObjs, the list has exactly one element

     newtype GenCmm d h g = Cmm [GenCmmTop d h g]  -- A whole .o file
     data GenCmmTop d h g
         = CmmProc h g           -- One procedure, graph d
         | CmmData <stuff> [d]   -- Initialised data, items d

  Old and new piplines use different representations
  	(CmmCvt.hs converts between the two)


-------------
OLD BACK END representations (OldCmm.hs):  
      type Cmm = GenCmm CmmStatic CmmInfo (ListGraph CmmStmt)
				-- A whole module
      newtype ListGraph i = ListGraph [GenBasicBlock i]

      data CmmStmt = Assign | Store | Return etc -- OLD BACK END ONLY


   Once the info tables are laid out, we replace CmmInfo with [CmmStatic]
      type RawCmm    = GenCmm CmmStatic [CmmStatic] (ListGraph CmmStmt)
   which represents the info tables as data, that should 
   immediately precede the code
  
-------------
NEW BACK END representations 
* Uses Hoopl library, a zero-boot package
* CmmNode defines a node of a flow graph.
* Cmm defines CmmGraph, CmmTop, Cmm
   - CmmGraph is a closed/closed graph + an entry node.

       data CmmGraph = CmmGraph { g_entry :: BlockId
                                , g_graph :: Graph CmmNode C C }

   - CmmTop is a top level chunk, specialization of GenCmmTop from CmmDecl.hs
       with CmmGraph as a flow graph.
   - Cmm is a collection of CmmTops.

       type Cmm          = GenCmm    CmmStatic CmmTopInfo CmmGraph
       type CmmTop       = GenCmmTop CmmStatic CmmTopInfo CmmGraph

   - CmmTop uses CmmTopInfo, which is a CmmInfoTable and CmmStackInfo

       data CmmTopInfo   = TopInfo {info_tbl :: CmmInfoTable, stack_info :: CmmStackInfo}

   - CmmStackInfo

       data CmmStackInfo = StackInfo {arg_space :: ByteOff, updfr_space :: Maybe ByteOff}

         * arg_space = SP offset on entry
         * updfr_space space = SP offset on exit
       Once the staci is manifested, we could drom CmmStackInfo, ie. get
         GenCmm CmmStatic CmmInfoTable CmmGraph, but we do not do that currently.


* MkGraph.hs: smart constructors for Cmm.hs
  Beware, the CmmAGraph defined here does not use AGraph from Hoopl,
  as CmmAGraph can be opened or closed at exit, See the notes in that module.

-------------
* SHARED stuff
  CmmDecl.hs - GenCmm and GenCmmTop types
  CmmExpr.hs - defines the Cmm expression types
             - CmmExpr, CmmReg, CmmLit, LocalReg, GlobalReg
             - Area, AreaId etc     (separate module?)
  CmmType.hs - CmmType, Width etc   (saparate module?)
  CmmMachOp.hs - MachOp and CallishMachOp types

  BlockId.hs defines  BlockId, BlockEnv, BlockSet
-------------