Notes on new codegen (Sept 09) ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Things to do: - SDM (2010-02-26) can we remove the Foreign constructor from Convention? Reason: we never generate code for a function with the Foreign calling convention, and the code for calling foreign calls is generated - All dataflow analyses are in the FuelMonad, even though they are guarnteed to consume no fuel. This seems silly - CmmContFlowOpt.runCmmContFlowOptZs is not called! - Why is runCmmOpts called from HscMain? Seems too "high up". In fact HscMain calls (runCmmOpts cmmCfgOptsZ) which is what runCmmContFlowOptZs does. Tidy up! - AsmCodeGen has a generic Cmm optimiser; move this into new pipeline - AsmCodeGen has post-native-cg branch elimiator (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 AsmCodeGen.fixAssignTop and cmmToCmm). 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. - Refactor Cmm so that it contains only shared stuff Add a module MoribundCmm which contains stuff from Cmm for old code gen path - 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 - Add type/newtype for CmmModule = [CmmGroup] -- A module CmmGroup = [CmmTop] -- A .o file CmmTop = Proc | Data -- A procedure or data - This is a *change*: currently a CmmGroup is one function's-worth of code regardless of SplitObjs. Question: can we *always* generate M.o if there is just one element in the list (rather than M/M1.o, M/M2.o etc) One SRT per group. - See "CAFs" below; we want to totally refactor the way SRTs are calculated - Change type CmmZ = GenCmm CmmStatic CmmInfo (CmmStackInfo, CmmGraph) to type CmmZ = GenCmm CmmStatic (CmmInfo, CmmStackInfo) CmmGraph -- And perhaps take opportunity to prune CmmInfo? - Clarify which fields of CmmInfo are still used - Maybe get rid of CmmFormals arg of CmmProc in all versions? - We aren't sure whether cmmToRawCmm is actively used by the new pipeline; check And what does CmmBuildInfoTables do?! - Nuke CmmZipUtil, move zipPreds into ZipCfg - Pull out Areas into its own module Parameterise AreaMap 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/ - 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 CmmProcPointZ.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 cmm/ ---------------------------------------------------- -------- Dead stuff ------------ CmmProcPoint Dead: Michael Adams CmmCPS Dead: Michael Adams CmmCPSGen.hs Dead: Michael Adams CmmBrokenBlock.hs Dead: Michael Adams CmmLive.hs Dead: Michael Adams CmmProcPoint.hs Dead: Michael Adams Dataflow.hs Dead: Michael Adams StackColor.hs Norman? StackPlacements.hs Norman? HscMain.optionallyConvertAndOrCPS testCmmConversion DynFlags: -fconvert-to-zipper-and-back, -frun-cps, -frun-cpsz -------- Moribund stuff ------------ CmmCvt.hs Conversion between old and new Cmm reps CmmOpt.hs Hopefully-redundant optimiser CmmZipUtil.hs Only one function; move elsewhere -------- Stuff to keep ------------ CmmCPSZ.hs Driver for new pipeline CmmLiveZ.hs Liveness analysis, dead code elim CmmProcPointZ.hs Identifying and splitting out proc-points CmmSpillReload.hs Save and restore across calls CmmCommonBlockElimZ.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 layout ---------- Cmm data types -------------- ZipCfgCmmRep.hs Cmm instantiations of dataflow graph framework MkZipCfgCmm.hs Cmm instantiations of dataflow graph framework Cmm.hs Key module; a mix of old and new stuff so needs tidying up in due course CmmExpr.hs CmmUtils.hs CmmLint.hs PprC.hs Pretty print Cmm in C syntax PprCmm.hs Pretty printer for Cmm PprCmmZ.hs Additional stuff for zipper rep CLabel.hs CLabel ---------- Dataflow modules -------------- Goal: separate library; for now, separate directory MkZipCfg.hs ZipCfg.hs ZipCfgExtras.hs ZipDataflow.hs CmmTx.hs Transactions OptimizationFuel.hs Fuel BlockId.hs BlockId, BlockEnv, BlockSet DFMonad.hs ---------------------------------------------------- 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) - Optimise: CmmContFlowOpt (simple optimisations, very self contained) - Cps convert: CmmCPSZ.protoCmmCPSZ - Optimise: CmmContFlowOpt again - Convert: CmmCvt.cmmOfZgraph (convert to old rep) very self contained * StgCmm.hs The new STG -> Cmm conversion code generator Lots of modules StgCmmXXX ---------------------------------------------------- CmmCPSZ.protoCmmCPSZ The new pipeline ---------------------------------------------------- CmmCPSZprotoCmmCPSZ: 1. Do cpsTop for each procedures separately 2. Build SRT representation; this spans multiple procedures (unless split-objs) cpsTop: * CmmCommonBlockElimZ.elimCommonBlocks: eliminate common blocks * CmmProcPointZ.minimalProcPointSet identify proc-points no change to graph * CmmProcPointZ.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: - CmmSpillReload.insertLateReloads - 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 - CmmProcPointZ.procPointAnalysis Given set of proc points, which blocks are reachable from each Claim: too few proc-points => code duplication, but program still works?? - CmmProcPointZ.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 paramter 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 CmmProcPointZ.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. CmmCPSZ.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 ZipCfgCmmRep! 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 ---------------------------------------------------- * Cmm.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 [d] -- Initialised data, items d Old and new piplines use different representations (CmmCvt.hs converts between the two) ------------- OLD BACK END representations (Cmm.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 * Not Cmm-specific at all ZipCfg.hs defines Graph, LGraph, FGraph, ZHead, ZTail, ZBlock ... classes LastNode, HavingSuccessors MkZipCfg.hs: AGraph: building graphs * ZipCfgCmmRep: instantiates ZipCfg for Cmm data Middle = ...CmmExpr... data Last = ...CmmExpr... type CmmGraph = Graph Middle Last type CmmZ = GenCmm CmmStatic CmmInfo (CmmStackInfo, CmmGraph) type CmmStackInfo = (ByteOff, Maybe ByteOff) -- (SP offset on entry, update frame space = SP offset on exit) -- The new codegen produces CmmZ, but once the stack is -- manifested we can drop that in favour of -- GenCmm CmmStatic CmmInfo CmmGraph Inside a CmmProc: - CLabel: used - CmmInfo: partly used by NEW - CmmFormals: not used at all PERHAPS NOT EVEN BY OLD PIPELINE! * MkZipCfgCmm.hs: smart constructors for ZipCfgCmmRep Depends on (a) MkZipCfg (Cmm-independent) (b) ZipCfgCmmRep (Cmm-specific) ------------- * SHARED stuff CmmExpr.hs defines the Cmm expression types - CmmExpr, CmmReg, Width, CmmLit, LocalReg, GlobalReg - CmmType, Width etc (saparate module?) - MachOp (separate module?) - Area, AreaId etc (separate module?) BlockId.hs defines BlockId, BlockEnv, BlockSet ------------- ------------- * Transactions indicate whether or not the result changes: CmmTx type Tx a = a -> TxRes a data TxRes a = TxRes ChangeFlag a