wiki:Commentary/Compiler/NewCodeGen

Version 6 (modified by simonpj, 6 years ago) (diff)

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Material about the new code generator

This page summarises work that Norman Ramsey, Simon M, and Simon PJ are doing on re-architecting GHC's back end.

Status:

  • Code generator: first draft done.
  • Control-flow opt: simple ones done
    • Common block elmination: to do
    • Block concatenation: to do
  • Adams optimisation: currently done somewhere but not modularly. I think.
  • Proc-point analysis and transformation: done?
  • Add spill/reload: done?
  • Stack slot alloction?
  • Make stack explicit: to do
  • Split into multiple CmmProcs: to do

ToDo: main issues

  • SRTs simply record live global variables. So we should use the same live-variable framework as for live local variables. That means we must be able to identify which globals are SRT-able. What about compression/encoding schemes?
  • Draining the Rep swamp.
  • How do we write continuations in the RTS? E.g. the update-frame continuation? Michael Adams had a syntax with two sets of parameters, the the ones on the stack and the return values.
  • Review code gen for calls with lots of args. In the existing codegen we push magic continuations that say "apply the return value to N more args". Do we want to do this? ToDo: how rare is it to have too many args?
  • Figure out how PAPs work. This may interact with the GC check and stack check at the start of a function call.
  • How do stack overflow checks work? (They are inserted by the CPS conversion, and must not generate a new info table etc.)
  • Was there something about sinking spills and hoisting reloads?

ToDo: small issues

  • Shall we rename Branch to GoTo?!
  • Where is the "push new continuation" middle node?
  • Get rid of CmmFormals on LastJump and LastCall in ZipCfgCmm in favour of CopyIn and CopyOut.
  • Change the C-- parser (which parses RTS .cmm files) to directly construct CmmGraph.
  • (SLPJ) See let-no-escape todos in StgCmmExpr.

The new Cmm data type

There is a new Cmm data type:

  • ZipCfg contains a generic zipper-based control-flow graph data type. It is generic in the sense that it's polymorphic in the type of middle nodes and last nodes of a block. (Middle nodes don't do control transfers; last nodes only do control transfers.) There are extensive notes at the start of the module.

    The key types it defines are:
    • Block identifiers: BlockId, BlockEnv, BlockSet
    • Control-flow blocks: Block
    • Control-flow graphs: Graph

  • ZipDataFlow contains a generic framework for solving dataflow problems over ZipCfg.

  • ZipCfgCmm instantiates ZipCfg for Cmm, by defining types Middle and Last and using these to instantiate the polymorphic fields of ZipCfg. It also defines a bunch of smart constructor (mkJump, mkAssign, mkCmmIfThenElse etc) which make it easy to build CmmGraph.

The pipeline

  • Code generator converts STG to CmmGraph. Implemented in StgCmm* modules (in directory codeGen).
  • Simple control flow optimisation, implemented in CmmContFlowOpt:
    • Branch chain elimination
    • Remove unreachable blocks
    • TODO block concatenation. branch to K; and this is the only use of K.
    • Consider: block duplication. branch to K; and K is a short block. Branch chain elimination is just a special case of this.
    • TODO Common block elimination (like CSE). This makes something else significantly simpler. (ToDo: what?).
  • The Adams optimisation. Given:
      call f returns to K
      K: CopyIn retvals; goto L
      L: <code>
    
    transform to
      call f returns to L
      L : CopyIn retvals; <code>
    
    and move CopyOut into L's other predecessors. ToDo: explain why this is a good thing.
  • Proc-point analysis and transformation, implemented in CmmProcPointZ. (Adams version is CmmProcPoint.) The transfomation part adds a CopyIn to the front of each proc-point, which expresses the idea that proc-points use a standard entry convention.
  • Add spill/reload, implemented in CmmSpillReload, to spill live C-- variables before a call and reload them afterwards. The middle node of the result is Middle (from ZipCfgCmm extended with Spill and Reload constructors. Invariant: (something like) all variables in a block are gotten from CopyIn or Reload.
  • Stack slot layout. Build inteference graph for variables live across calls, and allocate a stack slot for such variables. That is, stack slot allocation is very like register allocation.
  • Make the stack explicit.
    • Convert CopyIn, CopyOut, Spill, Reload to hardware-register and stack traffic.
    • Add stack-pointer adjustment instructions.
    • Avoid memory traffic at joins. (What does this mean?)
  • Split into multiple CmmProcs.

Runtime system

  • Garbage collector entry points: see Note [Heap checks] in StgCmmHeapery.
  • PAPs

  • Update frames and exception handling. Also STM frames.
  • Primitives can be rewritten:
    • Use parameters
    • In a few cases, use native calls (notably eval)