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

Norman's plan

1. New code to check invariants of output from ZipDataflow
2. Finish debugging ZipDataflow
3. Use Simon PJ's 'common-blockifier' to move the Adams optimization outside CmmProcProintZ
4. ProcPointZ does not insert CopyOut nodes; this omission must be rectified and will require some general infrastructure for inserting predecessors.
5. Simple optimizations on CopyIn and CopyOut may be required
6. Define an interface for calling conventions and invariants for the output of frame layout [will require help from Simon M]
7. Stack layout

Items 1-5 look like a few days apiece. Items 6 and 7 are more scary...

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?
• 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.
  
  
• CmmExpr contains the data types for Cmm expressions, registers, and the like. It does not depend on the dataflow framework at all.

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)