|Version 3 (modified by simonpj, 7 years ago) (diff)|
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.
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
- 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`.
- 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.
- Code generator converts STG to CmmGraph.
- 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.
- 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.