|Version 5 (modified by 9 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.
- 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
- Shall we rename Branch to GoTo?!
- Where is the "push new continuation" middle node?
- 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.)
- Get rid of
ZipCfgCmmin favour of
- Change the C-- parser (which parses RTS .cmm files) to directly construct
- Was there something about sinking spills and hoisting reloads?
- (SLPJ) See let-no-escape todos in
The new Cmm data type
There is a new Cmm data type:
ZipCfgcontains 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:
- Control-flow blocks:
- Control-flow graphs:
- Block identifiers:
ZipCfgfor Cmm, by defining types
Lastand using these to instantiate the polymorphic fields of
ZipCfg. It also defines a bunch of smart constructor (
etc) which make it easy to buildCmmGraph`.
- Code generator converts STG to
CmmGraph. Implemented in
StgCmm*modules (in directory
- Simple control flow optimisation, implemented in
- 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
CopyOutinto 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
CopyInto 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
Reloadconstructors. Invariant: (something like) all variables in a block are gotten from
- 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.
Reloadto hardware-register and stack traffic.
- Add stack-pointer adjustment instructions.
- Avoid memory traffic at joins. (What does this mean?)
- Split into multiple CmmProcs.
- Garbage collector entry points: see
Note [Heap checks]in
- Update frames and exception handling. Also STM frames.
- Primitives can be rewritten:
- Use parameters
- In a few cases, use native calls (notably eval)