|Version 1 (modified by dias, 7 years ago) (diff)|
The pipeline: Make the new code generator work with the existing native codeGen
- 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.
- Block concatenation. branch to K; and this is the only use of K.
- Common Block Elimination (like CSE). This essentially implements the Adams optimisation, we believe.
- Consider (sometime): block duplication. branch to K; and K is a short block. Branch chain elimination is just a special case of this.
- Proc-point analysis and transformation, implemented in CmmProcPointZ. (Adams version is CmmProcPoint.) The transformation part adds a CopyIn to the front of each proc-point, which expresses the idea that proc-points use a standard entry convention.
- The analysis produces a set of BlockId that should become proc-point
- The transformation inserts a CopyIn at the start of each proc-point, and a CopyOut just before each branch to a proc-point.
- 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.
- Lay out the stack
- A SlotId is the offset of a stack slot from the old end (high address) of the frame. It doesn't vary as the physical stack pointer moves.
- A particular variable has one and only one SlotId.
- The stack layout pass produces a mapping of: (variable -> slotid, label -> slotid)
- Plus, it transforms the code by eliminating CopyIn and CopyOut in favour of assigments
- Make the stack explicit.
- Convert 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.
The Adams optimisation is done by stuff above. Given:
call f returns to K K: CopyIn retvals; goto L L: <code>
transform tocall f returns to L L : CopyIn retvals; <code>
and move CopyOut into L's other predecessors. ToDo: explain why this is a good thing. In fact Common Block Elimination does this, we think.