|Version 18 (modified by dias, 5 years ago)|
Design of the new code generator
This page contains notes about the design of the new code generator
The new Cmm data type
There is a new Cmm data type:
- compiler/cmm/ZipCfg.hs 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. It allows you to define a new optimization simply by defining a lattice of dataflow facts (akin to a specialized logic) and then writing the dataflow-transfer functions found in compiler textbooks. Handing these functions to the dataflow engine produces a new optimization that is not only useful on its own, but that can easily be composed with other optimizations to create an integrated "superoptimization" that is strictly more powerful than any sequence of individual optimizations, no matter how many times they are re-run. The dataflow engine is based on (Lerner, Grove, and Chambers 2002); you can find a functional implementation of the dataflow engine presented in (Ramsey and Dias 2005).
- compiler/cmm/ZipCfgCmmRep.hs 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 the Cmm pipeline
Code generation now has three stages:
- Convert STG to Cmm, with implicit stack implicit, and native Cmm calls.
- Optimise the Cmm, and CPS-convert it to have an explicit stack, and no native calls. This part of the pipeline is stitched together in cmm/CmmCPSZ.hs.
- Feed the CPS-converted Cmm to the existing, unmodified native code generators.
The first two steps are described in more detail here:
- Code generator converts STG to CmmGraph. Implemented in StgCmm* modules (in directory codeGen).
- Parameter passing is made explicit. Parameters are passed in virtual registers R1, R2 etc. Overflow parameters are passed on the stack using explicit memory stores, to locations described abstractly using the ''Stack Area'' abstraction..
- That includes a store of the return address, which is stored explicitly on the stack in the same way as overflow parameters.
- No CopyIn, CopyOut nodes any more; instead "smart constructors" lower the calling convention to loads/stores/register transfers, using stack area abstraction.
- But we still have LastCall, LastReturn, LastBranch, LastJump as Last nodes.
- 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 function prologue to the front of each proc-point, following a standard entry convention.
- The analysis produces a set of BlockId that should become proc-points
- The transformation inserts a function prologue at the start of each proc-point, and a function epilogue 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 spill and reload instructions are simply memory stores and loads respectively, using symbolic stack offsets (see stack layout). For example, a spill of variable 'x' would look like Ptr32[SS(x)] = x.
- Figure out the stack layout
- Each variable 'x', and each proc-point label 'K', has an associated Area, written SS(x) and SS(k) resp, that names a contiguous portion of the stack frame.
- The stack layout pass produces a mapping of: (Area -> StackOffset). For more detail, see the description of stack layout.
- A StackOffset is the byte offset of a stack slot from the old end (high address) of the frame. It doesn't vary as the physical stack pointer moves.
- Manifest the stack pointer. Once the stack layout mapping has been determined, a second pass walks over the graph, making the stack pointer, Sp explicit. Before this pass, there is no Sp at all. After this, Sp is completely manifest.
- replacing references to Areas with offsets from Sp.
- adding adjustments to Sp.
- Split into multiple CmmProcs. At this point we build an info-table for each of the CmmProcs, including SRTs. Done on the basis of the live local variables (by now mapped to stack slots) and live CAF statics.
- LastCall and LastReturn nodes are replaced by Jumps.
- Build info tables.
- Find each safe MidForeignCall node, "lowers" it into the suspend/call/resume sequence (see Note [Foreign calls] in CmmNode.hs.), and build an info table for them.
- Convert the CmmInfo for each CmmProc into a [CmmStatic], using the live variable information computed just before "Figure out stack layout".
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.
- Garbage collector entry points: see Note [Heap checks] in StgCmmHeapery.
- Update frames and exception handling. Also STM frames.
- Primitives can be rewritten:
- Use parameters
- In a few cases, use native calls (notably eval)