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diff --git a/chapter-interprocedural-sccp/README.md b/chapter-interprocedural-sccp/README.md deleted file mode 100644 index 7310de80..00000000 --- a/chapter-interprocedural-sccp/README.md +++ /dev/null @@ -1,86 +0,0 @@ -# SCCP Across Functions - -English | [日本語](README.ja.md) - -[Previous: SCCP Within a Function](../chapter-sccp/README.md) | -[Next: SSA Before Types Are Known](../chapter-incomplete-types/README.md) | - -The previous chapter runs SCCP with each function, and assumes very -conservative typing across calls. This chapter lets the same worklist -discover call edges and propagate values between functions. - -## Discovering calls while solving types - -In **Simple**, we let our SCCP run interprocedurally, again by (nearly) doing nothing: -`CallNodes` call `FunNodes`, `ReturnNodes` return to `CallEndNodes` and they -pass their type information along in exactly the normal way... except for one issue: -there are no edges between `Calls` and `Funs` (nor `Returns` and `CallEnds`). -Such edges make up a [Call Graph](https://en.wikipedia.org/wiki/Call_graph). - -We extend the normal SCCP algorithm to observe when a `Call`s function pointer -input changes - when it picks up new function targets. There are only a finite -number so we represent them exactly with *function indices*, or *fidx* -internally, as described earlier chapters. (Separate compilation adds a -special index for the infinite unknown functions that are called outside this -compilation unit). - -Once we observe a new *fidx* at a `Call`, we *link* the `Call` with the `Fun`, -and the `Return` with the `CallEnd`. Such linkage acts exactly as-if we added -a CFG edge from the `Call` to the `Fun`, and the `Call`s arguments to the -`Fun`s `ParmNodes`. `FunNodes` extend `RegionNodes` (and `ParmNodes` extend -`PhiNodes`), so this linkage uses the same semantics. `Fun`s become reachable -when called; `Parm`s merge (*meet*) all calling arguments, and so on. -Similarly, `CallEnd`s merge results from all called functions, computing the -*meet* over all the `Return`s. - -This *link* step adds a few checks to our algorithm's inner loop, and also -builds a precise Call Graph which allows our SCCP to run interprocedurally. -Other than discovering a few new edges as the algorithm proceeds, the core -algorithm is unchanged. We are guaranteed to terminate with a fixed point -solution, and may discover e.g. certain call parameters are constants (or -e.g. not-null) on all calling paths. - -## Recursion is a loop in disguise - -We use the notion of *widening* integer ranges at loop heads, see Chapter XXX. -This allows us to have a sharper notion of integers over `BOTTOM` without also -allowing the lattice to fall 2^32 times. The same issue arises during -recursive calls: an argument can flow from a Parm through arithmetic into a -recursive call, then back to the same Parm. - -```simple -val fact = { int x -> x ? 0 : f(x-1)+1; }; -``` - -Here if we pass in `f(999)` , parameter `x` will have the integer range -`[999]`. We'll then track the recursive call to `f(998)` via normal SCCP -and realize the parameter `x` has a range `[999-998]`. Lather, rinse, repeat -and we're at `[999-997]`. We need to *widen* integer parameters at `Parms` -not just loop-head `Phis`. - -The same issue arises during the recursive return: the first return will be -with `0` when `x` is zero, then we return `0+1` or the range `[0-1]` then range -`[0-2]`, etc. `CallEnds` also need to widen. - -Loop Phis, Parms, and CallEnds get widening points, and since this same issue -hits in 3 unrelated places, their comments all cross-reference each other. - - -## Unknown callers - -A `FunPtrNode` (function address) that survives outside a direct call may be -called by code we cannot see. We have to assume the address escapes into -unknown code, which later calls it. We preserve its unknown-caller input, -since we cannot tell what that unknown caller will pass. Precise escape -analysis is introduced later exactly to inmprove this. - -## Building and checking - -Run `make lib` once for Java test dependencies, then `make tests` in this -directory. From the repository root, use -`make tests CHAPTERS=chapter-interprocedural-sccp`. -The inherited tests retain their historical class names. - -The [grammar](docs/24-grammar.md) includes chained comparisons inherited from -the extraction baseline. Their planned teaching home is with short-circuit -expressions. Allocator tuning is covered separately by the planned appendix. |