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diff --git a/chapter10b/README.md b/chapter10b/README.md new file mode 100644 index 00000000..795d2233 --- /dev/null +++ b/chapter10b/README.md @@ -0,0 +1,190 @@ +# Chapter 10b: Splitting Memory + +[Previous: Chapter 10a](../chapter10a/README.md) | +[Next: Chapter 11](../chapter11/README.md) + +Chapter 10a made memory effects explicit with one SSA memory value. This +chapter keeps exactly that parser model and recovers independent memory chains +through graph rewrites. There is no new language syntax; the +[grammar](docs/10-grammar.md), pointer types, and null checks are unchanged. + +## Why split memory? + +```java +struct Point { int x; int y; } +Point p = new Point; +p.x = 42; +while (arg > 0) { + p.y = p.y + arg; + arg = arg - 1; +} +return p.x; +``` + +The loop changes `y`, so it cannot affect `x`. With one memory chain the load +of `x` follows the loop's stores. Splitting the fields lets ordinary +load-after-store forwarding turn the return value into 42. + + + +These are schematic dependency graphs, with arrows from uses to their definitions. +In horizontal diagrams, definitions appear on the left and uses on the right; +in vertical diagrams, definitions appear above uses. +Control, pointer, and value edges are omitted when they do not explain memory. + +## Equivalence-class aliasing + +Each struct field has an integer alias identity. Two different fields receive +different identities, even if they have the same spelling in different structs. +Two instances of the same struct share their field aliases. Thus different +classes never alias; accesses in the same class **may alias**. Same-class does +not mean same-address: forwarding still checks pointer identity. + +The parser assigns field identities at struct declarations, beginning at 2. +These identities are independent of Start's output indices. The default slot +of a memory aggregate is 1; precise slots are greater than 1. + +We do not allocate a Start projection, scope binding, or loop Phi for every +declared field. Start supplies one memory value, Scope holds one `$mem` binding, +and Return consumes one complete memory value. The optimizer introduces slices +only where graph operations require them. + +## Three memory nodes + +| Node | Meaning | +|---|---| +| `MemMerge` | A complete memory state: a default plus precise alias overrides | +| `MemPhi` | One alias selected according to the incoming control path | +| `BulkMemPhi` | All aliases except an explicit exclusion set, selected by control | + +`MemMerge` combines independent pieces of memory at one program point. It is +not a control-flow merge. For example, after storing to alias `x`: + +```text +prior = current memory +store = Store_x(prior, pointer, value) +current memory = MemMerge(default=prior, x=store) +``` + +The Store selects its precise input from any incoming MemMerge. Loads and +precise memory Phis do the same. A missing precise entry falls back to the +default, provided that default still covers the requested alias. + +The explicit entries override the default: the semantic slices are disjoint, +and their union covers all memory. Flattening stacked aggregates preserves +entries that the outer aggregate has not replaced. This allows an `x` store +and a later `y` store to remain visible together at Return. + + + +## Splitting a memory Phi + +Scope initially creates a BulkMemPhi wherever the ordinary SSA algorithm needs +to merge memory. Its empty exclusion set means that it covers every alias. +After the Region is complete, the Phi inspects its memory inputs and users. +When they require a precise alias, it splits out that alias: + +```text +BulkPhi(R, left, right) + becomes +MemMerge( + default = BulkPhi_except{x}(R, left, right), + x = MemPhi_x(R, slice(left,x), slice(right,x))) +``` + +The new precise Phi and the remaining bulk Phi share a Region and matching +control-path indices. A loop uses the same transformation, with the backedge +among those inputs. Each further split preserves all previously extracted +slices; replacing a bulk Phi must never drop their parallel precise Phis. + + + +A bulk Phi can bypass an incoming aggregate only when all of that aggregate's +explicit slices are already excluded from the Phi. A precise consumer can +select one entry. A whole-memory consumer cannot just select the default: +that would lose the explicit updates. + +## Types and the worklist + +The type lattice retains Chapter 10a's six domains, colors, and notation: +`⊤` means top, `⊥` means bottom, and a suffix names the domain. The other five +domains are unchanged; memory now adds a flat element `MEM#N` for each precise +alias between memory top (`TypeMem.TOP`) and memory bottom (`TypeMem.BOT`). + + + +The parallel `MEM#2`, `MEM#3`, and further alias elements describe independent +field slices. For the first `Point` declaration above, these two aliases identify +`Point.x` and `Point.y`. Meeting different aliases gives memory bottom; joining +them gives memory top. Each precise alias is its own dual. The ellipsis stands +for more parallel aliases, not a type representing a set of aliases. + +Memory bottom covers all memory. The default aggregate slot numbered 1 is a +graph input position, not a precise field alias in this diagram. Types do not +track field values, private objects, or escape information. Bulk exclusion sets +and the default-plus-overrides structure of MemMerge belong to graph nodes, +not to an expanded type lattice. As in Chapter 10a, diagram edges show lattice +order, with top above bottom; field and tuple-element type lattices are elided. + +Scalar Phis remain ordinary PhiNodes. The scope's Phi factory chooses a +BulkMemPhi for whole memory; splitting constructs MemPhis explicitly. Generic +arithmetic factoring through Phis must not factor a memory aggregate. + +Memory splitting can increase node count. Progress comes from extracting an +alias from a bulk Phi and simplifying its precise chain, rather than from +requiring every rewrite to shrink the graph. Nodes must have conservative +initial types before introducing cyclic inputs. New bulk and precise Phis are +queued, so a recursive peephole cannot start another split halfway through the +first one. + +This also extends the worklist lesson from Chapter 9. A bulk Phi inspects its +users, so changes to a user's partition must wake the producer. Selecting a +slice from an aggregate can expose a new bulk-Phi user; that selected definition +must be queued too. The optimizer's worklist completeness assertion remains +enabled and checks these dependencies. + +## Evaluation and ordering + +MemMerge is a dependency node; it emits no heap operation. Its evaluator result +is a memory token, while Store mutates the object's fields. The scheduler must +distinguish aggregation from overwriting memory. A MemMerge is not a clobber, +and stores in different alias classes do not impose load/store anti-dependencies. + + + +Tests cover independent-field folding, parallel memory Phis, multiple field +updates surviving at Return, early returns, nested loops with break/continue, +and reads before subsequent stores through potentially aliased pointers. + +## What remains for later chapters? + +Chapter 10b knows every access's alias while parsing. What is lazy here is the +construction of memory partitions. Chapter 25 also allows an existing access's +alias to become known later, using the same representation. Its private +constructor memory, escape tracking, incomplete types, and module alias +remapping are separate extensions and are not needed here. + +Chapters 11-24 currently retain their older parser-managed memory chains. +Forwarding this representation through those snapshots is tracked in the +[backport queue](../docs/chapter-backports.md). Chapter 11's scheduling lesson +therefore still shows the older graph construction, with the same language. + +## Build and test + +With JDK 21 or newer, from this directory: + +```sh +make lib +make tests +make release +make view +``` + +Make builds this complete snapshot without Maven. `make release` writes +`build/release/simple.jar`; `make view` launches the shared graph viewer. +Both the root Maven reactor and the supplied IDEA module descriptor include +the chapter. From the repository root, test both halves with: + +```sh +make tests CHAPTERS="chapter10a chapter10b" +``` |