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diff --git a/chapter18/README.md b/chapter18/README.md index 1727c938..42f76227 100644 --- a/chapter18/README.md +++ b/chapter18/README.md @@ -7,6 +7,7 @@ 3. [Functions](#functions) 4. [CodeGen - The Compile Driver](#codegen---the-compile-driver) 5. [Graph Visualizer](#graph-visualizer) +6. [Memory across functions](#memory-across-functions) You can also read [this chapter](https://github.com/SeaOfNodes/Simple/tree/linear-chapter18) in a linear Git revision history on the [linear](https://github.com/SeaOfNodes/Simple/tree/linear) branch and [compare](https://github.com/SeaOfNodes/Simple/compare/linear-chapter17...linear-chapter18) it to the previous chapter. @@ -236,3 +237,32 @@ Nodes are color coded according to type, same as the lattice diagrams. Nodes are shaped according to node function as well. At the bottom are `ScopeNode`s, which only exist for the Parser but are actual Nodes and have `use->def` edges into the IR. + +## Memory across functions + +Each function has one `$mem` parameter and the parser keeps one memory binding. +Branches, loops, and multiple returns merge whole memory with BulkMemPhi. Field +uses discover precise MemPhis, while MemMerge packages a default memory state +and its alias overrides. MemMerge is an ordinary graph node; it no longer doubles +as a mutable parser alias table. + +| Boundary | Memory convention | +|---|---| +| Function entry | Parm 1 carries whole memory; heap contents are unknown | +| Call inputs | `{ctrl, $mem, arguments..., function pointer}` | +| CallEnd outputs | `{ctrl, $mem, value}` | +| Return inputs | `{ctrl, $mem, value, rpc}` | +| New inputs | `{ctrl, $mem, size, field values...}` | +| New outputs | `{ptr, $mem}`, covering only the allocated struct's aliases | + +A function may receive an existing object, so its entry memory cannot be treated +as an empty heap. Calls conservatively affect every alias. The returned memory +becomes the caller's new whole-memory state; there is no per-function alias +summary. Trivial inlining removes this boundary and lets the ordinary memory +rewrites see through the function body. + +Scheduling follows the relevant alias through MemMerge when looking for writes +or calls that must follow a Load. Calls end their basic block, so moving the Load +before that terminator supplies the required order without appending scheduling +edges to the argument list. Phi factoring keeps the simple one-step Load guard; +a direct Call user blocks that rewrite. |