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diff --git a/chapter24/README.md b/chapter24/README.md index 00d4c58c..07c63ee3 100644 --- a/chapter24/README.md +++ b/chapter24/README.md @@ -282,7 +282,7 @@ 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, and allows our SCCP to run interprocedurally. +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 @@ -292,68 +292,32 @@ e.g. not-null) on all calling paths. ## RegAlloc improvements: try cold loop splits first A loop Phi and its backedge can belong to one live range even though their -values must coexist at some instruction. This self-conflict requires splitting -the range. Chapter 23 split the Phi's definition and incoming values right away. +values must coexist at some instruction. This self-conflict requires splitting +the range. Chapter 23 split the Phi's definition and incoming values right away. This chapter first leaves the hot definition/backedge alone and tries the -outside-loop copies. If coloring succeeds, it avoids moves on every iteration. +outside-loop copies. If coloring succeeds, it avoids moves on every iteration. Other conflicts, including two-address operations, keep their existing handling. -The allocator remembers which Phis have had this chance. If the same Phi +The allocator remembers which Phis have had this chance. If the same Phi self-conflicts again, the next attempt uses the ordinary aggressive splitting. -The record uses Phi identity because live ranges are rebuilt each round. It +The record uses Phi identity because live ranges are rebuilt each round. It also covers a backedge-only conflict where the first attempt inserts no copy: -that attempt must still consume the one permitted delay. A regression checks +that attempt must still consume the one permitted delay. A regression checks both the cheaper first attempt and the mandatory fallback. -Chapter 21's adjacent-copy forwarding also carries forward: after coloring, -a sole use in the next instruction can read the source register when its -operand mask permits it and no two-address tie is broken. The table below -includes this cleanup and the lazy memory forward port. Cohort 22 now also includes the two zero-move C return -ABI checks added since the earlier audit (26 entries instead of 24). - -Run `make spill-stats`. All rows use **Chapter 24's compiler**, optimizer seed -123, and the same source/target combinations as the earlier cohort tables. -These Windows results combine x86 SystemV/Win64 and RISC-V/ARM SystemV, -remeasured after the lazy memory forward port. - -| Program cohort | Compilations | Retained moves | Loop-weighted moves | -|---|---:|---:|---:| -| Chapter 20 | 39 | 320 | 439 | -| Chapter 21 | 52 | 420 | 952 | -| Chapter 22 | 26 | 67 | 67 | -| Chapter 23 | 30 | 78 | 225 | -| Chapter 24 | 67 | 422 | 1,325 | -| **Total** | **214** | **1,307** | **3,008** | - -`_spills` counts retained SplitNodes, including register moves. `_spillScaled` -weights those moves by `8^loopDepth`; it is a cost estimate, not measured memory -traffic. Diagnostic machine graphs contribute no allocations to this table. -The reporter checks allocation legality and native/emulated results even when -spill expectations differ, and exits unsuccessfully for either kind of failure. - -In the original 2026-09-20 allocator audit, before subsequent compiler fixes, -the table totaled 1,332 / 2,956. With only the cold-first rule disabled, that -compiler and the same programs produced -**1,338 moves / 2,962 weighted moves**. The rule saves six of each. A RISC-V -MergeSort case adds one move, offset by improvements in other MergeSort cases, -Sieve, and an ARM loop case. Aggregate measurements justify accepting that -local regression; the improvement here is modest, not universal. - -On cohorts 20-23 alone in that audit, Chapter 23 produced 933 / 1,745; Chapter 24 produced -900 / 1,614. Only six of each reduction comes from this allocator change. The -rest comes from other compiler changes, including SCCP; comparing chapters -alone would overstate the heuristic's benefit. - -The older cohorts retain the 64-bit Person, guarded String, and smaller emulator -argument list. Revised Jig, BubbleSort, Newton, and stack-argument inputs belong -to cohort 24 alongside this chapter's existing tests. The frozen String workload -also exposed a missing null check in the inlining dominator walk; that correctness -fix is included on both sides of the heuristic comparison. Area/cost spill -ranking remains for Chapter 25. - -The current function-pointer rule keeps unknown callers whenever an address is -used as a value rather than only as a direct call target. This preserves callable -bodies without escape analysis, but Phi-selected functions retain general -parameter types. The no-return String case now drops its unused hash bodies and -needs no splits. Source/target membership remains unchanged; these graph changes -are separate from the cold-first allocator comparison above. +Rows are fixed test cohorts; columns are the compiler/allocator chapters. Each +cell is the sum of `_spillScaled`: retained split moves, including register +copies, weighted by loopDepth. Lower is better. **# tests counts +compilations** (program/CPU/ABI cases), including zero-spill cases, rather than +JUnit methods. The count and membership of each row stay fixed across columns. + +<!-- spill-matrix:start --> +| Test cohort | # tests | Ch 20 | Ch 21 | Ch 22 | Ch 23 | Ch 24 | +|---|---:|---:|---:|---:|---:|---:| +| Ch 20 | 39 | 360 | 576 | 430 | 442 | 441 | +| Ch 21 | 52 | | 1,072 | 960 | 961 | 948 | +| Ch 22 | 26 | | | 63 | 63 | 63 | +| Ch 23 | 24 | | | | 103 | 103 | +| Ch 24 | 58 | | | | | 1,387 | +<!-- spill-matrix:end --> + |