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diff --git a/chapter25/README.md b/chapter25/README.md index 971c4746..b6a7ded5 100644 --- a/chapter25/README.md +++ b/chapter25/README.md @@ -122,3 +122,66 @@ the environment; the Makefile currently defaults to x86-64/win64. Compiler changes can invalidate both serialized IR and native code in `sys.o`. Rebuild it before interpreting linked-program test results. A source-only subset is not equivalent to this chapter's full `make tests`. + +## RegAlloc improvements: live-range area and split cost + +Chapter 24 tried cold copies before splitting a loop Phi's hot edges. This +chapter also estimates how much scheduled code a spill frees from register +pressure, divided by the estimated cost of the new copies. Callee-save ranges +cover a whole function. Cloneable constants can be rebuilt nearer their uses. +Copies inside loops cost more: the estimate weights them by `8^loopDepth`. +These are inexpensive approximations, especially for values with many uses. + +The other Chapter 25 conflict-handling rules remain here: direct splitting of +several fixed-register uses and deeper splits for some multi-definition ranges. +Their size thresholds are heuristics, not general invariants to backport to the +first allocator. Shared legality fixes still start in the earliest affected +chapter. The final audit corrected fixed-neighbor color bias in 20-25, narrow +store masks in 20-21, and ARM register-bank/float-memory encoding in 21-25. + +Run `make spill-stats`. Each row uses **Chapter 25's compiler**. The first five +rows preserve the earlier 212 program/target entries, adapted to this chapter's +syntax and library organization; the [fixture notes](src/test/java/com/seaofnodes/simple/spill/README.md) +list those adaptations and the measurement limits. They use optimizer seed 123. +The last row includes nine allocations from `Chapter25Test` at its existing +seeds plus a fresh system-library compilation at the driver's seed 456. Reusing +`sys.o` must not make that substantial workload disappear from the measurements. +These results are from Windows x86-64; earlier cohorts also include RISC-V/ARM +SystemV and x86 SystemV. The reporter prints per-target totals as well. + +| Program cohort | Compilations | Split/move count | Loop-weighted count | +|---|---:|---:|---:| +| Chapter 20 | 39 | 381 | 612 | +| Chapter 21 | 52 | 464 | 1,185 | +| Chapter 22 | 24 | 103 | 131 | +| Chapter 23 | 30 | 122 | 346 | +| Chapter 24 | 67 | 744 | 1,479 | +| Chapter 25 | 10 | 783 | 1,826 | +| **Total** | **222** | **2,597** | **5,579** | + +`_spills` counts retained SplitNodes, including register-to-register moves; +`_spillScaled` applies the loop weight. Neither measures just memory traffic. +The old cohorts are allocation replays with register-legality checks, not reruns +of their native harnesses. `Chapter25Test` still runs its native/result checks. +Use `make -j 4 tests` for the full chapter, including inherited emulator tests. + +### Comments on the measurements + +There is no uniform improvement to claim. Substituting Chapter 24's ranking +while keeping the rest of this compiler unchanged saves five moves on the 221 +client compilations (2,206 / 4,719 versus 2,211 / 4,724), but fails to allocate +the system library within eight rounds. It is therefore an incomplete comparison, +not a lower valid whole-suite total. The existing area/cost ranking is retained; +the round limit and correctness checks are unchanged. The system library itself +contributes 386 moves / 855 weighted moves. + +A trial that favored all multi-use cloneable constants saved one move against +the earlier ranking on those clients, but also failed the fresh library build. +It was rejected. This is why both aggregate statistics and complete compilation +and execution checks matter; a favorable subtotal cannot justify a change. + +The earlier-cohort total rises from Chapter 24's 1,332 / 2,956 to 1,814 / 3,753. +Chapter 25 adds class initializers, module ownership, different escape analysis, +and syntax/library adaptations. These are different machine graphs; the increase +cannot be attributed to allocator quality alone. Compare a heuristic within one +compiler and fixed corpus before drawing conclusions from the chapter tables. |