optimize arithmetics

docs/pi_spigot_perf_baseline.md

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+## Pi Spigot JVM Baseline
+
+Saved on April 4, 2026 before the `List<Int>` indexed-access follow-up fix.
+
+Benchmark target:
+- [examples/pi-bench.py](/home/sergeych/dev/lyng/examples/pi-bench.py)
+- [examples/pi-bench.lyng](/home/sergeych/dev/lyng/examples/pi-bench.lyng)
+
+Execution path:
+- Python: `python3 examples/pi-bench.py`
+- Lyng JVM: `./gradlew :lyng:runJvm --args='/home/sergeych/dev/lyng/examples/pi-bench.lyng'`
+- Constraint: do not use Kotlin/Native `lyng` CLI for perf comparisons
+
+Baseline measurements:
+- Python full script: `167 ms`
+- Lyng JVM full script: `1.287097604 s`
+- Python warm function average over 5 runs: `126.126 ms`
+- Lyng JVM warm function average over 5 runs: about `1071.6 ms`
+
+Baseline ratio:
+- Full script: about `7.7x` slower on Lyng JVM
+- Warm function only: about `8.5x` slower on Lyng JVM
+
+Primary finding at baseline:
+- The hot `reminders[j]` accesses in `piSpigot` were still lowered through boxed object index ops and boxed arithmetic.
+- Newly added `GET_INDEX_INT` and `SET_INDEX_INT` only reached `pi`, not `reminders`.
+- Root cause: initializer element inference handled list literals, but not `List.fill(boxes) { 2 }`, so `reminders` did not become known `List<Int>` at compile time.
+
+## After Optimizations 1-4
+
+Follow-up change:
+- propagate inferred lambda return class into bytecode compilation
+- infer `List.fill(...)` element type from the fill lambda
+- lower `reminders[j]` reads and writes to `GET_INDEX_INT` and `SET_INDEX_INT`
+- add primitive-backed `ObjList` storage for all-int lists
+- lower `List.fill(Int) { Int }` to `LIST_FILL_INT`
+- stop boxing the integer index inside `GET_INDEX_INT` / `SET_INDEX_INT`
+
+Verification:
+- `piSpigot` disassembly now contains typed ops for `reminders`, for example:
+  - `GET_INDEX_INT s5(reminders), s10(j), ...`
+  - `SET_INDEX_INT s5(reminders), s10(j), ...`
+
+Post-change measurements using `jlyng`:
+- Full script: `655.819559 ms`
+- Warm 5-run total: `1.430945810 s`
+- Warm average per run: about `286.2 ms`
+
+Observed improvement vs baseline:
+- Full script: about `1.96x` faster (`1.287 s -> 0.656 s`)
+- Warm function: about `3.74x` faster (`1071.6 ms -> 286.2 ms`)
+
+Residual gap vs Python baseline:
+- Full script: Lyng JVM is still about `3.9x` slower than Python (`655.8 ms` vs `167 ms`)
+- Warm function: Lyng JVM is still about `2.3x` slower than Python (`286.2 ms` vs `126.126 ms`)
+
+Current benchmark-test snapshot (`n=200`, JVM test harness):
+- `optimized-int-division-rval-off`: `135 ms`
+- `optimized-int-division-rval-on`: `125 ms`
+- `piSpigot` bytecode now contains:
+  - `LIST_FILL_INT` for both `pi` and `reminders`
+  - `GET_INDEX_INT` / `SET_INDEX_INT` for the hot indexed loop