CPU Hotpath Overview (bench profile)
====================================

Context
-------
- Build/profile: `HAKMEM_PROFILE=bench`, `HAKMEM_TINY_PROFILE=full`, `HAKMEM_WARM_TLS_BIND_C7=2`.
- Workloads sampled:
  - 16–1024B (`./bench_random_mixed_hakmem 1000000 256 42`)
  - 129–1024B (`HAKMEM_BENCH_MIN_SIZE=129 HAKMEM_BENCH_MAX_SIZE=1024 ./bench_random_mixed_hakmem 1000000 256 42`)
- Target: identify user‑space hot spots to guide C7 flattening work.

Sampling attempt (perf)
-----------------------
- `perf record -g -e cycles:u` and `perf record -g -e cpu-clock:u` both fall back to `page-faults` on this host (likely perf_event_paranoid). The captures show:
  - ~97% of page-fault samples in `__memset_avx2_unaligned_erms` during warmup/zeroing.
  - Callers were `tiny_tls_sll_drain.part.0.constprop.0` and `adaptive_sizing_init` (warmup path).
- No steady‑state cycle profile was available without elevated perf permissions. `perf data` removed after inspection (`rm perf.data`) to keep tree clean.

What we can infer despite the limitation
----------------------------------------
- Warmup zeroing dominates page‑fault samples; steady‑state alloc/free is not represented.
- Hot candidates for the next pass (from previous code inspection and bench intuition):
  - `tiny_alloc_fast` / `malloc_tiny_fast` (C7 fast path)
  - `hak_tiny_free_fast_v2`
  - `tiny_unified_cache` hit path helpers
  - `tls_sll_pop_impl` / `tiny_tls_sll_drain`

Next measurement options
------------------------
- If perf cycles are still blocked:
  - Use `perf stat -e cycles,instructions,branches,branch-misses -r 5 -- ...` to get aggregate IPC per workload.
  - Add temporary user‑space counters (Box‑guarded) around C7 alloc/free hot sections to estimate per‑op cycles.
  - Run perf with elevated permissions or lower `perf_event_paranoid` if available.

Aggregate perf stat snapshot (bench profile)
--------------------------------------------
- Env: `HAKMEM_PROFILE=bench HAKMEM_TINY_PROFILE=full HAKMEM_WARM_TLS_BIND_C7=2`
- Workloads (Release, 3× perf stat):
  - 16–1024B: cycles≈109.8M, instructions≈233.2M → IPC≈2.12, branches≈49.4M, branch-miss≈2.89%
  - 129–1024B: cycles≈109.6M, instructions≈230.3M → IPC≈2.10, branches≈48.8M, branch-miss≈2.90%
  - 16–1024B with `HAKMEM_TINY_C7_HOT=1` (UC hit-only + flat TLS→UC→cold):
    - cycles≈111.8M, instructions≈242.1M → IPC≈2.16, branches≈52.0M, branch-miss≈2.75%
    - RSS≈7.1MB; throughput ≈47.4–47.6M ops/s (hot=1) vs ≈47.2M (hot=0) on the same runset.

Action items flowing from this note
-----------------------------------
- Proceed with design notes for C7 alloc/free flattening and UC hit simplification based on code structure.
- Keep warmup zeroing out of the steady‑state loop when profiling (consider `HAKMEM_BENCH_FAST_MODE` for future captures).

Conclusion (current state)
--------------------------
- `HAKMEM_TINY_C7_HOT` は実験用フラグとして残し、デフォルト OFF のまま運用する。ON にしても branch-miss はわずかに改善する程度で、ops/s は同等〜微減。
- ひとまず「安全＋そこそこ速い」現行経路を基準とし、さらなるフラット化は別途必要性を見て検討する。***