Phase 62A: C7 ULTRA Alloc Dependency Chain Trim - NEUTRAL (-0.71%)

Implemented C7 ULTRA allocation hotpath optimization attempt as per Phase 62A instructions.

Objective: Reduce dependency chain in tiny_c7_ultra_alloc() by:
1. Eliminating per-call tiny_front_v3_c7_ultra_header_light_enabled() checks
2. Using TLS headers_initialized flag set during refill
3. Reducing branch count and register pressure

Implementation:
- New ENV box: core/box/c7_ultra_alloc_depchain_opt_box.h
- HAKMEM_C7_ULTRA_ALLOC_DEPCHAIN_OPT=0/1 gate (default OFF)
- Modified tiny_c7_ultra_alloc() with optimized path
- Preserved original path for compatibility

Results (Mixed benchmark, 10-run):
- Baseline (OPT=0): 59.300 M ops/s (CV 1.98%)
- Treatment (OPT=1): 58.879 M ops/s (CV 1.83%)
- Delta: -0.71% (NEUTRAL, within ±1.0% threshold but negative)
- Status: NEUTRAL → Research box (default OFF)

Root Cause Analysis:
1. LTO optimization already inlines header_light function (call cost = 0)
2. TLS access (memory load + offset) not cheaper than function call
3. Layout tax from code addition (I-cache disruption pattern from Phases 43/46A/47)
4. 5.18% stack % is not optimizable hotspot (already well-optimized)

Key Lessons:
- LTO-optimized function calls can be cheaper than TLS field access
- Micro-optimizations on already-optimized paths show diminishing/negative returns
- 48.34% gap to mimalloc is likely algorithmic, not micro-architectural
- Layout tax remains consistent pattern across attempted micro-optimizations

Decision:
- NEUTRAL verdict → kept as research box with ENV gate (default OFF)
- Not adopted as production default
- Next phases: Option B (production readiness pivot) likely higher ROI than further micro-opts

Box Theory Compliance: ✅ Compliant (single point, reversible, clear boundary)
Performance Compliance: ❌ No (-0.71% regression)

Documentation:
- PHASE62A_C7_ULTRA_DEPCHAIN_OPT_RESULTS.md: Full A/B test analysis
- CURRENT_TASK.md: Updated with results and next phase options

🤖 Generated with Claude Code

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

core/box/c7_ultra_alloc_depchain_opt_box.h

@@ -0,0 +1,45 @@
+#ifndef C7_ULTRA_ALLOC_DEPCHAIN_OPT_BOX_H
+#define C7_ULTRA_ALLOC_DEPCHAIN_OPT_BOX_H
+
+// Phase 62A: C7 ULTRA Alloc Dependency Chain Trim
+//
+// 目的:
+//   - tiny_c7_ultra_alloc() の hot hit path の dependency chain を縮める
+//   - per-call header_light check を排除、TLS headers_initialized を活用
+//   - tiny_region_id_write_header() 呼び出しを最小化
+//   - Mixed 10-run で +1.0% GO、失敗時は NEUTRAL/NO-GO で research box化
+//
+// 最適化:
+//   1. header_light check を per-call から排除 → TLS headers_initialized で固定
+//   2. tiny_region_id_write_header() を必要時のみに（already initialized なら skip）
+//   3. refill 後の retry block を同じロジックで共有（レジスタ効率化）
+//
+// ENV:
+//   - HAKMEM_C7_ULTRA_ALLOC_DEPCHAIN=0/1 (default: 0, OFF)
+//
+// Box Theory:
+//   - Single conversion point: tiny_c7_ultra_alloc() 関数
+//   - Reversible: ENV gate で OFF に戻す
+//   - No side effects: Pure optimization, 新しいデータ構造なし
+
+#ifndef HAKMEM_C7_ULTRA_ALLOC_DEPCHAIN_OPT
+#define HAKMEM_C7_ULTRA_ALLOC_DEPCHAIN_OPT 0
+#endif
+
+#include <stdlib.h>
+
+// ENV gate (compile-time constant in BENCH_MINIMAL, runtime otherwise)
+static inline int c7_ultra_alloc_depchain_opt_enabled(void) {
+#if HAKMEM_BENCH_MINIMAL
+    return HAKMEM_C7_ULTRA_ALLOC_DEPCHAIN_OPT;  // FAST: compile-time constant
+#else
+    static int g_enable = -1;
+    if (__builtin_expect(g_enable == -1, 0)) {
+        const char* e = getenv("HAKMEM_C7_ULTRA_ALLOC_DEPCHAIN_OPT");
+        g_enable = (e && *e && *e != '0') ? 1 : 0;  // default OFF
+    }
+    return g_enable;
+#endif
+}
+
+#endif // C7_ULTRA_ALLOC_DEPCHAIN_OPT_BOX_H