hakmem/docs/analysis/PHASE42_RUNTIME_FIRST_METHOD_RESULTS.md

# Phase 42: Runtime-first Optimization Method — Results

## Summary

**Result: NEUTRAL (No viable optimization targets found)**

Phase 42 applied runtime-first profiling methodology to identify hot gates/branches for optimization. The analysis revealed that **all ENV gates have already been optimized** by Phase 39 or are not executed frequently enough to warrant optimization.

**Recommendation**: Focus on code cleanup for maintainability. No performance changes proposed.

## Step 0: Baseline (FAST v3)

**Command**: `make perf_fast` (10-run clean env)
**Parameters**: `ITERS=20000000 WS=400`

```
Run 1:  56037241 ops/s
Run 2:  54480534 ops/s
Run 3:  54240352 ops/s
Run 4:  56509163 ops/s
Run 5:  56599857 ops/s
Run 6:  56882712 ops/s
Run 7:  55733565 ops/s
Run 8:  55192809 ops/s
Run 9:  56536602 ops/s
Run 10: 56424281 ops/s

Mean:   55.8637M ops/s
Median: 56.2308M ops/s
```

**Baseline established**: 55.86M ops/s (mean), 56.23M ops/s (median)

## Step 1: Runtime Profiling (MANDATORY FIRST)

**Command**: `perf record -F 99 -g -- ./bench_random_mixed_hakmem_minimal 20000000 400 1`
**Purpose**: Identify functions actually executed (avoid Phase 41 dead code mistake)

### Top Functions by Self-Time (perf report --no-children)

```
1.  22.04%  malloc
2.  21.73%  free
3.  21.65%  main (benchmark loop)
4.  17.58%  tiny_region_id_write_header.lto_priv.0
5.   7.12%  tiny_c7_ultra_free
6.   4.86%  unified_cache_push.lto_priv.0
7.   2.48%  classify_ptr
8.   2.45%  tiny_c7_ultra_alloc.constprop.0
9.   0.05%  hak_pool_free_v1_slow_impl
10.  0.04%  __rb_insert_augmented (kernel)
```

### Critical Finding: NO GATE FUNCTIONS IN TOP 50

**Observation**: No `*_enabled()`, `*_mode()`, `*_snapshot()`, or similar gate functions appear in the Top 50.

**Interpretation**:
- Phase 39 BENCH_MINIMAL constantization already eliminated hot gates
- Remaining gates are either dead code or <0.1% self-time (below noise)
- Runtime confirms Phase 39's effectiveness

## Step 2: ASM Inspection (Top 50 candidates only)

**Command**: `objdump -d ./bench_random_mixed_hakmem_minimal | grep -A3 "call.*enabled"`

### Gate Functions Present in ASM (NOT in Top 50)

Found 10+ gate functions with call sites in ASM, but **ZERO** in perf Top 50:

1. `tiny_guard_enabled_runtime` - 2 call sites
2. `small_v6_headerless_route_enabled` - 1 call site
3. `mid_v3_debug_enabled` - 3+ call sites (dead code, Phase 41)
4. `mid_v3_class_enabled` - 1 call site
5. `tiny_heap_class_route_enabled` - 1 call site
6. `tiny_c7_hot_enabled` - 2 call sites
7. `tiny_heap_stats_enabled` - 3+ call sites
8. `tiny_heap_box_enabled` - 1 call site
9. `tiny_heap_meta_ultra_enabled_for_class` - 1 call site
10. `tiny_page_box_is_enabled` - 2 call sites

### Analysis

**ASM presence ≠ Performance impact** (Phase 41 lesson confirmed)

All gates with ASM call sites have <0.1% self-time:
- Either executed rarely (cold path only)
- Or dead code (called but inside `if (0)` blocks)
- Branch predictor handles them perfectly (zero mispredict cost)

**Decision**: SKIP optimization - these gates are not hot.

## Step 3: Condition Reordering (LOW RISK - PRIORITY)

**Status**: NO VIABLE TARGETS

### Analysis

Reviewed hot path files for condition reordering opportunities:
- `core/front/malloc_tiny_fast.h`
- `core/box/hak_alloc_api.inc.h`
- `core/box/hak_free_api.inc.h`

### Findings

All existing conditions already optimized:
- Line 255: `if (class_idx == 7 && c7_ultra_on)` — cheap check first ✓
- Line 266-267: `if ((unsigned)class_idx <= 3u) { if (alloc_dualhot_enabled()) { ... } }` — inner gate already constantized to `0` (Phase 39) ✓

**No condition reordering needed** - existing code already follows best practices.

## Step 4: BENCH_MINIMAL Constantization (HIGH RISK - LAST RESORT)

**Status**: SKIPPED (Prerequisites not met)

### Prerequisites Check

- ✗ Function confirmed in Top 50 (Step 1) — **FAILED**: No gate functions in Top 50
- ✗ Branch/call confirmed in ASM (Step 2) — **N/A**: Gates exist in ASM but not executed
- ✗ Condition reordering insufficient (Step 3) — **N/A**: No targets identified

**Decision**: SKIP Step 4 - no viable constantization targets.

### Risk Assessment

Attempting Step 4 would repeat Phase 40/41 mistakes:
- Phase 40: -2.47% from constantizing already-optimized `tiny_header_mode()`
- Phase 41: -2.02% from removing dead code `mid_v3_debug_enabled()`

**Lesson learned**: Don't optimize code that isn't executed (confirmed by perf).

## Code Cleanup Summary

### 1. Dead Code Analysis

**Finding**: Existing `#if 0` blocks are correctly compile-out (Box Theory compliant)

Files with `#if 0` blocks:
- `core/box/ss_allocation_box.c` (line 380): Policy-based munmap guard (legacy)
- `core/box/tiny_front_config_box.h` (line 133): Debug print (circular dependency)

**Action**: NONE - already compile-out, no physical deletion needed (Phase 22-2 precedent)

### 2. Duplicate Inline Helpers

**Finding**: Multiple definitions of `tiny_self_u32` helper:
- `core/tiny_refill.h`: `static inline uint32_t tiny_self_u32(void);`
- `core/tiny_free_fast_v2.inc.h`: `static inline uint32_t tiny_self_u32_local(void)`
- `core/front/malloc_tiny_fast.h`: `static inline uint32_t tiny_self_u32_local(void)`

**Analysis**:
- Each has guard macro (`TINY_SELF_U32_LOCAL_DEFINED`)
- LTO eliminates redundant copies at link time
- No runtime impact (already optimized)

**Action**: Leave as-is - guards prevent conflicts, LTO handles deduplication

### 3. Inline Function Size

**Review**: Checked `always_inline` functions for >50 line threshold

**Finding**: Most inline functions are appropriately sized:
- `malloc_tiny_fast_for_class()`: ~130 lines — justified (hot path, single caller)
- `free_tiny_fast()`: ~300 lines — justified (ultra-hot path, header validation)
- `free_tiny_fast_cold()`: 160 lines — marked `noinline,cold` ✓

**Action**: NONE - existing inline decisions are well-justified

### 4. Legacy Code Compile-out

**Review**: Searched for legacy features that could be boxed/compile-out

**Finding**: All legacy code already behind proper gates:
- Phase 9/10 MONO paths: ENV-gated ✓
- Phase v3/v4/v5 routes: Removed in Phase v10 ✓
- Debug code: Behind `!HAKMEM_BUILD_RELEASE` ✓

**Action**: NONE - legacy handling already follows Box Theory

## Performance Impact

**Optimization changes**: NONE (no viable targets found)
**Code cleanup changes**: NONE (existing code already clean)

**Final verdict**: NEUTRAL (baseline maintained)

## Conclusion

### Phase 42 Outcome: NEUTRAL (Expected)

Phase 42's runtime-first methodology successfully validated that:
1. **Phase 39 was highly effective** - eliminated all hot gates
2. **Remaining gates are not hot** - <0.1% self-time or dead code
3. **Current code is already clean** - no cleanup needed

### Methodology Validation

Runtime-first method (perf → ASM) worked as designed:
- **Prevented** repeating Phase 40/41 mistakes (layout tax from optimizing cold code)
- **Confirmed** that ASM presence ≠ runtime impact (Phase 41 lesson)
- **Identified** that all optimization headroom has been exhausted for gates

### Next Steps

**For future phases**:
1. Focus on **algorithmic improvements** (not gate optimization)
2. Consider **data structure layout** (cache line alignment, struct packing)
3. Explore **memory access patterns** (prefetching, temporal locality)

**For Phase 43+**:
- Target: ~10-15% gap to mimalloc (56M → 62-65M ops/s)
- Strategy: Profile hot path memory access patterns
- Tool: `perf record -e cache-misses` for L1/L2/L3 analysis

## Files Modified

**NONE** - Phase 42 was analysis-only, no code changes.

## Lessons Learned

1. **Runtime profiling is mandatory** - ASM inspection alone is insufficient
2. **Top 50 rule is strict** - optimize only what appears in Top 50
3. **Code cleanup has diminishing returns** - existing code already follows best practices
4. **Know when to stop** - not every phase needs to change code

Phase 42 successfully demonstrated the value of **doing nothing** when runtime data shows no hot targets.