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Phase 86: Free Path Legacy Mask (NO-GO, +0.25%)

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## Summary

Implemented Phase 86 "mask-only commit" optimization for free path:
- Bitset mask (0x7f for C0-C6) to identify LEGACY classes
- Direct call to tiny_legacy_fallback_free_base_with_env()
- No indirect function pointers (avoids Phase 85's -0.86% regression)
- Fail-fast on LARSON_FIX=1 (cross-thread validation incompatibility)

## Results (10-run SSOT)

**NO-GO**: +0.25% improvement (threshold: +1.0%)
- Control:    51,750,467 ops/s (CV: 2.26%)
- Treatment:  51,881,055 ops/s (CV: 2.32%)
- Delta:      +0.25% (mean), -0.15% (median)

## Root Cause

Competing optimizations plateau:
1. Phase 9/10 MONO LEGACY (+1.89%) already capture most free path benefit
2. Remaining margin insufficient to overcome:
   - Two branch checks (mask_enabled + has_class)
   - I-cache layout tax in hot path
   - Direct function call overhead

## Phase 85 vs Phase 86

| Metric | Phase 85 | Phase 86 |
|--------|----------|----------|
| Approach | Indirect calls + table | Bitset mask + direct call |
| Result | -0.86% | +0.25% |
| Verdict | NO-GO (regression) | NO-GO (insufficient) |

Phase 86 correctly avoided indirect call penalties but revealed architectural
limit: can't escape Phase 9/10 overlay without restructuring.

## Recommendation

Free path optimization layer has reached practical ceiling:
- Phase 9/10 +1.89% + Phase 6/19/FASTLANE +16-27% ≈ 18-29% total
- Further attempts on ceremony elimination face same constraints
- Recommend focus on different optimization layers (malloc, etc.)

## Files Changed

### New
- core/box/free_path_legacy_mask_box.h (API + globals)
- core/box/free_path_legacy_mask_box.c (refresh logic)

### Modified
- core/bench_profile.h (added refresh call)
- core/front/malloc_tiny_fast.h (added Phase 86 fast path check)
- Makefile (added object files)
- CURRENT_TASK.md (documented result)

All changes conditional on HAKMEM_FREE_PATH_LEGACY_MASK=1 (default OFF).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>
This commit is contained in:
Moe Charm (CI)
2025-12-18 22:05:34 +09:00
parent 89a9212700
commit e4c5f05355
15 changed files with 1518 additions and 3 deletions
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35
core/front/malloc_tiny_fast.h
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@ -74,6 +74,8 @@
#include "../box/free_cold_shape_stats_box.h" // Phase 5 E5-3a: Free cold shape stats
#include "../box/free_tiny_fast_mono_dualhot_env_box.h" // Phase 9: MONO DUALHOT ENV gate
#include "../box/free_tiny_fast_mono_legacy_direct_env_box.h" // Phase 10: MONO LEGACY DIRECT ENV gate
#include "../box/free_path_commit_once_fixed_box.h" // Phase 85: Free path commit-once (LEGACY-only)
#include "../box/free_path_legacy_mask_box.h" // Phase 86: Free path legacy mask (mask-only, no indirect calls)
#include "../box/alloc_passdown_ssot_env_box.h" // Phase 60: Alloc pass-down SSOT
// Helper: current thread id (low 32 bits) for owner check
@ -955,6 +957,39 @@ static inline int free_tiny_fast(void* ptr) {
// Phase 19-3b: Consolidate ENV snapshot reads (capture once per free_tiny_fast call).
const HakmemEnvSnapshot* env = hakmem_env_snapshot_enabled() ? hakmem_env_snapshot() : NULL;
// Phase 86: Free path legacy mask - Direct early exit for LEGACY classes (no indirect calls)
// Conditions:
// - ENV: HAKMEM_FREE_PATH_LEGACY_MASK=1
// - class_idx in legacy_mask (LEGACY route, not ULTRA/MID/V7)
// - LARSON_FIX=0 (checked at startup, fail-fast if enabled)
if (__builtin_expect(free_path_legacy_mask_enabled_fast(), 0)) {
if (__builtin_expect(free_path_legacy_mask_has_class((unsigned)class_idx), 0)) {
// Direct path: Call legacy handler without policy snapshot, route, or mono checks
tiny_legacy_fallback_free_base_with_env(base, (uint32_t)class_idx, env);
return 1;
}
}
// Phase 85: Free path commit-once (LEGACY-only) - Skip policy/route/mono ceremony for committed C4-C7
// Conditions:
// - ENV: HAKMEM_FREE_PATH_COMMIT_ONCE=1
// - class_idx in C4-C7 (129-256B LEGACY classes)
// - Pre-computed at startup that class can use commit-once
// - LARSON_FIX=0 (checked at startup, fail-fast if enabled)
if (__builtin_expect(free_path_commit_once_enabled_fast(), 0)) {
if (__builtin_expect((unsigned)class_idx >= 4u && (unsigned)class_idx <= 7u, 0)) {
const unsigned cache_idx = (unsigned)class_idx - 4u;
const struct FreePatchCommitOnceEntry* entry = &g_free_path_commit_once_entries[cache_idx];
if (__builtin_expect(entry->can_commit, 0)) {
// Direct path: Call handler without policy snapshot, route, or mono checks
FREE_PATH_STAT_INC(commit_once_hit);
entry->handler(base, (uint32_t)class_idx, env);
return 1;
}
}
}
// Phase 9: MONO DUALHOT early-exit for C0-C3 (skip policy snapshot, direct to legacy)
// Conditions:
// - ENV: HAKMEM_FREE_TINY_FAST_MONO_DUALHOT=1