hakmem/core/box/ss_hot_cold_box.h

// ss_hot_cold_box.h - Phase 3d-C: Hot/Cold Slab Split Box
// Purpose: Cache locality optimization via hot/cold slab separation
// License: MIT
// Date: 2025-11-20

#ifndef SS_HOT_COLD_BOX_H
#define SS_HOT_COLD_BOX_H

#include "../superslab/superslab_types.h"
#include <stdbool.h>

// ============================================================================
// Phase 3d-C: Hot/Cold Split Box API
// ============================================================================
//
// Goal: Improve L1D cache hit rate by separating hot (high utilization) and
//       cold (low utilization) slabs within a SuperSlab.
//
// Strategy:
// - Hot slabs (used > 50%): Prioritized for allocation → better cache locality
// - Cold slabs (used ≤ 50%): Used as fallback → delayed deallocation
//
// Expected: +8-12% throughput from improved cache line locality
//
// Box Contract:
// - ss_is_slab_hot(): Returns true if slab should be considered "hot"
// - ss_update_hot_cold_indices(): Rebuilds hot/cold index arrays
// - ss_init_hot_cold(): Initializes hot/cold fields on SuperSlab creation
//
// ============================================================================

// Phase 3d-C: Hot/Cold判定閾値
#define HOT_UTILIZATION_THRESHOLD 50  // 使用率50%以上でホット判定

// Phase 12-1.1: EMPTY判定ロジック（最優先再利用）
// Returns: true if slab is completely EMPTY (used == 0, highest reuse priority)
static inline bool ss_is_slab_empty(const TinySlabMeta* meta) {
    return (meta->capacity > 0 && meta->used == 0);
}

// Phase 3d-C: Hot判定ロジック
// Returns: true if slab is "hot" (high utilization, should be prioritized)
static inline bool ss_is_slab_hot(const TinySlabMeta* meta) {
    // ヒューリスティック: 使用率 > 50% → ホット
    // 理由: 使用率が高い = 頻繁にアクセスされている = キャッシュに載せたい
    if (meta->capacity == 0) {
        return false;  // Uninitialized slab
    }
    return (meta->used * 100 / meta->capacity) > HOT_UTILIZATION_THRESHOLD;
}

// Phase 12-1.1: EMPTY mask更新ヘルパー
// Marks a slab as EMPTY (highest reuse priority)
static inline void ss_mark_slab_empty(SuperSlab* ss, int slab_idx) {
    if (!ss || slab_idx < 0 || slab_idx >= SLABS_PER_SUPERSLAB_MAX) return;

    uint32_t bit = (1u << slab_idx);
    if (!(ss->empty_mask & bit)) {
        ss->empty_mask |= bit;
        ss->empty_count++;
    }
}

// Phase 12-1.1: EMPTY mask クリアヘルパー
// Removes a slab from EMPTY state (when reactivated)
static inline void ss_clear_slab_empty(SuperSlab* ss, int slab_idx) {
    if (!ss || slab_idx < 0 || slab_idx >= SLABS_PER_SUPERSLAB_MAX) return;

    uint32_t bit = (1u << slab_idx);
    if (ss->empty_mask & bit) {
        ss->empty_mask &= ~bit;
        ss->empty_count--;
    }
}

// Phase 3d-C: Hot/Cold インデックス更新
// Rebuilds hot_indices[] and cold_indices[] arrays based on current slab state
static inline void ss_update_hot_cold_indices(SuperSlab* ss) {
    if (!ss) return;

    ss->hot_count = 0;
    ss->cold_count = 0;
    // Phase 12-1.1: Reset empty tracking
    ss->empty_mask = 0;
    ss->empty_count = 0;

    uint32_t max_slabs = (1u << ss->lg_size) / SLAB_SIZE;
    if (max_slabs > SLABS_PER_SUPERSLAB_MAX) {
        max_slabs = SLABS_PER_SUPERSLAB_MAX;
    }

    // Scan active slabs and classify as EMPTY / hot / cold
    for (uint32_t i = 0; i < max_slabs && i < ss->active_slabs; i++) {
        TinySlabMeta* meta = &ss->slabs[i];

        // Skip uninitialized slabs (capacity == 0)
        if (meta->capacity == 0) {
            continue;
        }

        // Phase 12-1.1: EMPTY slabs have highest reuse priority
        if (ss_is_slab_empty(meta)) {
            ss_mark_slab_empty(ss, (int)i);
            continue;  // Don't add to hot/cold arrays
        }

        if (ss_is_slab_hot(meta)) {
            // Hot slab: high utilization
            if (ss->hot_count < 16) {
                ss->hot_indices[ss->hot_count++] = (uint8_t)i;
            }
        } else {
            // Cold slab: low utilization
            if (ss->cold_count < 16) {
                ss->cold_indices[ss->cold_count++] = (uint8_t)i;
            }
        }
    }
}

// Phase 3d-C: SuperSlab初期化時にhot/cold fieldsをゼロクリア
static inline void ss_init_hot_cold(SuperSlab* ss) {
    if (!ss) return;

    ss->hot_count = 0;
    ss->cold_count = 0;
    // Phase 12-1.1: Initialize EMPTY tracking
    ss->empty_mask = 0;
    ss->empty_count = 0;

    // Initialize index arrays to 0 (defensive programming)
    for (int i = 0; i < 16; i++) {
        ss->hot_indices[i] = 0;
        ss->cold_indices[i] = 0;
    }
}

#endif // SS_HOT_COLD_BOX_H