hakmem/core/tiny_alloc_fast_sfc.inc.h

// tiny_alloc_fast_sfc.inc.h - Box 5-NEW: Super Front Cache (SFC)
// Purpose: Ultra-fast TLS cache with 128-256 slots (vs old 16 slots)
// Performance: 3-4 instruction fast path, 95%+ hit rate, refill -98.5%
// Box Theory: Clear ownership, same-thread only, A/B testable
#pragma once

#include <stdint.h>
#include <stddef.h>
#include <stdio.h>    // For debug output (getenv, fprintf, stderr)
#include <stdlib.h>   // For getenv
#include "hakmem_tiny.h"

// ============================================================================
// Box 5-NEW: Super Front Cache - Global Config
// ============================================================================

// Default capacities (can be overridden per-class)
#define SFC_DEFAULT_CAPACITY 128
#define SFC_DEFAULT_REFILL_COUNT 64
#define SFC_DEFAULT_SPILL_THRESH 90  // Spill when >90% full

// Per-class capacity limits
#define SFC_MIN_CAPACITY 16
#define SFC_MAX_CAPACITY 256

// ============================================================================
// Box 5-NEW: Super Front Cache - TLS Data Structures
// ============================================================================

// TLS arrays (one per class, zero-initialized at thread start)
extern __thread void* g_sfc_head[TINY_NUM_CLASSES];        // Head of linked list
extern __thread uint32_t g_sfc_count[TINY_NUM_CLASSES];    // Current count
extern uint32_t g_sfc_capacity[TINY_NUM_CLASSES];          // Target capacity (shared, read-only)

// ============================================================================
// Box 5-NEW: Super Front Cache - Statistics (optional, compile-time gated)
// ============================================================================

#if HAKMEM_DEBUG_COUNTERS

typedef struct {
    // Fast path metrics
    uint64_t alloc_hits;       // Fast path hits (3-4 inst)
    uint64_t alloc_misses;     // Fast path misses → refill
    uint64_t free_hits;        // Push success
    uint64_t free_full;        // Push failed (full) → spill

    // Slow path metrics
    uint64_t refill_calls;     // Refill invocations
    uint64_t refill_blocks;    // Total blocks refilled
    uint64_t spill_calls;      // Spill invocations
    uint64_t spill_blocks;     // Total blocks spilled

    // Learning metrics (Phase 3)
    uint64_t alloc_window;     // Allocs in current window
    uint64_t miss_window;      // Misses in current window
    double miss_rate;          // miss_window / alloc_window
    int hotness;               // 0=cold, 1=warm, 2=hot, 3=scorching

    // Adaptive config (Phase 3)
    int adaptive_capacity;     // Current capacity (16-256)
    int adaptive_refill;       // Current refill count (16-128)
} sfc_stats_t;

extern sfc_stats_t g_sfc_stats[TINY_NUM_CLASSES];

#endif // HAKMEM_DEBUG_COUNTERS

// ============================================================================
// Box 5-NEW: Super Front Cache - Fast Path (3-4 instructions)
// ============================================================================

// Alloc: Pop from SFC cache (inline for zero-cost)
// Returns: pointer on success, NULL on miss
// Contract: Caller owns returned pointer
// Invariants: count ≥ 0, all pointers belong to correct class
static inline void* sfc_alloc(int cls) {
    void* base = g_sfc_head[cls];

    if (__builtin_expect(base != NULL, 1)) {
#if HAKMEM_TINY_HEADER_CLASSIDX
        const size_t next_offset = (cls == 7) ? 0 : 1;
#else
        const size_t next_offset = 0;
#endif
        // Pop: header-aware next
        g_sfc_head[cls] = *(void**)((uint8_t*)base + next_offset);
        g_sfc_count[cls]--;               // count--

#if HAKMEM_DEBUG_COUNTERS
        g_sfc_stats[cls].alloc_hits++;
#endif

        return base;  // 🚀 SFC HIT! (returns base)
    }

#if HAKMEM_DEBUG_COUNTERS
    g_sfc_stats[cls].alloc_misses++;
#endif

    return NULL;  // Miss → caller should refill
}

// Free: Push to SFC cache (inline for zero-cost)
// Returns: 1 on success (cached), 0 on full (caller should spill)
// Contract: ptr must belong to cls, same-thread only
// Invariants: count ≤ capacity, linked list integrity
static inline int sfc_free_push(int cls, void* ptr) {
    uint32_t cap = g_sfc_capacity[cls];
    uint32_t cnt = g_sfc_count[cls];

    // Debug: Always log sfc_free_push calls when SFC_DEBUG is set
    static __thread int free_debug_count = 0;
    if (getenv("HAKMEM_SFC_DEBUG") && free_debug_count < 20) {
        free_debug_count++;
        extern int g_sfc_enabled;
        fprintf(stderr, "[SFC_FREE_PUSH] cls=%d, ptr=%p, cnt=%u, cap=%u, will_succeed=%d, enabled=%d\n",
                cls, ptr, cnt, cap, (cnt < cap), g_sfc_enabled);
    }

    if (__builtin_expect(cnt < cap, 1)) {
#if HAKMEM_TINY_HEADER_CLASSIDX
        const size_t next_offset = (cls == 7) ? 0 : 1;
#else
        const size_t next_offset = 0;
#endif
        // Push: header-aware next placement
        *(void**)((uint8_t*)ptr + next_offset) = g_sfc_head[cls];
        g_sfc_head[cls] = ptr;           // head = base
        g_sfc_count[cls] = cnt + 1;      // count++

#if HAKMEM_DEBUG_COUNTERS
        g_sfc_stats[cls].free_hits++;
#endif

        return 1;  // Success
    }

#if HAKMEM_DEBUG_COUNTERS
    g_sfc_stats[cls].free_full++;
#endif

    return 0;  // Full → caller should spill
}

// ============================================================================
// Box 5-NEW: Super Front Cache - Public API (slow path, not inline)
// ============================================================================

// Initialize SFC (called once at startup)
void sfc_init(void);

// Shutdown SFC (called at exit, optional)
void sfc_shutdown(void);

// Refill: Batch refill from backend (Magazine/SuperSlab)
// Returns: number of blocks refilled (0 on failure)
// Contract: Transfers ownership from backend to SFC
int sfc_refill(int cls, int target_count);

// Spill: Batch spill to backend when cache too full
// Returns: number of blocks spilled
// Contract: Transfers ownership from SFC to backend
int sfc_spill(int cls, int spill_count);

// ============================================================================
// Box 5-NEW: Super Front Cache - Configuration (tuning)
// ============================================================================

typedef struct {
    int capacity;      // Target capacity (128-256)
    int refill_count;  // Batch refill size (64-128)
    int spill_thresh;  // Spill when count > capacity * thresh%
} sfc_config_t;

sfc_config_t sfc_get_config(int cls);
void sfc_set_config(int cls, sfc_config_t cfg);

// ============================================================================
// Box 5-NEW: Super Front Cache - Statistics (debugging)
// ============================================================================

#if HAKMEM_DEBUG_COUNTERS

sfc_stats_t sfc_get_stats(int cls);
void sfc_reset_stats(int cls);
void sfc_print_stats(void);  // Human-readable dump

#endif // HAKMEM_DEBUG_COUNTERS

// ============================================================================
// Box 5-NEW: Super Front Cache - Feature Flag (A/B testing)
// ============================================================================

// Global enable flag (set via ENV: HAKMEM_SFC_ENABLE)
extern int g_sfc_enabled;

// ============================================================================
// Box 5-NEW: Super Front Cache - Box Theory Compliance
// ============================================================================

// Invariants (enforced by design):
// - INVARIANT 1: g_sfc_count[cls] <= g_sfc_capacity[cls] (always)
// - INVARIANT 2: All pointers in cache belong to correct class (caller responsibility)
// - INVARIANT 3: SFC contains only same-thread allocations (TLS)
// - INVARIANT 4: Linked list integrity (*ptr points to valid node or NULL)
// - INVARIANT 5: g_sfc_count[cls] >= 0 (always)

// Ownership Rules:
// - SFC owns: Cached pointers in g_sfc_head[cls] linked list
// - Transfer IN: sfc_refill() from Magazine/SuperSlab (batch)
// - Transfer OUT: sfc_alloc() to application (single)
// - Return: sfc_free_push() from application (single)
// - Spill: sfc_spill() to Magazine/SuperSlab (batch)

// Boundaries:
// - Minimal coupling: Reuses existing sll_refill_small_from_ss()
// - Fallback: Old Box 5 (TLS SLL 16 slots) remains for backward compat
// - A/B testable: HAKMEM_SFC_ENABLE=0/1 switches between old/new

// ============================================================================
// End of tiny_alloc_fast_sfc.inc.h
// ============================================================================