hakmem/core/tiny_alloc_fast.inc.h

// tiny_alloc_fast.inc.h - Box 5: Allocation Fast Path (3-4 instructions)
// Purpose: Ultra-fast TLS freelist pop (inspired by System tcache & Mid-Large HAKX +171%)
// Invariant: Hit rate > 95% → 3-4 instructions, Miss → refill from backend
// Design: "Simple Front + Smart Back" - Front is dumb & fast, Back is smart
#pragma once
#include "tiny_atomic.h"
#include "hakmem_tiny.h"

// ========== Debug Counters (compile-time gated) ==========
#if HAKMEM_DEBUG_COUNTERS
// Refill-stage counters (defined in hakmem_tiny.c)
extern unsigned long long g_rf_total_calls[];
extern unsigned long long g_rf_hit_bench[];
extern unsigned long long g_rf_hit_hot[];
extern unsigned long long g_rf_hit_mail[];
extern unsigned long long g_rf_hit_slab[];
extern unsigned long long g_rf_hit_ss[];
extern unsigned long long g_rf_hit_reg[];
extern unsigned long long g_rf_mmap_calls[];

// Publish hits (defined in hakmem_tiny.c)
extern unsigned long long g_pub_mail_hits[];
extern unsigned long long g_pub_bench_hits[];
extern unsigned long long g_pub_hot_hits[];

// Free pipeline (defined in hakmem_tiny.c)
extern unsigned long long g_free_via_tls_sll[];
#endif

// ========== Box 5: Allocation Fast Path ==========
// 箱理論の Fast Allocation 層。TLS freelist から直接 pop（3-4命令）。
// 不変条件:
// - TLS freelist が非空なら即座に return (no lock, no sync)
// - Miss なら Backend (Box 3: SuperSlab) に委譲
// - Cross-thread allocation は考慮しない（Backend が処理）

// External TLS variables (defined in hakmem_tiny.c)
extern __thread void* g_tls_sll_head[TINY_NUM_CLASSES];
extern __thread uint32_t g_tls_sll_count[TINY_NUM_CLASSES];

// External backend functions
extern int sll_refill_small_from_ss(int class_idx, int max_take);
extern void* hak_tiny_alloc_slow(size_t size, int class_idx);
extern int hak_tiny_size_to_class(size_t size);

// External macros
#ifndef HAK_RET_ALLOC
#define HAK_RET_ALLOC(cls, ptr) return (ptr)
#endif

// ========== Fast Path: TLS Freelist Pop (3-4 instructions) ==========

// Allocation fast path (inline for zero-cost)
// Returns: pointer on success, NULL on miss (caller should try refill/slow)
//
// Assembly (x86-64, optimized):
//   mov    rax, QWORD PTR g_tls_sll_head[class_idx]  ; Load head
//   test   rax, rax                                   ; Check NULL
//   je     .miss                                      ; If empty, miss
//   mov    rdx, QWORD PTR [rax]                       ; Load next
//   mov    QWORD PTR g_tls_sll_head[class_idx], rdx  ; Update head
//   ret                                               ; Return ptr
// .miss:
//   ; Fall through to refill
//
// Expected: 3-4 instructions on hit (1 load, 1 test, 1 load, 1 store)
static inline void* tiny_alloc_fast_pop(int class_idx) {
    // Optional one-shot front-path diag (env: HAKMEM_TINY_FRONT_DIAG=1)
    do { static int en=-1, once=0; if (en==-1){ const char* s=getenv("HAKMEM_TINY_FRONT_DIAG"); en=(s&&*s&&*s!='0')?1:0; }
         if (en && !once){ fprintf(stderr, "[FRONT] tiny_alloc_fast_pop active (class=%d)\n", class_idx); once=1; } } while(0);
    // Box Boundary: TLS freelist の先頭を pop
    // Ownership: TLS なので所有権チェック不要（同一スレッド保証）
    void* head = g_tls_sll_head[class_idx];
    if (__builtin_expect(head != NULL, 1)) {
        // Fast path hit: 3 instructions
        g_tls_sll_head[class_idx] = *(void**)head;  // Pop: next = *head

        // Optional: update count (for stats, can be disabled)
        if (g_tls_sll_count[class_idx] > 0) {
            g_tls_sll_count[class_idx]--;
        }

#if HAKMEM_DEBUG_COUNTERS
        // Track TLS freelist hits (compile-time gated, zero runtime cost when disabled)
        g_free_via_tls_sll[class_idx]++;
#endif

        return head;
    }

    // Fast path miss → NULL (caller should refill)
    return NULL;
}

// ========== Refill Path: Backend Integration ==========

// Refill TLS freelist from backend (SuperSlab/ACE/Learning layer)
// Returns: number of blocks refilled
//
// This integrates with existing HAKMEM infrastructure:
// - SuperSlab provides memory chunks
// - ACE provides adaptive capacity learning
// - L25 provides mid-large integration
//
// Refill count is tunable via HAKMEM_TINY_REFILL_COUNT (default: 32)
// - Smaller count (8-16): better for diverse workloads, faster warmup
// - Larger count (64-128): better for homogeneous workloads, fewer refills
static inline int tiny_alloc_fast_refill(int class_idx) {
    // Optional one-shot diag (env)
    do { static int en=-1, once=0; if (en==-1){ const char* s=getenv("HAKMEM_TINY_FRONT_DIAG"); en=(s&&*s&&*s!='0')?1:0; }
         if (en && !once){ fprintf(stderr, "[FRONT] tiny_alloc_fast_refill enter (class=%d)\n", class_idx); once=1; } } while(0);
    // Tunable refill count (cached in TLS for performance)
    static __thread int s_refill_count = 0;
    if (__builtin_expect(s_refill_count == 0, 0)) {
        int def = 128;  // Phase 1 Quick Win: 32 → 128 (reduce refill overhead)
        char* env = getenv("HAKMEM_TINY_REFILL_COUNT");
        int v = (env ? atoi(env) : def);

        // Clamp to sane range (avoid pathological cases)
        if (v < 8) v = 8;        // Minimum: avoid thrashing
        if (v > 256) v = 256;    // Maximum: avoid excessive TLS memory

        s_refill_count = v;
    }

#if HAKMEM_DEBUG_COUNTERS
    // Track refill calls (compile-time gated)
    g_rf_total_calls[class_idx]++;
#endif

    // Box Boundary: Delegate to Backend (Box 3: SuperSlab)
    // This gives us ACE, Learning layer, L25 integration for free!
    // Note: g_rf_hit_slab counter is incremented inside sll_refill_small_from_ss()
    int refilled = sll_refill_small_from_ss(class_idx, s_refill_count);

    return refilled;
}

// ========== Combined Fast Path (Alloc + Refill) ==========

// Complete fast path allocation (inline for zero-cost)
// Returns: pointer on success, NULL on failure (OOM or size too large)
//
// Flow:
// 1. TLS freelist pop (3-4 instructions) - Hit rate ~95%
// 2. Miss → Refill from backend (~5% cases)
// 3. Refill success → Retry pop
// 4. Refill failure → Slow path (OOM or new SuperSlab allocation)
//
// Example usage:
//   void* ptr = tiny_alloc_fast(64);
//   if (!ptr) {
//       // OOM handling
//   }
static inline void* tiny_alloc_fast(size_t size) {
    // 1. Size → class index (inline, fast)
    int class_idx = hak_tiny_size_to_class(size);
    if (__builtin_expect(class_idx < 0, 0)) {
        return NULL;  // Size > 1KB, not Tiny
    }

    // 2. Fast path: TLS freelist pop (3-4 instructions, 95% hit rate)
    void* ptr = tiny_alloc_fast_pop(class_idx);
    if (__builtin_expect(ptr != NULL, 1)) {
        HAK_RET_ALLOC(class_idx, ptr);
    }

    // 3. Miss: Refill from backend (Box 3: SuperSlab)
    int refilled = tiny_alloc_fast_refill(class_idx);
    if (__builtin_expect(refilled > 0, 1)) {
        // Refill success → retry pop
        ptr = tiny_alloc_fast_pop(class_idx);
        if (ptr) {
            HAK_RET_ALLOC(class_idx, ptr);
        }
    }

    // 4. Refill failure or still empty → slow path (OOM or new SuperSlab)
    // Box Boundary: Delegate to Slow Path (Box 3 backend)
    ptr = hak_tiny_alloc_slow(size, class_idx);
    if (ptr) {
        HAK_RET_ALLOC(class_idx, ptr);
    }

    return ptr;  // NULL if OOM
}

// ========== Push to TLS Freelist (for free path) ==========

// Push block to TLS freelist (used by free fast path)
// This is a "helper" for Box 6 (Free Fast Path)
//
// Invariant: ptr must belong to current thread (no ownership check here)
// Caller (Box 6) is responsible for ownership verification
static inline void tiny_alloc_fast_push(int class_idx, void* ptr) {
    // Box Boundary: Push to TLS freelist
    *(void**)ptr = g_tls_sll_head[class_idx];
    g_tls_sll_head[class_idx] = ptr;
    g_tls_sll_count[class_idx]++;
}

// ========== Statistics & Diagnostics ==========

// Get TLS freelist stats (for debugging/profiling)
typedef struct {
    int class_idx;
    void* head;
    uint32_t count;
} TinyAllocFastStats;

static inline TinyAllocFastStats tiny_alloc_fast_stats(int class_idx) {
    TinyAllocFastStats stats = {
        .class_idx = class_idx,
        .head = g_tls_sll_head[class_idx],
        .count = g_tls_sll_count[class_idx]
    };
    return stats;
}

// Reset TLS freelist (for testing/benchmarking)
// WARNING: This leaks memory! Only use in controlled test environments.
static inline void tiny_alloc_fast_reset(int class_idx) {
    g_tls_sll_head[class_idx] = NULL;
    g_tls_sll_count[class_idx] = 0;
}

// ========== Performance Notes ==========
//
// Expected metrics (based on System tcache & HAKX +171% results):
// - Fast path hit rate: 95%+ (workload dependent)
// - Fast path latency: 3-4 instructions (1-2 cycles on modern CPUs)
// - Miss penalty: ~20-50 instructions (refill from SuperSlab)
// - Throughput improvement: +10-25% vs current multi-layer design
//
// Key optimizations:
// 1. `__builtin_expect` for branch prediction (hot path first)
// 2. `static inline` for zero-cost abstraction
// 3. TLS variables (no atomic ops, no locks)
// 4. Minimal work in fast path (defer stats/accounting to backend)
//
// Comparison with current design:
// - Current: 20+ instructions (Magazine → SuperSlab → ACE → ...)
// - New: 3-4 instructions (TLS freelist pop only)
// - Reduction: -80% instructions in hot path
//
// Inspired by:
// - System tcache (glibc malloc) - 3-4 instruction fast path
// - HAKX Mid-Large (+171%) - "Simple Front + Smart Back"
// - Box Theory - Clear boundaries, minimal coupling