hakmem/core/box/front_fastlane_box.h

#ifndef HAK_FRONT_FASTLANE_BOX_H
#define HAK_FRONT_FASTLANE_BOX_H

// ============================================================================
// Phase 6: Front FastLane Box (Hot Inline / Try API)
// ============================================================================
//
// Purpose: Single-box entry point for malloc/free hot paths
//          Collapses wrapper→gate→policy→route layers into one
//
// API:
//   void* front_fastlane_try_malloc(size_t size)
//     - Returns non-NULL on success (handled by FastLane)
//     - Returns NULL on failure (fallback to existing wrapper path)
//
//   bool front_fastlane_try_free(void* ptr)
//     - Returns true if handled (success)
//     - Returns false if not handled (fallback to existing wrapper path)
//
// Box Theory:
//   - L0: ENV gate (front_fastlane_env_box.h)
//   - L1: This file (hot inline handlers)
//   - L2: Stats (front_fastlane_stats_box.h, cold helpers in .c)
//
// Strategy:
//   - Read existing "winning boxes" only once
//   - Call existing hot handlers (malloc_tiny_fast_for_class, free_tiny_fast)
//   - No duplicate checks (deduplicate existing wrapper logic)
//   - Fail-fast: Any uncertainty → return not-handled
//
// Safety:
//   - ENV-gated (default OFF)
//   - Single fallback boundary (FastLane → ColdFallback)
//   - Reversible (ENV toggle)
//
// ============================================================================

#include <stddef.h>
#include <stdbool.h>
#include <stdint.h>
#include "front_fastlane_env_box.h"
#include "front_fastlane_stats_box.h"
#include "../hakmem_tiny.h"                // hak_tiny_size_to_class, tiny_get_max_size
#include "../front/malloc_tiny_fast.h"     // malloc_tiny_fast_for_class

// FastLane is only safe after global init completes.
// Before init, wrappers must handle recursion guards + syscall init.
extern int g_initialized;

// ============================================================================
// Hot Inline: try_malloc
// ============================================================================

// Patch 4: Actual Tiny routing implementation
// Strategy: Read existing winning boxes only once, call existing hot handlers
// No duplicate checks (deduplicate existing wrapper logic)
static inline void* front_fastlane_try_malloc(size_t size) {
    FRONT_FASTLANE_STAT_INC(malloc_total);

    // Fail-fast: do not enter FastLane before init completes.
    if (__builtin_expect(!g_initialized, 0)) {
        FRONT_FASTLANE_STAT_INC(malloc_fallback_other);
        return NULL;
    }

    // Fast path: Size check (Tiny range only)
    // Use cached max size (typically 256 or 1024)
    size_t max_size = tiny_get_max_size();
    if (__builtin_expect(size > max_size, 0)) {
        FRONT_FASTLANE_STAT_INC(malloc_fallback_size);
        return NULL;  // Not Tiny → fallback
    }

    // Class calculation (single LUT lookup, no branches)
    int class_idx = hak_tiny_size_to_class(size);
    if (__builtin_expect(class_idx < 0 || class_idx >= 8, 0)) {
        FRONT_FASTLANE_STAT_INC(malloc_fallback_class);
        return NULL;  // Invalid class → fallback
    }

    // Class mask check (gradual rollout support)
    uint8_t mask = front_fastlane_class_mask();
    if (__builtin_expect(((mask >> class_idx) & 1) == 0, 0)) {
        FRONT_FASTLANE_STAT_INC(malloc_fallback_other);
        return NULL;  // Class not enabled → fallback
    }

    // Call existing hot handler (no duplication)
    // This is the winning path from E5-4 / Phase 4 E2
    void* ptr = malloc_tiny_fast_for_class(size, class_idx);
    if (__builtin_expect(ptr != NULL, 1)) {
        FRONT_FASTLANE_STAT_INC(malloc_hit);
        return ptr;  // Success
    }

    // Allocation failed (refill needed, TLS exhausted, etc.)
    FRONT_FASTLANE_STAT_INC(malloc_fallback_alloc);
    return NULL;  // Fallback to cold path
}

// ============================================================================
// Hot Inline: try_free
// ============================================================================

// Patch 6: Actual Tiny direct path implementation
// Strategy: Header validation → direct Tiny free (same pattern as E5-1)
// Only handle cases where we have high confidence (Tiny header magic)
static inline bool front_fastlane_try_free(void* ptr) {
    FRONT_FASTLANE_STAT_INC(free_total);

    // Fail-fast: do not enter FastLane before init completes.
    if (__builtin_expect(!g_initialized, 0)) {
        FRONT_FASTLANE_STAT_INC(free_fallback_other);
        return false;
    }

#if HAKMEM_TINY_HEADER_CLASSIDX
    // Page boundary guard: ptr must not be page-aligned
    // (Accessing ptr-1 when ptr is page-aligned could segfault)
    uintptr_t off = (uintptr_t)ptr & 0xFFFu;
    if (__builtin_expect(off == 0, 0)) {
        FRONT_FASTLANE_STAT_INC(free_fallback_aligned);
        return false;  // Page-aligned → fallback (unsafe to read header)
    }

    // Fast header validation (1 load, 1 compare)
    uint8_t header = *((uint8_t*)ptr - 1);
    uint8_t magic = header & 0xF0u;

    if (__builtin_expect(magic != 0xA0u, 0)) {
        // Not Tiny header (could be Mid/Pool/Large or external allocation)
        if (magic != 0) {
            FRONT_FASTLANE_STAT_INC(free_fallback_header);
        }
        return false;  // Not Tiny → fallback
    }

    // Extract class index from header (lower 4 bits)
    int class_idx = (int)(header & 0x0Fu);
    if (__builtin_expect(class_idx >= 8, 0)) {
        FRONT_FASTLANE_STAT_INC(free_fallback_class);
        return false;  // Invalid class → fallback
    }

    // Class mask check (gradual rollout support)
    uint8_t mask = front_fastlane_class_mask();
    if (__builtin_expect(((mask >> class_idx) & 1) == 0, 0)) {
        FRONT_FASTLANE_STAT_INC(free_fallback_other);
        return false;  // Class not enabled → fallback
    }

    // Call existing hot handler (no duplication)
    // This is the winning path from E5-1 (free_tiny_fast returns 1 on success)
    // Forward declaration needed - free_tiny_fast is in malloc_tiny_fast.h
    extern int free_tiny_fast(void* ptr);
    if (__builtin_expect(free_tiny_fast(ptr), 1)) {
        FRONT_FASTLANE_STAT_INC(free_hit);
        return true;  // Success
    }

    // Free failed (cold path needed - refill, full TLS, etc.)
    FRONT_FASTLANE_STAT_INC(free_fallback_failure);
    return false;  // Fallback to cold path
#else
    // No header support → always fallback
    FRONT_FASTLANE_STAT_INC(free_fallback_other);
    return false;
#endif
}

#endif  // HAK_FRONT_FASTLANE_BOX_H