第5章 内存管理
WebAssembly 的内存系统是其高性能的关键基础。本章将深入探讨 WebAssembly 的线性内存模型、内存操作指令、以及高效的内存管理策略。
线性内存模型
5.1.1 内存基础概念
WebAssembly 使用线性内存模型，这是一个连续的字节数组：
(module
;; 定义最小1页（64KB），最大10页的内存
(memory $main 1 10)
;; 导出内存供外部访问
(export "memory" (memory $main))
;; 简单的内存读写演示
(func $memory_demo (param $offset i32) (param $value i32)
;; 写入32位整数到指定偏移
(i32.store (local.get $offset) (local.get $value))
;; 读取并验证
(i32.load (local.get $offset))
drop)
(export "memory_demo" (func $memory_demo)))
内存组织结构：
内存页 = 64KB (65536 字节)
地址空间 = 0 到 (页数 * 64KB - 1)
地址 0x0000: [字节 0] [字节 1] [字节 2] [字节 3] ...
地址 0x0004: [字节 4] [字节 5] [字节 6] [字节 7] ...
...
地址 0xFFFC: [倒数第4字节] [倒数第3字节] [倒数第2字节] [最后字节]
5.1.2 内存布局策略
有效的内存布局对性能至关重要：
(module
(memory 1) ;; 64KB 内存
;; 内存布局规划
;; 0x0000 - 0x0100: 系统保留区域 (256字节)
;; 0x0100 - 0x1000: 栈空间 (3840字节)
;; 0x1000 - 0x8000: 堆空间 (28KB)
;; 0x8000 - 0xFFFF: 静态数据区 (32KB)
(global $RESERVED_START i32 (i32.const 0x0000))
(global $STACK_START i32 (i32.const 0x0100))
(global $HEAP_START i32 (i32.const 0x1000))
(global $DATA_START i32 (i32.const 0x8000))
;; 当前栈指针
(global $stack_ptr (mut i32) (i32.const 0x1000))
;; 堆分配指针
(global $heap_ptr (mut i32) (i32.const 0x1000))
;; 栈操作：压栈
(func $push (param $value i32)
;; 移动栈指针
(global.set $stack_ptr
(i32.sub (global.get $stack_ptr) (i32.const 4)))
;; 存储值
(i32.store (global.get $stack_ptr) (local.get $value)))
;; 栈操作：弹栈
(func $pop (result i32)
(local $value i32)
;; 读取值
(local.set $value (i32.load (global.get $stack_ptr)))
;; 移动栈指针
(global.set $stack_ptr
(i32.add (global.get $stack_ptr) (i32.const 4)))
local.get $value)
;; 简单的堆分配器
(func $malloc (param $size i32) (result i32)
(local $ptr i32)
;; 获取当前堆指针
(local.set $ptr (global.get $heap_ptr))
;; 对齐到4字节边界
(local.set $size
(i32.and
(i32.add (local.get $size) (i32.const 3))
(i32.const 0xFFFFFFFC)))
;; 更新堆指针
(global.set $heap_ptr
(i32.add (global.get $heap_ptr) (local.get $size)))
;; 检查是否超出堆空间
(if (i32.gt_u (global.get $heap_ptr) (global.get $DATA_START))
(then
;; 分配失败，返回 NULL
(global.set $heap_ptr (local.get $ptr))
(return (i32.const 0))))
local.get $ptr)
(export "push" (func $push))
(export "pop" (func $pop))
(export "malloc" (func $malloc))
(export "memory" (memory 0)))
5.1.3 内存增长机制
WebAssembly 支持运行时动态增长内存：
(module
(memory 1 100) ;; 初始1页，最大100页
;; 内存状态监控
(func $get_memory_size (result i32)
(memory.size)) ;; 返回当前页数
;; 增长内存
(func $grow_memory (param $pages i32) (result i32)
(local $old_size i32)
;; 获取增长前的大小
(local.set $old_size (memory.size))
;; 尝试增长内存
(memory.grow (local.get $pages))
;; memory.grow 返回增长前的页数，失败时返回 -1
(if (result i32)
(i32.eq (memory.grow (i32.const 0)) (i32.const -1))
(then (i32.const 0)) ;; 失败
(else (i32.const 1)))) ;; 成功
;; 检查内存是否足够
(func $ensure_memory (param $required_bytes i32) (result i32)
(local $current_bytes i32)
(local $required_pages i32)
(local $current_pages i32)
;; 计算当前总字节数
(local.set $current_pages (memory.size))
(local.set $current_bytes
(i32.mul (local.get $current_pages) (i32.const 65536)))
;; 检查是否需要增长
(if (i32.ge_u (local.get $current_bytes) (local.get $required_bytes))
(then (return (i32.const 1)))) ;; 已经足够
;; 计算需要的页数
(local.set $required_pages
(i32.div_u
(i32.add (local.get $required_bytes) (i32.const 65535))
(i32.const 65536)))
;; 尝试增长到所需大小
(call $grow_memory
(i32.sub (local.get $required_pages) (local.get $current_pages))))
(export "get_memory_size" (func $get_memory_size))
(export "grow_memory" (func $grow_memory))
(export "ensure_memory" (func $ensure_memory))
(export "memory" (memory 0)))
内存操作指令
5.2.1 加载指令详解
WebAssembly 提供多种粒度的内存加载指令：
(module
(memory 1)
;; 初始化测试数据
(data (i32.const 0) "\01\02\03\04\05\06\07\08\09\0A\0B\0C\0D\0E\0F\10")
(func $load_operations (param $addr i32)
;; 8位加载
(i32.load8_u (local.get $addr)) ;; 无符号8位 → i32
(i32.load8_s (local.get $addr)) ;; 有符号8位 → i32
;; 16位加载
(i32.load16_u (local.get $addr)) ;; 无符号16位 → i32
(i32.load16_s (local.get $addr)) ;; 有符号16位 → i32
;; 32位加载
(i32.load (local.get $addr)) ;; 32位 → i32
;; 64位加载（到 i64）
(i64.load8_u (local.get $addr)) ;; 无符号8位 → i64
(i64.load8_s (local.get $addr)) ;; 有符号8位 → i64
(i64.load16_u (local.get $addr)) ;; 无符号16位 → i64
(i64.load16_s (local.get $addr)) ;; 有符号16位 → i64
(i64.load32_u (local.get $addr)) ;; 无符号32位 → i64
(i64.load32_s (local.get $addr)) ;; 有符号32位 → i64
(i64.load (local.get $addr)) ;; 64位 → i64
;; 浮点数加载
(f32.load (local.get $addr)) ;; 32位浮点
(f64.load (local.get $addr)) ;; 64位浮点
;; 清空栈
drop drop drop drop drop drop drop drop
drop drop drop drop drop drop)
(export "load_operations" (func $load_operations))
(export "memory" (memory 0)))
5.2.2 存储指令详解
相应的存储指令支持不同的数据宽度：
(module
(memory 1)
(func $store_operations (param $addr i32) (param $value i32)
;; 8位存储
(i32.store8 (local.get $addr) (local.get $value))
;; 16位存储
(i32.store16 (local.get $addr) (local.get $value))
;; 32位存储
(i32.store (local.get $addr) (local.get $value)))
;; 批量数据复制
(func $memory_copy (param $dest i32) (param $src i32) (param $size i32)
(local $i i32)
(local.set $i (i32.const 0))
(loop $copy_loop
;; 检查是否完成
(if (i32.ge_u (local.get $i) (local.get $size))
(then (br $copy_loop)))
;; 复制一个字节
(i32.store8
(i32.add (local.get $dest) (local.get $i))
(i32.load8_u
(i32.add (local.get $src) (local.get $i))))
;; 递增计数器
(local.set $i (i32.add (local.get $i) (i32.const 1)))
(br $copy_loop)))
;; 内存填充
(func $memory_fill (param $addr i32) (param $value i32) (param $size i32)
(local $i i32)
(local.set $i (i32.const 0))
(loop $fill_loop
(if (i32.ge_u (local.get $i) (local.get $size))
(then (br $fill_loop)))
(i32.store8
(i32.add (local.get $addr) (local.get $i))
(local.get $value))
(local.set $i (i32.add (local.get $i) (i32.const 1)))
(br $fill_loop)))
(export "store_operations" (func $store_operations))
(export "memory_copy" (func $memory_copy))
(export "memory_fill" (func $memory_fill))
(export "memory" (memory 0)))
5.2.3 批量内存操作（WASM 扩展）
现代 WebAssembly 支持高效的批量内存操作：
(module
(memory 1)
;; 使用 memory.copy 批量复制（需要 bulk-memory 提案）
(func $fast_memory_copy (param $dest i32) (param $src i32) (param $size i32)
(memory.copy
(local.get $dest)
(local.get $src)
(local.get $size)))
;; 使用 memory.fill 批量填充
(func $fast_memory_fill (param $addr i32) (param $value i32) (param $size i32)
(memory.fill
(local.get $addr)
(local.get $value)
(local.get $size)))
;; 初始化内存段
(func $init_data_segment (param $dest i32) (param $segment_index i32) (param $size i32)
(memory.init $segment_index
(local.get $dest)
(i32.const 0)
(local.get $size)))
(export "fast_memory_copy" (func $fast_memory_copy))
(export "fast_memory_fill" (func $fast_memory_fill))
(export "init_data_segment" (func $init_data_segment))
(export "memory" (memory 0)))
数据段初始化
5.3.1 静态数据段
数据段用于在模块实例化时初始化内存：
(module
(memory 1)
;; 字符串数据段
(data $hello (i32.const 0x1000) "Hello, WebAssembly!")
(data $numbers (i32.const 0x1020) "\01\02\03\04\05\06\07\08")
;; 结构化数据
(data $person_record (i32.const 0x1100)
;; 年龄 (4字节)
"\1A\00\00\00" ;; 26岁
;; 身高 (4字节浮点)
"\00\00\B4\42" ;; 90.0
;; 名字长度 (4字节)
"\05\00\00\00" ;; 5个字符
;; 名字数据
"Alice")
;; 查找表数据
(data $lookup_table (i32.const 0x2000)
;; 平方表：0² 到 15²
"\00\00\00\00" ;; 0
"\01\00\00\00" ;; 1
"\04\00\00\00" ;; 4
"\09\00\00\00" ;; 9
"\10\00\00\00" ;; 16
"\19\00\00\00" ;; 25
"\24\00\00\00" ;; 36
"\31\00\00\00" ;; 49
"\40\00\00\00" ;; 64
"\51\00\00\00" ;; 81
"\64\00\00\00" ;; 100
"\79\00\00\00" ;; 121
"\90\00\00\00" ;; 144
"\A9\00\00\00" ;; 169
"\C4\00\00\00" ;; 196
"\E1\00\00\00") ;; 225
;; 读取字符串
(func $get_string_char (param $index i32) (result i32)
(if (result i32)
(i32.ge_u (local.get $index) (i32.const 20))
(then (i32.const 0)) ;; 超出范围
(else
(i32.load8_u
(i32.add (i32.const 0x1000) (local.get $index))))))
;; 快速平方查找
(func $fast_square (param $n i32) (result i32)
(if (result i32)
(i32.ge_u (local.get $n) (i32.const 16))
(then (i32.mul (local.get $n) (local.get $n))) ;; 计算
(else ;; 查表
(i32.load
(i32.add
(i32.const 0x2000)
(i32.mul (local.get $n) (i32.const 4)))))))
;; 读取人员记录
(func $get_person_age (result i32)
(i32.load (i32.const 0x1100)))
(func $get_person_height (result f32)
(f32.load (i32.const 0x1104)))
(func $get_person_name_length (result i32)
(i32.load (i32.const 0x1108)))
(export "get_string_char" (func $get_string_char))
(export "fast_square" (func $fast_square))
(export "get_person_age" (func $get_person_age))
(export "get_person_height" (func $get_person_height))
(export "get_person_name_length" (func $get_person_name_length))
(export "memory" (memory 0)))
5.3.2 动态数据段（被动段）
被动数据段可以在运行时按需初始化：
(module
(memory 1)
;; 被动数据段（不会自动初始化）
(data $template_data passive "Template: ")
(data $error_messages passive
"Error 1: Invalid input\00"
"Error 2: Out of memory\00"
"Error 3: Division by zero\00")
;; 消息偏移表
(data $message_offsets passive
"\00\00\00\00" ;; 错误1偏移: 0
"\14\00\00\00" ;; 错误2偏移: 20
"\2B\00\00\00") ;; 错误3偏移: 43
;; 字符串工作缓冲区基址
(global $string_buffer i32 (i32.const 0x3000))
;; 初始化错误消息系统
(func $init_error_system
;; 初始化错误消息到缓冲区
(memory.init $error_messages
(global.get $string_buffer) ;; 目标地址
(i32.const 0) ;; 源偏移
(i32.const 60)) ;; 大小
;; 初始化偏移表
(memory.init $message_offsets
(i32.add (global.get $string_buffer) (i32.const 1000))
(i32.const 0)
(i32.const 12))
;; 删除数据段以释放内存
(data.drop $error_messages)
(data.drop $message_offsets))
;; 构建带模板的错误消息
(func $build_error_message (param $error_code i32) (param $dest i32)
(local $template_len i32)
(local $msg_offset i32)
(local $msg_addr i32)
;; 复制模板前缀
(memory.init $template_data
(local.get $dest)
(i32.const 0)
(i32.const 10)) ;; "Template: " 长度
;; 获取错误消息偏移
(local.set $msg_offset
(i32.load
(i32.add
(i32.add (global.get $string_buffer) (i32.const 1000))
(i32.mul (local.get $error_code) (i32.const 4)))))
;; 计算消息实际地址
(local.set $msg_addr
(i32.add (global.get $string_buffer) (local.get $msg_offset)))
;; 复制错误消息（简化版，假设长度为20）
(call $copy_string
(i32.add (local.get $dest) (i32.const 10))
(local.get $msg_addr)
(i32.const 20)))
;; 辅助：复制字符串
(func $copy_string (param $dest i32) (param $src i32) (param $len i32)
(local $i i32)
(loop $copy_loop
(if (i32.ge_u (local.get $i) (local.get $len))
(then (br $copy_loop)))
(i32.store8
(i32.add (local.get $dest) (local.get $i))
(i32.load8_u
(i32.add (local.get $src) (local.get $i))))
(local.set $i (i32.add (local.get $i) (i32.const 1)))
(br $copy_loop)))
(export "init_error_system" (func $init_error_system))
(export "build_error_message" (func $build_error_message))
(export "memory" (memory 0)))
内存对齐与性能
5.4.1 对齐原理
内存对齐对性能有重要影响：
(module
(memory 1)
;; 演示对齐访问
(func $alignment_demo
;; 对齐的32位访问（地址是4的倍数）
(i32.store align=4 (i32.const 0x1000) (i32.const 0x12345678))
(i32.load align=4 (i32.const 0x1000))
;; 非对齐访问（可能较慢，但仍然有效）
(i32.store align=1 (i32.const 0x1001) (i32.const 0x87654321))
(i32.load align=1 (i32.const 0x1001))
;; 64位对齐访问
(i64.store align=8 (i32.const 0x2000) (i64.const 0x123456789ABCDEF0))
(i64.load align=8 (i32.const 0x2000))
drop drop drop)
;; 数据结构对齐
(func $aligned_struct_demo
(local $base_addr i32)
(local.set $base_addr (i32.const 0x3000))
;; 结构体：
;; struct Person {
;; i32 id; // 0-3
;; f64 height; // 8-15 (对齐到8字节)
;; i32 age; // 16-19
;; } // 总大小：24字节（末尾填充到8字节对齐）
;; 存储 ID
(i32.store align=4
(local.get $base_addr)
(i32.const 12345))
;; 存储身高（注意8字节对齐）
(f64.store align=8
(i32.add (local.get $base_addr) (i32.const 8))
(f64.const 175.5))
;; 存储年龄
(i32.store align=4
(i32.add (local.get $base_addr) (i32.const 16))
(i32.const 25)))
;; 内存对齐工具函数
(func $align_up (param $addr i32) (param $alignment i32) (result i32)
(local $mask i32)
;; 计算掩码（对齐-1）
(local.set $mask (i32.sub (local.get $alignment) (i32.const 1)))
;; 对齐公式：(addr + mask) & ~mask
(i32.and
(i32.add (local.get $addr) (local.get $mask))
(i32.xor (local.get $mask) (i32.const -1))))
;; 检查地址是否对齐
(func $is_aligned (param $addr i32) (param $alignment i32) (result i32)
(i32.eqz
(i32.rem_u (local.get $addr) (local.get $alignment))))
(export "alignment_demo" (func $alignment_demo))
(export "aligned_struct_demo" (func $aligned_struct_demo))
(export "align_up" (func $align_up))
(export "is_aligned" (func $is_aligned))
(export "memory" (memory 0)))
5.4.2 性能优化技巧
(module
(memory 1)
;; 缓存友好的数据访问
(func $cache_friendly_sum (param $array_ptr i32) (param $length i32) (result i32)
(local $sum i32)
(local $i i32)
;; 顺序访问，利用缓存局部性
(loop $sum_loop
(if (i32.ge_u (local.get $i) (local.get $length))
(then (br $sum_loop)))
(local.set $sum
(i32.add
(local.get $sum)
(i32.load
(i32.add
(local.get $array_ptr)
(i32.mul (local.get $i) (i32.const 4))))))
(local.set $i (i32.add (local.get $i) (i32.const 1)))
(br $sum_loop))
local.get $sum)
;; 批量操作优化：一次处理多个元素
(func $vectorized_add (param $a_ptr i32) (param $b_ptr i32) (param $result_ptr i32) (param $length i32)
(local $i i32)
(local $end i32)
;; 计算向量化结束位置（4的倍数）
(local.set $end
(i32.and (local.get $length) (i32.const 0xFFFFFFFC)))
;; 向量化循环：一次处理4个元素
(loop $vector_loop
(if (i32.ge_u (local.get $i) (local.get $end))
(then (br $vector_loop)))
;; 处理4个连续的32位整数
(i32.store
(i32.add (local.get $result_ptr) (i32.mul (local.get $i) (i32.const 4)))
(i32.add
(i32.load (i32.add (local.get $a_ptr) (i32.mul (local.get $i) (i32.const 4))))
(i32.load (i32.add (local.get $b_ptr) (i32.mul (local.get $i) (i32.const 4))))))
(local.set $i (i32.add (local.get $i) (i32.const 1)))
(br $vector_loop))
;; 处理剩余元素
(loop $remainder_loop
(if (i32.ge_u (local.get $i) (local.get $length))
(then (br $remainder_loop)))
(i32.store
(i32.add (local.get $result_ptr) (i32.mul (local.get $i) (i32.const 4)))
(i32.add
(i32.load (i32.add (local.get $a_ptr) (i32.mul (local.get $i) (i32.const 4))))
(i32.load (i32.add (local.get $b_ptr) (i32.mul (local.get $i) (i32.const 4))))))
(local.set $i (i32.add (local.get $i) (i32.const 1)))
(br $remainder_loop)))
;; 内存预取模拟（通过提前访问实现）
(func $prefetch_demo (param $data_ptr i32) (param $length i32) (result i32)
(local $sum i32)
(local $i i32)
(local $prefetch_distance i32)
(local.set $prefetch_distance (i32.const 64)) ;; 提前64个元素
(loop $prefetch_loop
(if (i32.ge_u (local.get $i) (local.get $length))
(then (br $prefetch_loop)))
;; 预取未来的数据
(if (i32.lt_u
(i32.add (local.get $i) (local.get $prefetch_distance))
(local.get $length))
(then
(i32.load ;; 触发缓存加载
(i32.add
(local.get $data_ptr)
(i32.mul
(i32.add (local.get $i) (local.get $prefetch_distance))
(i32.const 4))))
drop))
;; 处理当前数据
(local.set $sum
(i32.add
(local.get $sum)
(i32.load
(i32.add
(local.get $data_ptr)
(i32.mul (local.get $i) (i32.const 4))))))
(local.set $i (i32.add (local.get $i) (i32.const 1)))
(br $prefetch_loop))
local.get $sum)
(export "cache_friendly_sum" (func $cache_friendly_sum))
(export "vectorized_add" (func $vectorized_add))
(export "prefetch_demo" (func $prefetch_demo))
(export "memory" (memory 0)))
高级内存管理
5.5.1 简单的内存分配器
(module
(memory 1)
;; 内存块头结构（8字节）
;; [0-3]: 块大小
;; [4-7]: 下一个块的偏移（0表示最后一个块）
(global $heap_start i32 (i32.const 0x1000))
(global $heap_end i32 (i32.const 0xF000))
;; 初始化堆
(func $init_heap
;; 创建初始空闲块
(i32.store (global.get $heap_start)
(i32.sub (global.get $heap_end) (global.get $heap_start)))
(i32.store (i32.add (global.get $heap_start) (i32.const 4))
(i32.const 0)))
;; 分配内存
(func $heap_alloc (param $size i32) (result i32)
(local $current i32)
(local $block_size i32)
(local $next_offset i32)
(local $aligned_size i32)
;; 对齐到8字节边界
(local.set $aligned_size
(i32.and
(i32.add (local.get $size) (i32.const 15))
(i32.const 0xFFFFFFF8)))
(local.set $current (global.get $heap_start))
;; 遍历空闲块链表
(loop $find_block
;; 检查当前块
(local.set $block_size (i32.load (local.get $current)))
(local.set $next_offset (i32.load (i32.add (local.get $current) (i32.const 4))))
;; 如果块足够大
(if (i32.ge_u (local.get $block_size)
(i32.add (local.get $aligned_size) (i32.const 8)))
(then
;; 分割块
(call $split_block (local.get $current) (local.get $aligned_size))
;; 返回数据指针（跳过头部）
(return (i32.add (local.get $current) (i32.const 8)))))
;; 移动到下一块
(if (i32.eqz (local.get $next_offset))
(then (return (i32.const 0)))) ;; 没有找到合适的块
(local.set $current (i32.add (global.get $heap_start) (local.get $next_offset)))
(br $find_block)))
;; 分割内存块
(func $split_block (param $block i32) (param $size i32)
(local $block_size i32)
(local $remaining_size i32)
(local $new_block i32)
(local.set $block_size (i32.load (local.get $block)))
(local.set $remaining_size
(i32.sub (local.get $block_size) (i32.add (local.get $size) (i32.const 8))))
;; 如果剩余大小足够创建新块
(if (i32.gt_u (local.get $remaining_size) (i32.const 16))
(then
;; 更新当前块大小
(i32.store (local.get $block) (i32.add (local.get $size) (i32.const 8)))
;; 创建新的空闲块
(local.set $new_block
(i32.add (local.get $block) (i32.add (local.get $size) (i32.const 8))))
(i32.store (local.get $new_block) (local.get $remaining_size))
(i32.store (i32.add (local.get $new_block) (i32.const 4))
(i32.load (i32.add (local.get $block) (i32.const 4))))
;; 更新链接
(i32.store (i32.add (local.get $block) (i32.const 4))
(i32.sub (local.get $new_block) (global.get $heap_start))))))
;; 释放内存
(func $heap_free (param $ptr i32)
(local $block i32)
;; 获取块头地址
(local.set $block (i32.sub (local.get $ptr) (i32.const 8)))
;; 简化版：直接添加到空闲链表头部
(i32.store (i32.add (local.get $block) (i32.const 4))
(i32.sub (global.get $heap_start) (global.get $heap_start)))
;; 这里应该实现块合并逻辑，但为简化省略
)
;; 获取堆统计信息
(func $heap_stats (param $total_size_ptr i32) (param $free_blocks_ptr i32)
(local $current i32)
(local $total_free i32)
(local $block_count i32)
(local.set $current (global.get $heap_start))
(loop $count_blocks
(if (i32.eqz (local.get $current))
(then (br $count_blocks)))
(local.set $total_free
(i32.add (local.get $total_free) (i32.load (local.get $current))))
(local.set $block_count (i32.add (local.get $block_count) (i32.const 1)))
(local.set $current
(i32.add (global.get $heap_start)
(i32.load (i32.add (local.get $current) (i32.const 4)))))
(br $count_blocks))
(i32.store (local.get $total_size_ptr) (local.get $total_free))
(i32.store (local.get $free_blocks_ptr) (local.get $block_count)))
(export "init_heap" (func $init_heap))
(export "heap_alloc" (func $heap_alloc))
(export "heap_free" (func $heap_free))
(export "heap_stats" (func $heap_stats))
(export "memory" (memory 0)))
5.5.2 内存池分配器
(module
(memory 1)
;; 固定大小内存池
(global $pool_16_start i32 (i32.const 0x2000)) ;; 16字节块池
(global $pool_32_start i32 (i32.const 0x4000)) ;; 32字节块池
(global $pool_64_start i32 (i32.const 0x6000)) ;; 64字节块池
(global $pool_16_free_head (mut i32) (i32.const 0))
(global $pool_32_free_head (mut i32) (i32.const 0))
(global $pool_64_free_head (mut i32) (i32.const 0))
;; 初始化内存池
(func $init_pools
;; 初始化16字节池（128个块 = 2KB）
(call $init_pool (global.get $pool_16_start) (i32.const 128) (i32.const 16))
(global.set $pool_16_free_head (global.get $pool_16_start))
;; 初始化32字节池（64个块 = 2KB）
(call $init_pool (global.get $pool_32_start) (i32.const 64) (i32.const 32))
(global.set $pool_32_free_head (global.get $pool_32_start))
;; 初始化64字节池（32个块 = 2KB）
(call $init_pool (global.get $pool_64_start) (i32.const 32) (i32.const 64))
(global.set $pool_64_free_head (global.get $pool_64_start)))
;; 初始化单个池
(func $init_pool (param $start_addr i32) (param $block_count i32) (param $block_size i32)
(local $i i32)
(local $current i32)
(local $next i32)
(local.set $current (local.get $start_addr))
(loop $init_loop
(if (i32.ge_u (local.get $i) (i32.sub (local.get $block_count) (i32.const 1)))
(then (br $init_loop)))
;; 计算下一个块的地址
(local.set $next (i32.add (local.get $current) (local.get $block_size)))
;; 在当前块的开头存储下一个块的地址
(i32.store (local.get $current) (local.get $next))
(local.set $current (local.get $next))
(local.set $i (i32.add (local.get $i) (i32.const 1)))
(br $init_loop))
;; 最后一个块指向NULL
(i32.store (local.get $current) (i32.const 0)))
;; 池分配
(func $pool_alloc (param $size i32) (result i32)
(local $ptr i32)
;; 根据大小选择合适的池
(if (i32.le_u (local.get $size) (i32.const 16))
(then
(local.set $ptr (global.get $pool_16_free_head))
(if (local.get $ptr)
(then
(global.set $pool_16_free_head (i32.load (local.get $ptr)))
(return (local.get $ptr))))))
(if (i32.le_u (local.get $size) (i32.const 32))
(then
(local.set $ptr (global.get $pool_32_free_head))
(if (local.get $ptr)
(then
(global.set $pool_32_free_head (i32.load (local.get $ptr)))
(return (local.get $ptr))))))
(if (i32.le_u (local.get $size) (i32.const 64))
(then
(local.set $ptr (global.get $pool_64_free_head))
(if (local.get $ptr)
(then
(global.set $pool_64_free_head (i32.load (local.get $ptr)))
(return (local.get $ptr))))))
;; 如果所有池都满了，返回NULL
i32.const 0)
;; 池释放
(func $pool_free (param $ptr i32) (param $size i32)
;; 根据大小确定池类型
(if (i32.le_u (local.get $size) (i32.const 16))
(then
(i32.store (local.get $ptr) (global.get $pool_16_free_head))
(global.set $pool_16_free_head (local.get $ptr))
(return)))
(if (i32.le_u (local.get $size) (i32.const 32))
(then
(i32.store (local.get $ptr) (global.get $pool_32_free_head))
(global.set $pool_32_free_head (local.get $ptr))
(return)))
(if (i32.le_u (local.get $size) (i32.const 64))
(then
(i32.store (local.get $ptr) (global.get $pool_64_free_head))
(global.set $pool_64_free_head (local.get $ptr)))))
;; 获取池状态
(func $pool_stats (param $pool_size i32) (result i32)
(local $free_count i32)
(local $current i32)
;; 根据池大小选择对应的空闲链表头
(if (i32.eq (local.get $pool_size) (i32.const 16))
(then (local.set $current (global.get $pool_16_free_head))))
(if (i32.eq (local.get $pool_size) (i32.const 32))
(then (local.set $current (global.get $pool_32_free_head))))
(if (i32.eq (local.get $pool_size) (i32.const 64))
(then (local.set $current (global.get $pool_64_free_head))))
;; 计算空闲块数量
(loop $count_loop
(if (i32.eqz (local.get $current))
(then (br $count_loop)))
(local.set $free_count (i32.add (local.get $free_count) (i32.const 1)))
(local.set $current (i32.load (local.get $current)))
(br $count_loop))
local.get $free_count)
(export "init_pools" (func $init_pools))
(export "pool_alloc" (func $pool_alloc))
(export "pool_free" (func $pool_free))
(export "pool_stats" (func $pool_stats))
(export "memory" (memory 0)))
本章小结
通过本章学习，你已经全面掌握了：
- 线性内存模型：WebAssembly 的内存组织和布局策略
- 内存操作：加载、存储指令及其对齐优化
- 数据段：静态和动态数据初始化技术
- 性能优化：缓存友好访问和向量化技术
- 内存管理：分配器设计和内存池实现
这些技能为构建高性能的 WebAssembly 应用奠定了重要基础。
📝 进入下一步：第6章 控制流
🎯 重点技能：
- ✅ 内存模型理解
- ✅ 内存操作优化
- ✅ 数据段应用
- ✅ 性能调优技巧
- ✅ 内存管理设计