iOS 底层alloc init new 源码流程示例分析

有问必答 發表於 2022-12-25 16:25:54

iOS 底层alloc init new 源码流程示例分析

<div id="navCategory"><h5 class="catalogue">目录</h5><ul class="first_class_ul"><li>alloc&init 的源码流程图</li><ul class="second_class_ul"><li>Init 源码探索</li><li>new 的源码探索</li></ul></ul></div><p class="maodian"></p><h2>alloc&init 的源码流程图</h2>
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<p style="text-align:center"><img alt="" src="https://img.jbzj.com/file_images/article/202212/20221225091111017.jpg" /></p>
<p>首先创建Person 类，在main函数创建Person 实例 <code>Person *p = ;</code> 1.进入到alloc 方法的源码实现</p>
<div class="jb51code"><pre class="brush:cpp;">+ (id)alloc {
return _objc_rootAlloc(self);
}
</pre></div>
<p>2.跳转到<code>_objc_rootAlloc</code> 源码实现</p>
<div class="jb51code"><pre class="brush:cpp;">id
_objc_rootAlloc(Class cls)
{
return callAlloc(cls, false/*checkNil*/, true/*allocWithZone*/);
}
</pre></div>
<p>3.跳转至 <code>callAlloc</code> 的源码实现</p>
<div class="jb51code"><pre class="brush:cpp;">static ALWAYS_INLINE id
callAlloc(Class cls, bool checkNil, bool allocWithZone=false)
{
#if __OBJC2__ //有可用的编译器优化
if (slowpath(checkNil && !cls)) return nil;
//判断是否自定义实现了 +allocWithZone 方法
if (fastpath(!cls->ISA()->hasCustomAWZ())) {
   return _objc_rootAllocWithZone(cls, nil);
}
#endif
// No shortcuts available.
if (allocWithZone) {
   return ((id(*)(id, SEL, struct _NSZone *))objc_msgSend)(cls, @selector(allocWithZone:), nil);
}
return ((id(*)(id, SEL))objc_msgSend)(cls, @selector(alloc));
</pre></div>
<p>该方法中有两个定义的宏</p>
<div class="jb51code"><pre class="brush:cpp;">#define fastpath(x) (__builtin_expect(bool(x), 1))
#define slowpath(x) (__builtin_expect(bool(x), 0))
</pre></div>
<p>其中__builtin_expect指令由gcc 引入，目的：1. 编译器可以对代码进行优化，以减少指令跳转带来的性能下降，2.作用：允许程序员将最有可能执行的分支告诉编译器；3.写法为： __builtin_expect(EXP, N) , 表示 EXP == N的概率很大；</p>
<p>fastPath 定义的__builtin_expect(bool(x), 1) 表示x 的值为真的可能性更大；</p>
<p>slowpath 定义的__builtin_expect(bool(x), 0) 表示x 的值为假的可能性更大；</p>
<p>日常开发中可以通过设置来优化编译器，达到性能优化的目的，设置路径： Build Settiing -> Optimization Level -> Debug -> 将None 改为fastest/smallest 4.跳转至 <code>_objc_rootAllocWithZone</code> 的源码实现</p>
<div class="jb51code"><pre class="brush:cpp;">id
_objc_rootAllocWithZone(Class cls, objc_zone_t zone __unused)
{
// allocWithZone under __OBJC2__ ignores the zone parameter
return _class_createInstanceFromZone(cls, 0, nil,
OBJECT_CONSTRUCT_CALL_BADALLOC);
}
</pre></div>
<p>5.跳转至 <code>_class_createInstanceFromZone</code> 源码实现</p>
<div class="jb51code"><pre class="brush:cpp;">static ALWAYS_INLINE id
_class_createInstanceFromZone(Class cls, size_t extraBytes, void *zone,int construct_flags = OBJECT_CONSTRUCT_NONE,bool cxxConstruct = true,size_t *outAllocatedSize = nil)
{
ASSERT(cls->isRealized());
// Read class's info bits all at once for performance
// 一次性读取累的的信息以提高性能
bool hasCxxCtor = cxxConstruct && cls->hasCxxCtor();
bool hasCxxDtor = cls->hasCxxDtor();
bool fast = cls->canAllocNonpointer();
size_t size;
size = cls->instanceSize(extraBytes);
if (outAllocatedSize) *outAllocatedSize = size;
id obj;
#if SUPPORT_ZONES
// 支持zone
// 早期的内存是通过zone申请的 ilo89i='
if (zone) {
   obj = (id)malloc_zone_calloc((malloc_zone_t *)zone, 1, size);
} else {
#endif
   obj = (id)calloc(1, size);
#if SUPPORT_ZONES
}
#endif
if (slowpath(!obj)) {
   if (construct_flags & OBJECT_CONSTRUCT_CALL_BADALLOC) {
         return _objc_callBadAllocHandler(cls);
   }
   return nil;
}
if (!zone && fast) {
   obj->initInstanceIsa(cls, hasCxxDtor);
} else {
   // Use raw pointer isa on the assumption that they might be
   // doing something weird with the zone or RR.
   obj->initIsa(cls);
}
if (fastpath(!hasCxxCtor)) {
   return obj;
}
construct_flags |= OBJECT_CONSTRUCT_FREE_ONFAILURE;
return object_cxxConstructFromClass(obj, cls, construct_flags);
}
</pre></div>
<p>该方法中有三个核心方法:</p>
<ul><li>cls->instanceSize：计算所需内存大小, 源码实现</li></ul>
<div class="jb51code"><pre class="brush:cpp;">inline size_t instanceSize(size_t extraBytes) const {
// 快速计算内存大小
   if (fastpath(cache.hasFastInstanceSize(extraBytes))) {
         return cache.fastInstanceSize(extraBytes);
   }
   size_t size = alignedInstanceSize() + extraBytes;
   // CF requires all objects be at least 16 bytes.
   if (size < 16) size = 16;
   return size;
}
</pre></div>
<p><code>fastInstanceSize</code> 的源码实现</p>
<div class="jb51code"><pre class="brush:cpp;">size_t fastInstanceSize(size_t extra) const
{
   ASSERT(hasFastInstanceSize(extra));
   if (__builtin_constant_p(extra) && extra == 0) {
         return _flags & FAST_CACHE_ALLOC_MASK16;
   } else {
         size_t size = _flags & FAST_CACHE_ALLOC_MASK;
         // remove the FAST_CACHE_ALLOC_DELTA16 that was added
         // by setFastInstanceSize
         return align16(size + extra - FAST_CACHE_ALLOC_DELTA16);
   }
}
</pre></div>
<p><code>align16</code>的源码实现</p>
<div class="jb51code"><pre class="brush:cpp;">static inline size_t align16(size_t x) {
return (x + size_t(15)) & ~size_t(15);
}
</pre></div>
<p>断点调试此处的参数x 为8 即： align16(8)</p>
<p style="text-align:center"><img alt="" src="https://img.jbzj.com/file_images/article/202212/20221225091111018.jpg" /></p>
<p>2.<code>calloc</code> 申请内存，返回地址指针向内存中申请大小为 <code>instanceSize</code>计算的内存，并将内存地址的指针返回，赋值给obj,<code>obj = (id)calloc(1, size);</code></p>
<p>3.<code>obj->initInstanceIsa(cls, hasCxxDtor);</code> : 初始化isa 指针并将类与isa 关联</p>
<p class="maodian"></p><h3>Init 源码探索</h3>
<p>通过查看 Init 源码</p>
<div class="jb51code"><pre class="brush:cpp;">- (id)init {
return _objc_rootInit(self);
}
</pre></div>
<div class="jb51code"><pre class="brush:cpp;">id
_objc_rootInit(id obj)
{
// In practice, it will be hard to rely on this function.
// Many classes do not properly chain -init calls.
return obj;
}
</pre></div>
<p>通过源码实现可以看到 Init 就是将传入的对象直接返回</p>
<p class="maodian"></p><h3>new 的源码探索</h3>
<p>日常开发中，对象的创建有 alloc Init 和new ，现在看下new的源码实现</p>
<div class="jb51code"><pre class="brush:cpp;">+ (id)new {
return ;
}
</pre></div>
<p>通过源码可以看出 new 相当于alloc init 过程，但是二者有何区别以下是其他博主总结的，引用一下</p>
<p style="text-align:center"><img alt="" src="https://img.jbzj.com/file_images/article/202212/20221225091111019.jpg" /></p>
<p>以上就是iOS 底层alloc init new 源码流程示例分析的详细内容，更多关于iOS 底层alloc init new分析的资料请关注琼殿技术社区其它相关文章！</p>

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iOS 底层alloc init new 源码流程示例分析