在上一篇 objc_msgSend 流程分析（快速查找） 中解析了消息的快速查找流程，那有快速肯定就有慢速查找，今天我们就来分析下消息的慢速查找流程
objc_msgSend 慢速查找流程
在快速查找流程中，如果没有找到方法实现，会走到 CheckMiss 还是 JumpMiss，因为带的参数为 NORMAL， 最终会走到 __objc_msgSend_uncached 汇编里面
__objc_msgSend_uncached 汇编源码如下
STATIC_ENTRY __objc_msgSend_uncached
UNWIND __objc_msgSend_uncached, FrameWithNoSaves
// THIS IS NOT A CALLABLE C FUNCTION
// Out-of-band p16 is the class to search
MethodTableLookup //查询方法列表
TailCallFunctionPointer x17
END_ENTRY __objc_msgSend_uncached
搜索 MethodTableLookup 的汇编实现，在 objc-msg-arm64.s 下，其源码的核心是 _lookUpImpOrForward
.macro MethodTableLookup
// push frame
SignLR
stp fp, lr, [sp, #-16]!
mov fp, sp
// save parameter registers: x0..x8, q0..q7
sub sp, sp, #(10*8 + 8*16)
stp q0, q1, [sp, #(0*16)]
stp q2, q3, [sp, #(2*16)]
stp q4, q5, [sp, #(4*16)]
stp q6, q7, [sp, #(6*16)]
stp x0, x1, [sp, #(8*16+0*8)]
stp x2, x3, [sp, #(8*16+2*8)]
stp x4, x5, [sp, #(8*16+4*8)]
stp x6, x7, [sp, #(8*16+6*8)]
str x8, [sp, #(8*16+8*8)]
// lookUpImpOrForward(obj, sel, cls, LOOKUP_INITIALIZE | LOOKUP_RESOLVER)
// receiver and selector already in x0 and x1
mov x2, x16
mov x3, #3
bl _lookUpImpOrForward
// IMP in x0
mov x17, x0
// restore registers and return
ldp q0, q1, [sp, #(0*16)]
ldp q2, q3, [sp, #(2*16)]
ldp q4, q5, [sp, #(4*16)]
ldp q6, q7, [sp, #(6*16)]
ldp x0, x1, [sp, #(8*16+0*8)]
ldp x2, x3, [sp, #(8*16+2*8)]
ldp x4, x5, [sp, #(8*16+4*8)]
ldp x6, x7, [sp, #(8*16+6*8)]
ldr x8, [sp, #(8*16+8*8)]
mov sp, fp
ldp fp, lr, [sp], #16
AuthenticateLR
.endmacro
验证
创建一个新工程，创建 LCPerson 类，声明一个实例方法，在main 函数中添加断点
运行项目，会走到断点处，此时打开汇编调试：Debug -> Debug worlflow -> Always show Disassembly，会出现如下界面
在 objc_msgSend 加一个断点，执行下一步，会断在 objc_msgSend 处，按住 control 点击 stepinto 键，进入 objc_msgSend 的汇编
在 _objc_msgSend_uncached 汇编处加一个断点，执行下一步，会断在 _objc_msgSend_uncached 处，按住 control，点击 stepinto 键，进入 _objc_msgSend_uncached 汇编
此时，可以看到，汇编最后走的就是 lookUpImpOrForward
慢速查找
- 根据上面汇编调试，在
objc-781源码中查找lookUpImpOrForward的实现，方法是在objc-runtime-new.mm文件中，是一个 C 函数的实现
IMP lookUpImpOrForward(id inst, SEL sel, Class cls, int behavior)
{
const IMP forward_imp = (IMP)_objc_msgForward_impcache;
IMP imp = nil;
Class curClass;
runtimeLock.assertUnlocked();
// 快速查找，如果找到则直接返回imp
//目的：防止多线程操作时，刚好调用函数，此时缓存进来了
// Optimistic cache lookup
if (fastpath(behavior & LOOKUP_CACHE)) {
imp = cache_getImp(cls, sel);
if (imp) goto done_nolock;
}
// runtimeLock is held during isRealized and isInitialized checking
// to prevent races against concurrent realization.
// runtimeLock is held during method search to make
// method-lookup + cache-fill atomic with respect to method addition.
// Otherwise, a category could be added but ignored indefinitely because
// the cache was re-filled with the old value after the cache flush on
// behalf of the category.
//保证读取的线程安全
runtimeLock.lock();
// We don't want people to be able to craft a binary blob that looks like
// a class but really isn't one and do a CFI attack.
//
// To make these harder we want to make sure this is a class that was
// either built into the binary or legitimately registered through
// objc_duplicateClass, objc_initializeClassPair or objc_allocateClassPair.
//
//判断是否是一个已知的类：判断当前类是否是已经被认可的类，即已经加载的类
checkIsKnownClass(cls);
//判断类是否实现
if (slowpath(!cls->isRealized())) {
cls = realizeClassMaybeSwiftAndLeaveLocked(cls, runtimeLock);
// runtimeLock may have been dropped but is now locked again
}
//判断类是否初始化
if (slowpath((behavior & LOOKUP_INITIALIZE) && !cls->isInitialized())) {
cls = initializeAndLeaveLocked(cls, inst, runtimeLock);
// runtimeLock may have been dropped but is now locked again
// If sel == initialize, class_initialize will send +initialize and
// then the messenger will send +initialize again after this
// procedure finishes. Of course, if this is not being called
// from the messenger then it won't happen. 2778172
}
runtimeLock.assertLocked();
curClass = cls;
// The code used to lookpu the class's cache again right after
// we take the lock but for the vast majority of the cases
// evidence shows this is a miss most of the time, hence a time loss.
//
// The only codepath calling into this without having performed some
// kind of cache lookup is class_getInstanceMethod().
for (unsigned attempts = unreasonableClassCount();;) {
// curClass method list.
//当前类方法列表（采用二分查找算法），如果找到，则返回，将方法缓存到cache中
Method meth = getMethodNoSuper_nolock(curClass, sel);
if (meth) {
imp = meth->imp;
goto done;
}
//当前类 = 当前类的父类，并判断父类是否为nil
if (slowpath((curClass = curClass->superclass) == nil)) {
// No implementation found, and method resolver didn't help.
// Use forwarding.
//未找到方法实现，方法解析器也不行，使用转发
imp = forward_imp;
break;
}
// Halt if there is a cycle in the superclass chain.
// 如果父类链中存在循环，则停止
if (slowpath(--attempts == 0)) {
_objc_fatal("Memory corruption in class list.");
}
// Superclass cache.
imp = cache_getImp(curClass, sel);
if (slowpath(imp == forward_imp)) {
// Found a forward:: entry in a superclass.
// Stop searching, but don't cache yet; call method
// resolver for this class first.
break;
}
if (fastpath(imp)) {
// Found the method in a superclass. Cache it in this class.
goto done;
}
}
// No implementation found. Try method resolver once.
if (slowpath(behavior & LOOKUP_RESOLVER)) {
//动态方法决议的控制条件，表示流程只走一次
behavior ^= LOOKUP_RESOLVER;
return resolveMethod_locked(inst, sel, cls, behavior);
}
done:
//存储到缓存
log_and_fill_cache(cls, imp, sel, inst, curClass);
runtimeLock.unlock();
done_nolock:
if (slowpath((behavior & LOOKUP_NIL) && imp == forward_imp)) {
return nil;
}
return imp;
}
lookupImpOrForward 主要就是查找方法并返回 imp
- 从缓存中查找方法
-
checkIsKnownClass判断当前类是否是系统的类或者是自定创建的类，只有是系统已知的才能执行查找 -
realizeClassMaybeSwiftAndLeaveLocked类是否实现，获得当前对象继承关系，方便从父类查找 -
initializeAndLeaveLocked是否初始化，主要是系统方法的自动调用，比如initalize等 - for 循环，按照类继承链或者元类继承链的顺序查找
- 从当前类查找方法
- 从父类查找方法，并将父类赋值给curClass
- 如果父类链中存在循环，则报错，终止循环
- 从父类缓存中查找方法：找不到继续循环查找；找到，直接走
done
- 判断是否执行过动态方法解析
- 没有，执行动态方法解析
- 执行过一次动态方法解析，则走到消息转发流程
getMethodNoSuper_nolock 方法解析
static method_t *
getMethodNoSuper_nolock(Class cls, SEL sel)
{
runtimeLock.assertLocked();
ASSERT(cls->isRealized());
auto const methods = cls->data()->methods();
for (auto mlists = methods.beginLists(),
end = methods.endLists();
mlists != end;
++mlists)
{
method_t *m = search_method_list_inline(*mlists, sel);
if (m) return m;
}
return nil;
}
继续查看 search_method_list_inline 的源码实现
ALWAYS_INLINE static method_t *
search_method_list_inline(const method_list_t *mlist, SEL sel)
{
int methodListIsFixedUp = mlist->isFixedUp();
int methodListHasExpectedSize = mlist->entsize() == sizeof(method_t);
if (fastpath(methodListIsFixedUp && methodListHasExpectedSize)) {
return findMethodInSortedMethodList(sel, mlist);
} else {
// Linear search of unsorted method list
for (auto& meth : *mlist) {
if (meth.name == sel) return &meth;
}
}
#if DEBUG
// sanity-check negative results
if (mlist->isFixedUp()) {
for (auto& meth : *mlist) {
if (meth.name == sel) {
_objc_fatal("linear search worked when binary search did not");
}
}
}
#endif
return nil;
}
findMethodInSortedMethodList 的源码实现
ALWAYS_INLINE static method_t *
findMethodInSortedMethodList(SEL key, const method_list_t *list)
{
ASSERT(list);
const method_t * const first = &list->first;
const method_t *base = first;
const method_t *probe;
uintptr_t keyValue = (uintptr_t)key;
uint32_t count;
for (count = list->count; count != 0; count >>= 1) {
probe = base + (count >> 1);
uintptr_t probeValue = (uintptr_t)probe->name;
if (keyValue == probeValue) {
// `probe` is a match.
// Rewind looking for the *first* occurrence of this value.
// This is required for correct category overrides.
while (probe > first && keyValue == (uintptr_t)probe[-1].name) {
probe--;
}
return (method_t *)probe;
}
if (keyValue > probeValue) {
base = probe + 1;
count--;
}
}
return nil;
}
方法列表是有序的，所以使用二分查找，更快速。在源码中两点需要注意
-
count >> 1，就是右移一位，变成原来的一半。例如00010001(17)右移一位就变成了00001000(8)。 -
while (probe > first && keyValue == (uintptr_t)probe[-1].name) { probe--; }是因为分类方法中可能出现同名方法
总结
-
- 从缓存中查找，如果有返回
imp
- 从缓存中查找，如果有返回
-
- 从方法列表中查找（使用二分查找），如果有，存储到缓存，返回
imp，如果没有，若父类中存在，从父类中查找，否则走动态决议
-
- 将父类赋值给当前类，重复上述动作，直到父类不存在
-
- 找到
imp则存入缓存中
- 找到
- 从方法列表中查找（使用二分查找），如果有，存储到缓存，返回
流程图