慢速查找的引入和定位
- 上篇文章分析了,方法的本质是消息的发送,并主要探究了方法的快速查找流程,即去类的缓存中查找。如果缓存中没有查找到,就会
JumpMis
s,进入__objc_msgSend_uncached
流程。 __objc_msgSend_uncached
,会走方法列表查询流程MethodTableLookup
。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
TailCallFunctionPointer x17
END_ENTRY __objc_msgSend_uncached
MethodTableLookup中
最重要的方法,即慢速查找流程的重点_lookUpImpOrForward
。这里有两个细节去找到_lookUpImpOrForward
方法。- 搜索
lookUpImpOrForward
在汇编代码中,我们是没有找到的。这里可以猜测在底层c/ c++
层,即objc
层面实现了lookUpImpOrForward
。有一个经验分享:在汇编层方法是__两个下划线,到c/c++
层去掉一个下划线_,到OC
层再去掉一个下划线。 - 也可以通过程序来验证汇编的流程找到lookUpImpOrForward在哪。具体操作为,
- 开启汇编调试【
Debug -- Debug worlflow -- 勾选Always show Disassembly】
,运行程序 - 运行到
objc_msgSend
,摁住ctrol+stepinto
,进入objc_msgSend
的汇编流程 - 运行到
_objc_msgSend_uncached
,摁住ctrol+stepinto
,进入_objc_msgSend_uncached
的汇编流程 _objc_msgSend_uncached
的汇编流程找到lookUpImpOrForward
的位置。
- 开启汇编调试【
- 搜索
.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
慢速查找的流程
lookUpImpOrForward
代码如下:
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();
// 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.
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.
//
// TODO: this check is quite costly during process startup.
checkIsKnownClass(cls);
if (slowpath(!cls->isRealized())) {
cls = realizeClassMaybeSwiftAndLeaveLocked(cls, runtimeLock);
// runtimeLock may have been dropped but is now locked again
}
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.
Method meth = getMethodNoSuper_nolock(curClass, sel);
if (meth) {
imp = meth->imp;
goto done;
}
if (slowpath((curClass = curClass->superclass) == nil)) {
// No implementation found, and method resolver didn't help.
// Use forwarding.
imp = forward_imp;
break;
}
// Halt if there is a cycle in the superclass chain.
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)) {
behavior ^= LOOKUP_RESOLVER;
return resolveMethod_locked(inst, sel, cls, behavior);
}
done:
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;
}
一、多线程防护再查找缓存
if (fastpath(behavior & LOOKUP_CACHE)) {
imp = cache_getImp(cls, sel);
if (imp) goto done_nolock;
}
- 这里会再进行一次快速查找流程,如果查到
IMP
,就返回IMP
。这里是对多线程的一个保护举动。如果慢速查找开始之前,有一个线程缓存了cache
,就再查找一遍。
二、确定类
确定类是已知类
checkIsKnownClass(cls);
static void
checkIsKnownClass(Class cls)
{
if (slowpath(!isKnownClass(cls))) {
_objc_fatal("Attempt to use unknown class %p.", cls);
}
}
- 确定是已知类。当前传入的
cls
是否是已经存在和声明的类。如果不是,就会报错。
确定类已经实现
if (slowpath(!cls->isRealized())) {
cls = realizeClassMaybeSwiftAndLeaveLocked(cls, runtimeLock);
// runtimeLock may have been dropped but is now locked again
}
- 确定当前类是否已经实现,如果没有实现,就对类的所有信息进行赋值,如前面分析的一样,
realizeClassWithoutSwift
方法包含了对类的class_rw_t,class_ro_t,superclass,initClassIsa,flags
,确定类
的继承链和mete
继承类等所有信息赋值,确定我们查询的类的信息的完整性。
确定类已经初始化
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
}
- 确定当前的类是否初始化,系统层会默认为我们初始化当前类。即
lookUpImpOrNil(nil, @selector(initialize), cls, LOOKUP_INITIALIZE)
; 本质上是调用了initialize
方法。
三、查找IMP
去本类的继承链方法列表查找
- 核心循环,去继承链循环。首先去继承链的方法列表查询,核心方法
getMethodNoSuper_nolock
。 getMethodNoSuper_nolock -> search_method_list_inline
。这里有一个细节是对类的cls->data()->methods()
方法列表查询,属于双层循环查找。可以简单推测类的方法在类中的method_array_t
中存储是二维数组。method_array_t -> method_list_t ->method_t
。- 方法的查找
findMethodInSortedMethodList
这里采用了经典二分法
查找方法列表。 - 如果查询到
sel
,就返回IMP
。并在当前类cache_fill
缓存方法,便于下次的快速查找。没有就向下。
去继承链的缓存查找
- 核心方法
cache_getImp
。- 如果父类的缓存中找到
sel
,就返回IMP
。并在当前类cache_fill
缓存方法,便于下次的快速查找。 - 如果没有找到就继续递归到父类的父类,直到nil。
imp = forward_imp
跳出递归。 - 有一种特殊情况,刚好父类缓存了本类的
forward_imp
,我们也不做处理,直接跳出。
- 如果父类的缓存中找到
动态方法决议查找
if (slowpath(behavior & LOOKUP_RESOLVER)) {
behavior ^= LOOKUP_RESOLVER;
return resolveMethod_locked(inst, sel, cls, behavior);
}
- 如果继承链的循环方法列表里和继承链的缓存中都没有,系统为我们设置了一次补救机会,
resolveMethod_locked
动态方法决议。-
这里通过与或控制了进入的条件,如果没进行到动态方法决议,则执行动态方法决议
-
如果执行过一次动态方法决议,则方法进入消息转发流程。
-
动态方法决议和消息的转发待续
- 这里时间不是很充裕了,有点累了,下次再针对动态方法决议和消息的转发详细研究,再写博客。