objc_msgSend-慢速查找流程

2021-05-17  本文已影响0人  伍骁辛

在objc_msgSend-快速查找流程中我们讲到，objc_msgSend首先通过汇编快速查找方法缓存，如果找到，调用TailCallCachedImp直接将方法缓存起来然后进行调用就可以了，如果查找不到就跳到CheckMiss，然后走慢速查找流程。接下来我们分析一下objc_msgSend慢速查找流程。

objc_msgSend查找流程：

在objc_msgSend-快速查找流程中我们分析了，先通过GetClassFromIsa_p16获取到传入对象所属的类，然后通过CacheLookup在方法缓存表中查找，如果缓存命中则走CacheHit方法，缓存没命中走CheckMiss方法。

一、CheckMiss 方法

  .macro CheckMiss
// miss if bucket->sel == 0
.if $0 == GETIMP
cbz p9, LGetImpMiss
.elseif $0 == NORMAL //传进来的是NORMAL,所以走这里
cbz p9, __objc_msgSend_uncached
.elseif $0 == LOOKUP
cbz p9, __objc_msgLookup_uncached
.else
.abort oops
.endif
.endmacro

传进来的是NORMAL，所以会走到 __objc_msgSend_uncached 方法

二、__objc_msgSend_uncached 方法

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

紧接着又会来到 MethodTableLookup 方法

三、MethodTableLookup 方法

.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

接着又会来到 lookUpImpOrForward 方法

四、lookUpImpOrForward 方法

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.lock();


// 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.assertLocked();
curClass = cls;

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)) {
        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); // 有问题???? cache_getImp - lookup - lookUpImpOrForward
    if (slowpath(imp == forward_imp)) {
        break;
    }
    if (fastpath(imp)) {
        goto done;
    }
}

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;
}
4.1 判断缓存是否存在，存在则直接通过cls和sel直接获取imp，并返回。
if (fastpath(behavior & LOOKUP_CACHE)) {
   imp = cache_getImp(cls, sel);
   if (imp) goto done_nolock;
 }
4.2 相关类信息判断
4.3 查找本类的方法列表
4.3.1 利用 getMethodNoSuper_nolock 查找本类的方法列表，如果找到了，进入 goto done;
4.3.2 getMethodNoSuper_nolock 方法

调用调用 search_method_list_inline 方法 对本类方法列表进行查找

static method_t *
getMethodNoSuper_nolock(Class cls, SEL sel)
{
runtimeLock.assertLocked();

ASSERT(cls->isRealized());
// fixme nil cls? 
// fixme nil sel?

auto const methods = cls->data()->methods();
for (auto mlists = methods.beginLists(),
          end = methods.endLists();
     mlists != end;
     ++mlists)
{
    // <rdar://problem/46904873> getMethodNoSuper_nolock is the hottest
    // caller of search_method_list, inlining it turns
    // getMethodNoSuper_nolock into a frame-less function and eliminates
    // any store from this codepath.
    method_t *m = search_method_list_inline(*mlists, sel);
    if (m) return m;
}

return nil;
}
4.3.3 search_method_list_inline 方法

调用 findMethodInSortedMethodList 方法 对本类方法列表进行二分查找

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;
}
4.3.4 findMethodInSortedMethodList 方法

对本类方法列表进行二分查找

  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;
}
4.4 done 方法
4.5 log_and_fill_cache 方法
4.6 递归查找父类的缓存
4.6.1 查找本类的方法列表 如果找不到，就递归查找父类的缓存
4.7 递归父类缓存查找不到，利用 imp = forward_imp
if (slowpath((curClass = curClass->superclass) == nil)) {
// No implementation found, and method resolver didn't help.
// Use forwarding.
imp = forward_imp;
break;
}
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;
}
4.7.1 forward_imp
4.7.2 _objc_msgForward_impcache
4.7.3 TailCallFunctionPointer 方法

TailCallFunctionPointer 方法 就是返回指针的值，返回 x17 的值，x17 的值是 __objc_forward_handler 方法 确定的

.macro TailCallFunctionPointer
// $0 = function pointer value
braaz   $0
.endmacro
4.7.4 __objc_forward_handler 方法
 objc_defaultForwardHandler(id self, SEL sel)
{
   _objc_fatal("%c[%s %s]: unrecognized selector sent to instance %p "
            "(no message forward handler is installed)", 
            class_isMetaClass(object_getClass(self)) ? '+' : '-', 
            object_getClassName(self), sel_getName(sel), self);
}
void *_objc_forward_handler = (void*)objc_defaultForwardHandler;

如果方法没有实现，imp 会置换成 forward_imp , forward_imp 最终会走到 __objc_forward_handler 方法 返回 unrecognized selector sent to instance ... 信息，我们查看一下方法没有实现的报错信息会发现，报错信息的模板原来在这。

Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[LGPerson say666]: unrecognized selector sent to instance 0x1007738f0'

4.8 动态方法决议

在4.7中将 imp 置换成 forward_imp 后，会 break 跳出循环，走到动态方法决议这里：

 if (slowpath(behavior & LOOKUP_RESOLVER)) {
    behavior ^= LOOKUP_RESOLVER;
    return resolveMethod_locked(inst, sel, cls, behavior);
 }
4.8.1 resolveMethod_locked 方法
resolveMethod_locked(id inst, SEL sel, Class cls, int behavior)
{
runtimeLock.assertLocked();
ASSERT(cls->isRealized());
// 方法没有你怎么不知道
// 报错
// 给你一次机会
runtimeLock.unlock();

if (! cls->isMetaClass()) {
    // try [cls resolveInstanceMethod:sel]
    resolveInstanceMethod(inst, sel, cls);
} 
else {
    // try [nonMetaClass resolveClassMethod:sel]
    // and [cls resolveInstanceMethod:sel]
    resolveClassMethod(inst, sel, cls);
    if (!lookUpImpOrNil(inst, sel, cls)) {
        resolveInstanceMethod(inst, sel, cls);
    }
}

// chances are that calling the resolver have populated the cache
// so attempt using it
return lookUpImpOrForward(inst, sel, cls, behavior | LOOKUP_CACHE);
}
4.8.2 resolveInstanceMethod 方法`
4.8.3 动态方法决议实现
#import "LGPerson.h"
#import <objc/message.h>

@implementation LGPerson

- (void)askHelp{
NSLog(@"%s",__func__);
}

+ (BOOL)resolveInstanceMethod:(SEL)sel{

if (sel == @selector(say666)) {
    NSLog(@"%@ 来了老弟",NSStringFromSelector(sel));
    
    IMP imp           = class_getMethodImplementation(self, @selector(askHelp));
    Method sayAskHelpMethod = class_getInstanceMethod(self, @selector(askHelp));
    const char *type  = method_getTypeEncoding(sayAskHelpMethod);
    return class_addMethod(self, sel, imp, type);
}

  return [super resolveInstanceMethod:sel];
}

5. 总结

最终到 _objc_forward_handler 方法 崩溃报错 unrecognized selector sent to instance ...。

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