iOS objc_msgSend 慢速查找流程分析

奉孝
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在 iOS objc_msgSend 流程 中我们讲到了， 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OK了，如果查找不到就跳到 CheckMiss ，然后走慢速查找流程。接下来我们一起分析一下 objc_msgSend 慢速查找流程。

objc_msgSend 查找流程：

  • 获取传入对象所属的类。
  • 获取该类的方法缓存表。
  • 使用传入的选择子在缓存中查询。
  • 如果缓存中不存在，则开始慢速查找流程。
  • 跳转至 IMP 映射位置的方法。

在 iOS objc_msgSend 流程 中我们分析过了，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

  • 根据所有已知类的列表检查给定的类，有问题直接内部抛出异常。
  • 判断类是否已经被实现，未实现则去实现，这部分后面类的加载章节会详细分析，主要是按照 superclass 和 isa 走向去递归实现父类和元类，同时准备好对象方法和类方法的查找链。
  • 判断类是否被初始化，未初始化则去初始化。
checkIsKnownClass(cls);

if (slowpath(!cls->isRealized())) {
    cls = realizeClassMaybeSwiftAndLeaveLocked(cls, runtimeLock);
}

if (slowpath((behavior & LOOKUP_INITIALIZE) && !cls->isInitialized())) {
    cls = initializeAndLeaveLocked(cls, inst, runtimeLock);
}

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;
for (unsigned attempts = unreasonableClassCount();;) {
        // curClass method list.
        Method meth = getMethodNoSuper_nolock(curClass, sel);
        if (meth) {
            imp = meth->imp;
            goto done;
        }
}
4.3.2 getMethodNoSuper_nolock 方法

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

  • 如果找到了，进入本方法，调用 log_and_fill_cache 方法
 done:
    log_and_fill_cache(cls, imp, sel, inst, curClass);
    runtimeLock.unlock();

4.5 log_and_fill_cache 方法

  • 利用 cache_fill 方法 写入到缓存里面，为了下次直接从缓存里面快速查找到。
static void
log_and_fill_cache(Class cls, IMP imp, SEL sel, id receiver, Class implementer)
{
#if SUPPORT_MESSAGE_LOGGING
    if (slowpath(objcMsgLogEnabled && implementer)) {
        bool cacheIt = logMessageSend(implementer->isMetaClass(), 
                                      cls->nameForLogging(),
                                      implementer->nameForLogging(), 
                                      sel);
        if (!cacheIt) return;
    }
#endif
    // objc_msgSend -> 二分查找自己 -> cache_fill -> objc_msgSend
    //
    cache_fill(cls, sel, imp, receiver);
}

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
  • 调用 cache_getImp 方法 找到父类
// Superclass cache.
imp = cache_getImp(curClass, sel); // 有问题???? cache_getImp - lookUpImpOrForward
  • cache_getImp 方法
STATIC_ENTRY _cache_getImp

	GetClassFromIsa_p16 p0
	CacheLookup GETIMP, _cache_getImp

LGetImpMiss:
	mov	p0, #0
	ret

	END_ENTRY _cache_getImp

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
  • const IMP forward_imp = (IMP)_objc_msgForward_impcache;
4.7.2 _objc_msgForward_impcache
  • _objc_msgForward_impcache 方法 调用 __objc_msgForward 方法
  • __objc_msgForward 方法 调用 TailCallFunctionPointer x17
STATIC_ENTRY __objc_msgForward_impcache

// No stret specialization.
b	__objc_msgForward

END_ENTRY __objc_msgForward_impcache


ENTRY __objc_msgForward

adrp	x17, __objc_forward_handler@PAGE
ldr	p17, [x17, __objc_forward_handler@PAGEOFF]
TailCallFunctionPointer x17

END_ENTRY __objc_msgForward
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`
  • 我们发现在 resolveInstanceMethod 方法 中将 IMP imp = lookUpImpOrNil(inst, sel, cls); ，所以我们跳进 lookUpImpOrNil 方法 看一下会发现又回到了 lookUpImpOrForward 方法 ，那对之前做了什么产生了好奇。
  • 往上走我们发现有下面两行代码
  • BOOL (*msg)(Class, SEL, SEL) = (typeof(msg))objc_msgSend; bool resolved = msg(cls, resolve_sel, sel);
  • 如果我们实现 resolveInstanceMethod 方法 将方法的 imp 进行赋值，然后再回到 lookUpImpOrForward 方法 之后 imp 有值，就不会报错了。
static void resolveInstanceMethod(id inst, SEL sel, Class cls)
{
    runtimeLock.assertUnlocked();
    ASSERT(cls->isRealized());
    SEL resolve_sel = @selector(resolveInstanceMethod:);

    if (!lookUpImpOrNil(cls, resolve_sel, cls->ISA())) {
        // Resolver not implemented.
        return;
    }

    BOOL (*msg)(Class, SEL, SEL) = (typeof(msg))objc_msgSend;
    bool resolved = msg(cls, resolve_sel, sel);

    // Cache the result (good or bad) so the resolver doesn't fire next time.
    // +resolveInstanceMethod adds to self a.k.a. cls
    IMP imp = lookUpImpOrNil(inst, sel, cls);

    if (resolved  &&  PrintResolving) {
        if (imp) {
            _objc_inform("RESOLVE: method %c[%s %s] "
                         "dynamically resolved to %p", 
                         cls->isMetaClass() ? '+' : '-', 
                         cls->nameForLogging(), sel_getName(sel), imp);
        }
        else {
            // Method resolver didn't add anything?
            _objc_inform("RESOLVE: +[%s resolveInstanceMethod:%s] returned YES"
                         ", but no new implementation of %c[%s %s] was found",
                         cls->nameForLogging(), sel_getName(sel), 
                         cls->isMetaClass() ? '+' : '-', 
                         cls->nameForLogging(), sel_getName(sel));
        }
    }
}

lookUpImpOrNil(id obj, SEL sel, Class cls, int behavior = 0)
{
    return lookUpImpOrForward(obj, sel, cls, behavior | LOOKUP_CACHE | LOOKUP_NIL);
}
4.8.3 动态方法决议实现
#import "LGPerson.h"
#import <objc/message.h>

@implementation LGPerson

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

+ (BOOL)resolveInstanceMethod:(SEL)sel{
    
    if (sel == @selector(say666)) {
        NSLog(@"%@ 来了",NSStringFromSelector(sel));
        
        IMP imp           = class_getMethodImplementation(self, @selector(sayMaster));
        Method sayMMethod = class_getInstanceMethod(self, @selector(sayMaster));
        const char *type  = method_getTypeEncoding(sayMMethod);
        return class_addMethod(self, sel, imp, type);
    }
    
    return [super resolveInstanceMethod:sel];
}

2020-09-22 23:32:48.798620+0800 KCObjc[29296:555622] say666 来了
2020-09-22 23:32:48.799018+0800 KCObjc[29296:555622] -[LGPerson sayMaster]

5. 总结

  1. 当在 objc_msgSend 缓存中没有找到方法，就会来到 CheckMiss -> __objc_msgSend_uncached -> MethodTableLookup -> lookUpImpOrForward 进行慢速查找流程。
  2. 在 lookUpImpOrForward 里面会先去本类当中查找方法 getMethodNoSuper_nolock ，本类没有找到就会去递归的去父类当中查找。
  3. 如果本类和父类都没有找到，就会进行动态方法决议 _class_resolveMethod ，这是苹果爸爸给我们的最后一次机会。
  4. 动态方法我们还不处理，最后就会走到 将 imp 置换成 forward_imp , 最终到 _objc_forward_handler 方法 崩溃报错 unrecognized selector sent to instance ...。
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