Assembly analysis swift polymorphism principle

 

polymorphic

First, let's take a look at the instance method call of the structure

Take a look at the compilation

 

After compiling, you can determine who will be called finally , Because the function address is written dead .

Switch to class Then let's take a look

Take another look at the compilation

 

You can find a lot more assembly code , So when inheritance is not needed , Use struct It will be more efficient . 

adopt si Go inside the method

It is certain that it is entering speak Method , Similarly, other methods are called in the same way . 

Now let's formally enter the link of polymorphism principle , Let's first look at what polymorphism is

The parent class pointer points to the child class .

 -0x48(%rbp) Is the address value of the global object ,%r13 Is the address value of the heap space object , The second sentence is to get the address of the object in the heap space 8 Bytes of data to %rax, therefore %rax It's in there dog Before the object memory 8 Bytes , That is, type information , The memory address of the method will be placed in the type information , That is, you know whether to call the methods of the parent class or the methods of the subclass .

dog When an object calls a method , First, I will get the type information , Then add the offset , Here is 0x50, It will go directly to the memory address of the method , Remove 8 A byte comes out , Namely speak Method .eat And other methods are called in the same way .

There is a function table in the type information , This function table is in Dog Class, for example

0x50 High memory 
Dog.speak() //8 Bytes 
Dog.eat()
Animal.sleep()
Dog.run()

  From these three method calls, we can see that the offset is 0x50.

No matter how many are created dog object , Before that 8 Each byte is the same , Are the same type information .

There are so many spaces in our code memory , Heap space 、 Global area 、 Stack space 、 Code section . So where does the type information exist , First, eliminate stack space , Because it always exists, and it doesn't come out of the stack after calling , You need to stay in memory all the time , Let's see which area it is through compilation .

0x102cce879 + 0x9867 =  0x102CD80E0, That is to say dog object （ Global variables ） Address value of , You can contrast Dog Type information address , It's very much like .

Heap space Dog Object address value , The address value of the code area is the leftmost address of the assembly , Like this 0x100c36800, In terms of address size, it is ： Code section 、 Global area 、 Heap space 、 Stack space . We can roughly see from the address , The type information is in the global area .

It can also be used MachOView Prying into mach-o file , Look at the type information in TEXT Paragraph or DATA paragraph .

summary ：

Swift The realization principle of polymorphism in , Is similar to the C++ The virtual table of , Directly put the method address of the object to be called in the future into the type information , And it can be determined after compilation .