我现在正在学习C++异常，遇到了一个麻烦，程序显示如下

#include<iostream>
#include<unistd.h>
#include<string>
#include<thread>
#include<vector>
#include<exception>
using namespace std;


vector<int> vec(20);


void fn()throw() {
        vec.at(10);
}

int main(){
        fn();

return 0;
}

我使用gdb来反汇编fn（），我们可以看到callq0x4008d0_unwind_resume@plt，这是一个对堆栈展开操作的调用，因为vector::at可能会抛出一个超出范围的异常

Dump of assembler code for function fn():
   0x00000000004009e6 <+0>:     push   %rbp
   0x00000000004009e7 <+1>:     mov    %rsp,%rbp
   0x00000000004009ea <+4>:     mov    $0xa,%esi
   0x00000000004009ef <+9>:     mov    $0x6020a0,%edi
   0x00000000004009f4 <+14>:    callq  0x400ba6 <std::vector<int, std::allocator<int> >::at(unsigned long)>
   0x00000000004009f9 <+19>:    jmp    0x400a11 <fn()+43>
   0x00000000004009fb <+21>:    cmp    $0xffffffffffffffff,%rdx
   0x00000000004009ff <+25>:    je     0x400a09 <fn()+35>
   0x0000000000400a01 <+27>:    mov    %rax,%rdi
   0x0000000000400a04 <+30>:    callq  0x4008d0 <_Unwind_Resume@plt>
   0x0000000000400a09 <+35>:    mov    %rax,%rdi
   0x0000000000400a0c <+38>:    callq  0x400880 <__cxa_call_unexpected@plt>
   0x0000000000400a11 <+43>:    pop    %rbp
   0x0000000000400a12 <+44>:    retq   
End of assembler dump.

但是，当我试图用调用函数将引发异常来模拟这个过程时，汇编代码调用<_unwind_resume>； 不存在，为什么？

#include<iostream>
#include<unistd.h>
#include<string>
#include<thread>
#include<vector>
#include<exception>
using namespace std;


class myException:public exception  
{  
        public:  
                myException(){ } 
};


void fn()throw() {
        throw myException();
}

void fn2()throw(){
        fn();
}


int main(){
        fn2();

        return 0;
}

函数fn2()的汇编代码，它不包括调用unwind_resume@plt，为什么？

(gdb) disassemble fn2
Dump of assembler code for function fn2():
   0x0000000000400aec <+0>:     push   %rbp
   0x0000000000400aed <+1>:     mov    %rsp,%rbp
   0x0000000000400af0 <+4>:     callq  0x400aa6 <fn()>
   0x0000000000400af5 <+9>:     nop
   0x0000000000400af6 <+10>:    pop    %rbp
   0x0000000000400af7 <+11>:    retq   
End of assembler dump.