[SOLVED] when does a function will include stack _Unwind_Resume call in c++?

Issue

I am now studying c++ exception and a trouble comes to me, program show as below

#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;
}

I use gdb to disassemble fn(), and we can see callq 0x4008d0 _Unwind_Resume@plt, it is a call to stack unwind operation since vector::at might throw an out of range exception

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.

however, when I try to imitate this progress with calling a function will throw exception , assembler code call <_Unwind_Resume> doesn't exist,why?

#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;
}

assembler code for function fn2(), it doesn't include call Unwind_Resume@plt,why?

(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.

Solution

Since fn2 only calls fn, which is declared as throw(), no exception can propagate to an active stack frame for fn. GCC recognizes this situation and optimizes the exception handler away.

In the original case, this is not possible because std::vector::at(size_type) can throw. The exception handler is only needed because of the throw() declaration, to call std::unexpected() in case of an exception.

_Unwind_Resume only shows up if the stack frame needs some special action when unwinding (such as calling destructors or std::unexpected()). Without that, the Itanium C++ ABI (which is used by GCC) does not require any per-frame action at all, which is why this implementation is sometimes called zero-cost exception handling.

Answered By - Florian Weimer