How do exceptions work (behind the scenes) in c++
I keep seeing people say that exceptions are slow, but I never see any proof. So, instead of asking if they are, I will ask how do exceptions work behind the scenes, so I can make decisions of when to use them and whether they are slow.
From what I know, exceptions are the same as doing a return bunch of times, except that it also checks after each return whether it needs to do another one or to stop. How does it check when to stop returning? I guess there is a second stack that holds the type of the exception and a stack location, it then does returns until it gets there. I am also guessing that the only time this second stack is touched is on a throw and on each try/catch. AFAICT implementing a similar behaviour with return codes would take the same amount of time. But this is all just a guess, so I want to know what really happens.
How do exceptions really work?
Answer
Instead of guessing, I decided to actually look at the generated code with a small piece of C++ code and a somewhat old Linux install.
class MyException
{
public:
MyException() { }
~MyException() { }
};
void my_throwing_function(bool throwit)
{
if (throwit)
throw MyException();
}
void another_function();
void log(unsigned count);
void my_catching_function()
{
log(0);
try
{
log(1);
another_function();
log(2);
}
catch (const MyException& e)
{
log(3);
}
log(4);
}
I compiled it with g++ -m32 -W -Wall -O3 -save-temps -c, and looked at the generated assembly file.
.file "foo.cpp"
.section .text._ZN11MyExceptionD1Ev,"axG",@progbits,_ZN11MyExceptionD1Ev,comdat
.align 2
.p2align 4,,15
.weak _ZN11MyExceptionD1Ev
.type _ZN11MyExceptionD1Ev, @function
_ZN11MyExceptionD1Ev:
.LFB7:
pushl %ebp
.LCFI0:
movl %esp, %ebp
.LCFI1:
popl %ebp
ret
.LFE7:
.size _ZN11MyExceptionD1Ev, .-_ZN11MyExceptionD1Ev
_ZN11MyExceptionD1Ev is MyException::~MyException(), so the compiler decided it needed a non-inline copy of the destructor.
.globl __gxx_personality_v0
.globl _Unwind_Resume
.text
.align 2
.p2align 4,,15
.globl _Z20my_catching_functionv
.type _Z20my_catching_functionv, @function
_Z20my_catching_functionv:
.LFB9:
pushl %ebp
.LCFI2:
movl %esp, %ebp
.LCFI3:
pushl %ebx
.LCFI4:
subl $20, %esp
.LCFI5:
movl $0, (%esp)
.LEHB0:
call _Z3logj
.LEHE0:
movl $1, (%esp)
.LEHB1:
call _Z3logj
call _Z16another_functionv
movl $2, (%esp)
call _Z3logj
.LEHE1:
.L5:
movl $4, (%esp)
.LEHB2:
call _Z3logj
addl $20, %esp
popl %ebx
popl %ebp
ret
.L12:
subl $1, %edx
movl %eax, %ebx
je .L16
.L14:
movl %ebx, (%esp)
call _Unwind_Resume
.LEHE2:
.L16:
.L6:
movl %eax, (%esp)
call __cxa_begin_catch
movl $3, (%esp)
.LEHB3:
call _Z3logj
.LEHE3:
call __cxa_end_catch
.p2align 4,,3
jmp .L5
.L11:
.L8:
movl %eax, %ebx
.p2align 4,,6
call __cxa_end_catch
.p2align 4,,6
jmp .L14
.LFE9:
.size _Z20my_catching_functionv, .-_Z20my_catching_functionv
.section .gcc_except_table,"a",@progbits
.align 4
.LLSDA9:
.byte 0xff
.byte 0x0
.uleb128 .LLSDATT9-.LLSDATTD9
.LLSDATTD9:
.byte 0x1
.uleb128 .LLSDACSE9-.LLSDACSB9
.LLSDACSB9:
.uleb128 .LEHB0-.LFB9
.uleb128 .LEHE0-.LEHB0
.uleb128 0x0
.uleb128 0x0
.uleb128 .LEHB1-.LFB9
.uleb128 .LEHE1-.LEHB1
.uleb128 .L12-.LFB9
.uleb128 0x1
.uleb128 .LEHB2-.LFB9
.uleb128 .LEHE2-.LEHB2
.uleb128 0x0
.uleb128 0x0
.uleb128 .LEHB3-.LFB9
.uleb128 .LEHE3-.LEHB3
.uleb128 .L11-.LFB9
.uleb128 0x0
.LLSDACSE9:
.byte 0x1
.byte 0x0
.align 4
.long _ZTI11MyException
.LLSDATT9:
Surprise! There are no extra instructions at all on the normal code path. The compiler instead generated extra out-of-line fixup code blocks, referenced via a table at the end of the function (which is actually put on a separate section of the executable). All the work is done behind the scenes by the standard library, based on these tables (_ZTI11MyException is typeinfo for MyException).
OK, that was not actually a surprise for me, I already knew how this compiler did it. Continuing with the assembly output:
.text
.align 2
.p2align 4,,15
.globl _Z20my_throwing_functionb
.type _Z20my_throwing_functionb, @function
_Z20my_throwing_functionb:
.LFB8:
pushl %ebp
.LCFI6:
movl %esp, %ebp
.LCFI7:
subl $24, %esp
.LCFI8:
cmpb $0, 8(%ebp)
jne .L21
leave
ret
.L21:
movl $1, (%esp)
call __cxa_allocate_exception
movl $_ZN11MyExceptionD1Ev, 8(%esp)
movl $_ZTI11MyException, 4(%esp)
movl %eax, (%esp)
call __cxa_throw
.LFE8:
.size _Z20my_throwing_functionb, .-_Z20my_throwing_functionb
Here we see the code for throwing an exception. While there was no extra overhead simply because an exception might be thrown, there is obviously a lot of overhead in actually throwing and catching an exception. Most of it is hidden within __cxa_throw, which must:
- Walk the stack with the help of the exception tables until it finds a handler for that exception.
- Unwind the stack until it gets to that handler.
- Actually call the handler.
Compare that with the cost of simply returning a value, and you see why exceptions should be used only for exceptional returns.
To finish, the rest of the assembly file:
.weak _ZTI11MyException
.section .rodata._ZTI11MyException,"aG",@progbits,_ZTI11MyException,comdat
.align 4
.type _ZTI11MyException, @object
.size _ZTI11MyException, 8
_ZTI11MyException:
.long _ZTVN10__cxxabiv117__class_type_infoE+8
.long _ZTS11MyException
.weak _ZTS11MyException
.section .rodata._ZTS11MyException,"aG",@progbits,_ZTS11MyException,comdat
.type _ZTS11MyException, @object
.size _ZTS11MyException, 14
_ZTS11MyException:
.string "11MyException"
The typeinfo data.
.section .eh_frame,"a",@progbits
.Lframe1:
.long .LECIE1-.LSCIE1
.LSCIE1:
.long 0x0
.byte 0x1
.string "zPL"
.uleb128 0x1
.sleb128 -4
.byte 0x8
.uleb128 0x6
.byte 0x0
.long __gxx_personality_v0
.byte 0x0
.byte 0xc
.uleb128 0x4
.uleb128 0x4
.byte 0x88
.uleb128 0x1
.align 4
.LECIE1:
.LSFDE3:
.long .LEFDE3-.LASFDE3
.LASFDE3:
.long .LASFDE3-.Lframe1
.long .LFB9
.long .LFE9-.LFB9
.uleb128 0x4
.long .LLSDA9
.byte 0x4
.long .LCFI2-.LFB9
.byte 0xe
.uleb128 0x8
.byte 0x85
.uleb128 0x2
.byte 0x4
.long .LCFI3-.LCFI2
.byte 0xd
.uleb128 0x5
.byte 0x4
.long .LCFI5-.LCFI3
.byte 0x83
.uleb128 0x3
.align 4
.LEFDE3:
.LSFDE5:
.long .LEFDE5-.LASFDE5
.LASFDE5:
.long .LASFDE5-.Lframe1
.long .LFB8
.long .LFE8-.LFB8
.uleb128 0x4
.long 0x0
.byte 0x4
.long .LCFI6-.LFB8
.byte 0xe
.uleb128 0x8
.byte 0x85
.uleb128 0x2
.byte 0x4
.long .LCFI7-.LCFI6
.byte 0xd
.uleb128 0x5
.align 4
.LEFDE5:
.ident "GCC: (GNU) 4.1.2 (Ubuntu 4.1.2-0ubuntu4)"
.section .note.GNU-stack,"",@progbits
Even more exception handling tables, and assorted extra information.
So, the conclusion, at least for GCC on Linux: the cost is extra space (for the handlers and tables) whether or not exceptions are thrown, plus the extra cost of parsing the tables and executing the handlers when an exception is thrown. If you use exceptions instead of error codes, and an error is rare, it can be faster, since you do not have the overhead of testing for errors anymore.
In case you want more information, in particular what all the __cxa_ functions do, see the original specification they came from: