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 scene so I can make a decisions of when to use them and if they are slow.

From what I know exceptions are the same thing as doing a bunch of return but it also checks when it needs to stop doing the return. How does it check when to do stop? I am taking a guess and saying there is a second stack which holds the type of exception and stack location then does returns until it gets there. I am also guessing the only time that stack is touch is on a throw and every try/catch. AFAICT implementing a similar behaviour with return code would take the same amount of time. But this is all a guess so I want to know.

How do exceptions really work?

Answers


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:


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