Re: [linux-audio-dev] [ot] [rant] gcc, you let me down one time toomany

On 6/9/05, Erik de Castro Lopo <erikd-lad@email-addr-hidden-nerd.com> wrote:
> David Cournapeau wrote:
>
> > eviltwin69@email-addr-hidden wrote:
> >
> > >
> > > I was under the impression that there was bounds checking going on with
> > >vectors. Is this not the case?
> > >
> > >
> > >
> > Not necesserally: if you are using operator (), yes, if you use operator
> > [], no.
>
> I think you are all guessing.

No, I am not. I cannot find the information on the C++ faq right now,
but If m pretty sure that it is written in the book of Stroustrup.

I did a small test mself: if you check the assembly code of g++ for
the functions

void c_array(int * a)
{
a[5]
}

void vector(vector<int> a)
{
a[5]
}

void vector(vector<int> a)
{
a.at(5)
}

you will see that the assembly code for c_array and vector are quite
similar ( I don't know much about assembly, but here is the output of
g++ -O3 -S for c_array :

.file "c_array.cpp"
        .text
        .align 2
        .p2align 4,,15
.globl _Z7c_arrayPi
        .type _Z7c_arrayPi, @function
_Z7c_arrayPi:
.LFB529:
        pushl %ebp
.LCFI0:
        movl %esp, %ebp
.LCFI1:
        movl 8(%ebp), %edx
        movl $0, 20(%edx)
        popl %ebp
        ret
.LFE529:
        .size _Z7c_arrayPi, .-_Z7c_arrayPi
        .weak pthread_mutex_unlock
        .weak pthread_mutex_trylock
        .weak pthread_mutex_lock
        .weak pthread_create
        .weak pthread_setspecific
        .weak pthread_getspecific
        .weak pthread_key_delete
        .weak pthread_key_create
        .weak pthread_once
        .section .note.GNU-stack,"",@progbits
        .ident "GCC: (GNU) 3.3.6 (Debian 1:3.3.6-5)"

and for void vector(vector<int> v)

        .file "vector.cpp"
        .text
        .align 2
        .p2align 4,,15
.globl _Z6vectorSt6vectorIiSaIiEE
        .type _Z6vectorSt6vectorIiSaIiEE, @function
_Z6vectorSt6vectorIiSaIiEE:
.LFB539:
.L2:
.L7:
        pushl %ebp
.LCFI0:
        movl %esp, %ebp
.LCFI1:
        popl %ebp
        ret
.LFE539:
        .size _Z6vectorSt6vectorIiSaIiEE, .-_Z6vectorSt6vectorIiSaIiEE
        .weak pthread_mutex_unlock
        .weak pthread_mutex_trylock
        .weak pthread_mutex_lock
        .weak pthread_create
        .weak pthread_setspecific
        .weak pthread_getspecific
        .weak pthread_key_delete
        .weak pthread_key_create
        .weak pthread_once
        .section .note.GNU-stack,"",@progbits
        .ident "GCC: (GNU) 3.3.6 (Debian 1:3.3.6-5)"

The assembly code using at() instead of [] is much longer.

Cheers,

David
Received on Thu Jun 9 20:15:04 2005