efficiency

efficiency

[Bob Rossi]

Hi,

I'm currently writing a patch to bison, and can not afford to effect
it's efficiency.

Theoretically, a function in bison uses a local variable named "lfoo" and 
a global variable named "gfoo". I need to pack both of these into a struct,
named "sfoo".

If the function currently uses the variables directly like 'lfoo = 0;',
will changing them to 'sfoo_obj->lfoo = 0;' effect the efficiency of the
program in regards to speed?

Thanks,
Bob Rossi

Re: efficiency

[Brian Budge]

Hi -

This is not really a gcc question, and it should really be asked
elsewhere... however, it's unlikely that this occasional dereference
will adversely affect performance.

  Brian

On 3/16/06, Bob Rossi <bob_rossi@cox.net> wrote:
> Hi,
>
> I'm currently writing a patch to bison, and can not afford to effect
> it's efficiency.
>
> Theoretically, a function in bison uses a local variable named "lfoo" and
> a global variable named "gfoo". I need to pack both of these into a struct,
> named "sfoo".
>
> If the function currently uses the variables directly like 'lfoo = 0;',
> will changing them to 'sfoo_obj->lfoo = 0;' effect the efficiency of the
> program in regards to speed?
>
> Thanks,
> Bob Rossi
>

Re: efficiency

[Bob Rossi]

On Thu, Mar 16, 2006 at 05:58:29PM -0800, Brian Budge wrote:
> Hi -
> 
> This is not really a gcc question, and it should really be asked
> elsewhere... however, it's unlikely that this occasional dereference
> will adversely affect performance.

OK, where should a question like this be asked? if not the compiler that
does the optimization? I forgot to mention, I only really care about the
efficiency regarding gcc's output.

Bob Rossi

Re: efficiency

[Andrew Haley]

Bob Rossi writes:
 > On Thu, Mar 16, 2006 at 05:58:29PM -0800, Brian Budge wrote:
 > > Hi -
 > > 
 > > This is not really a gcc question, and it should really be asked
 > > elsewhere... however, it's unlikely that this occasional dereference
 > > will adversely affect performance.
 > 
 > OK, where should a question like this be asked? if not the compiler that
 > does the optimization?

I agree with you, Bob -- it's a perfectly reasonable question.

 > I forgot to mention, I only really care about the efficiency
 > regarding gcc's output.

Well, if you really want to know, use "gcc -S".

Access to a local is something like

        movl    8(%ebp), %eax

and to a global via a pointer

        movl    sfoo, %eax
        movl    4(%eax), %eax
 
Andrew.