linux-kernel - Re: [PATCH v3 02/11] tools/nolibc: add new crt.h with _start

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Date:   Thu, 13 Jul 2023 08:34:18 +0200
From:   Willy Tarreau <w@....eu>
To:     Zhangjin Wu <falcon@...ylab.org>
Cc:     thomas@...ch.de, arnd@...db.de, linux-kernel@...r.kernel.org,
        linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v3 02/11] tools/nolibc: add new crt.h with _start_c

Hi Zhangjin,

I haven't reviewed the rest yet but regarding this point:

On Thu, Jul 13, 2023 at 02:12:27PM +0800, Zhangjin Wu wrote:
> > > +	/* find auxv */
> > > +	i = 0;
> > > +	while (envp[i])
> > > +		i++;
> > > +	_auxv = (void *)(envp + i + 1);
> > 
> > Could be simplified a bit:
> > 
> > _auxv = (void *) envp;
> > while (_auxv)
> > 	_auxv++;
> >
> 
> Yeah, it is better, but needs a little change.
> 
>     _auxv = (void *) envp;
>     while (*_auxv)
> 	_auxv++;
>     _auxv++;

Or just:

    _auxv = (void*)environ;
    while (*_auxv++)
         ;

or:

    for (_auxv = (void*)environ; *_auxv++; )
         ;

Please also have a look at the output code, because at low optimization
levels, compilers sometimes produce a better code with a local variable
than with a global variable in a loop. Thus I wouldn't be that much
surprised if at -O0 or -O1 you'd see slightly more compact code using:

       /* find envp */
       environ = argv + argc + 1;
        
       /* find auxv */
       for (auxv = environ; *auxv++)
                ;
       _auxv = auxv;

than:
       /* find envp */
       envp = argv + argc + 1;
       environ = envp;
        
       /* find auxv */
       for (_auxv = environ; *_auxv++)
                ;

Since it's going to become generic code, it's worth running a few tests
to see how to best polish it.

Thanks,
Willy