Re: drivers/char/sysrq.c

[Jörn Engel (joern@wohnheim.fh-wedel.de)]

On Fri, 30 May 2003 17:13:17 +0200, J.A. Magallon wrote:
> On 05.30, Jörn Engel wrote:
> > On Fri, 30 May 2003 16:44:55 +0200, Margit Schubert-While wrote:
> > > 
> > > In drivers/char/sysrq.c (2.4 and 2.5) we have :
> > > 
> > >         if ((key >= '0') & (key <= '9')) {
> > >                 retval = key - '0';
> > >         } else if ((key >= 'a') & (key <= 'z')) {
> > > 
> > > Shouldn't the "&" be (pedantically) "&&" ?
> > 
> > It is semantically the same.  If you can show that gcc optimization
> > also creates the same assembler code, few people will object to a
> > patch.
> > 
> 
> I see a diff:
> - & is bitwise and you always perform the op
> - && is logical and gcc must shortcut it
> 
> I think people use & 'cause they prefer the extra argument calculation
> than the branch for the shortcut (AFAIR...)

Yes, it is an optimization, nothing more.  Either code will behave the
same, but one version might be a little faster, depending on the cpu,
unless the compiler is already clever enough to do this himself.

Just wrote a small test program with both variants and tested on i386
with gcc 3.2.3 and 2.95.4, with -O2 and -Os.  The code generated was
identical in all eight cases.  So if ever this zero optimization
reduces readability of the code, write a patch and make it better.
Beats any spelling fixes.

Jörn

-- 
Eighty percent of success is showing up.
-- Woody Allen
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