Date: Fri, 22 Jun 2001 08:37:04 +0200 From: Jim Segrave <jes@jes-2.demon.nl> To: questions@FreeBSD.ORG Subject: Re: What does this macro do? Message-ID: <20010622083703.A12305@jes-2.demon.nl> In-Reply-To: <bulk.53264.20010620141603@hub.freebsd.org>; from "questions-digest" on Wed 20 Jun 2001 (14:16 -0700) References: <bulk.53264.20010620141603@hub.freebsd.org>
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> Date: Wed, 20 Jun 2001 09:49:25 -0700
> From: John Merryweather Cooper <jmcoopr@webmail.bmi.net>
> Subject: Re: what does this define do?
>
> On 2001.06.20 09:38 j mckitrick wrote:
> >
> > at the risk of making a fool of myself (one which i am never hesitant to
> > take ;-) could someone explain to me how this macro
> >
> > #define n(flags) (~(flags) & (flags))
> >
> > is different from this one?
> >
> > #define n(flags) 0
> >
> >
>
> Potentially, in the code generated . . .
>
> This is, of course, highly compiler dependent. Looks like somebody is
> concerned with the efficiency of a MOV AX, 0 as opposed to an XOR AX, AX.
> But IMHO, this is a waste of effort in C, since God only knows what the
> compiler is going to puke out for this define. The first macro might even
> result in a performance "hit" if the compiler has to make some internal
> calls on the flags pseudo parameter to do bitwise operations on it. It
> might be interesting (for the curious) to see how above and the following
> differ in code output:
>
> #define n(flags) ((flags) - (flags))
> #define n(flags) ((flags) ^ (flags))
I think people are overlooking a more significant difference between:
#define n(flags) (~(flags) & (flags))
and
#define n(flags) 0
Not that I can see it being useful, but...
If you invoke this macro with flags being a function call or (even
worse), something like ++i, then the first version will generate two
function calls or two increments (and undefined behaviour).
Or try
n(rand())
which probably won't evaluate to 0.
A good compiler should see, for integer variables 'a' and 'x':
a = x - x;
a = x & ~x;
a = x ^ x;
as being reducible at compile time to zero and should generate the
same code for all three, hopefully the most efficient for the target
processor method of setting a to zero. The C standard does not require
compilers to blindly execute every arithmetic operation in the source
- it can totally delete code if the result is never used, it can
evaluate constant expressions like the above at compile time and
substitue the results.
int increment (i)
{
return
--
Jim Segrave jes@jes-2.demon.nl
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