Date: Wed, 20 Jun 2001 10:47:18 -0700 From: John Merryweather Cooper <jmcoopr@webmail.bmi.net> To: j mckitrick <jcm@FreeBSD-uk.eu.org> Cc: John Merryweather Cooper <jmcoopr@webmail.bmi.net>, freebsd-questions@FreeBSD.ORG Subject: Re: what does this define do? Message-ID: <20010620104718.C14541@johncoop> In-Reply-To: <20010620182353.A3958@dogma.freebsd-uk.eu.org>; from jcm@FreeBSD-uk.eu.org on Wed, Jun 20, 2001 at 10:23:53 -0700 References: <20010620173855.A2941@dogma.freebsd-uk.eu.org> <20010620094925.A14541@johncoop> <20010620182353.A3958@dogma.freebsd-uk.eu.org>
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On 2001.06.20 10:23 j mckitrick wrote:
> | 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.
>
> I wondered about that. But i agree, with no way of knowing what the
> compiler is doing anyway, it doesn't make much sense.
>
> The way it is being used seems to invalid the potential benefit:
>
> static char var = FLAG1 | FLAG2 | n(FLAG3)
>
> because the compiler might drop the n() values anyway, since ORing with 0
> does nothing anyway. I can understand the n() values being in the code
> as
> placeholders, so you can see what lines are hi or low on the hardware
> register, but if the compiler drops them anyway, optimizing the macro
> rather
> than just using 0 doesn't seem to buy any benefits.
>
>
> Jonathon
> --
> "It is through will alone I set my mind in motion...."
>
Because I qualify as among the curious, I ran a little test program to look
at the assembly. A couple of observations:
1) the "not/and" macro breaks on doubles;
2) the "xor" macro also breaks on doubles;
3) the compiler doesn't know about fldz . . . :)
See attached . . .
jmc
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#include <stdlib.h>
#define n1(flags) (~(flags) & (flags))
#define n2(flags) (0)
#define n3(flags) ((flags) ^ (flags))
#define n4(flags) ((flags) - (flags))
#define INIT_IVAR (-1)
#define INIT_DVAR (3.1425)
int main (int argc, char **argv)
{
int ivar;
double dvar;
ivar = INIT_IVAR;
ivar = n1(ivar);
ivar = INIT_IVAR;
ivar = n2(ivar);
ivar = INIT_IVAR;
ivar = n3(ivar);
ivar = INIT_IVAR;
ivar = n4(ivar);
/* dvar = INIT_DVAR; causes compiler error */
/* dvar = n1(dvar); */
dvar = INIT_DVAR;
dvar = n2(dvar);
/* dvar = INIT_DVAR; causes compiler error */
/* dvar = n3(dvar); */
dvar = INIT_DVAR;
dvar = n4(dvar);
exit(0);
}
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