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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[-- Attachment #1 --]
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
[-- Attachment #2 --]
#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);
}
[-- Attachment #3 --]
testzero.o: file format elf32-i386
Disassembly of section .text:
00000000 <main>:
;; prologue
0: 55 push %ebp
1: 89 e5 mov %esp,%ebp
3: 83 ec 18 sub $0x18,%esp
;; n1(flags)
6: c7 45 fc ff ff ff ff movl $0xffffffff,0xfffffffc(%ebp)
d: 8b 45 fc mov 0xfffffffc(%ebp),%eax
10: f7 d0 not %eax
12: 21 45 fc and %eax,0xfffffffc(%ebp)
;; n2(flags)
15: c7 45 fc ff ff ff ff movl $0xffffffff,0xfffffffc(%ebp)
1c: c7 45 fc 00 00 00 00 movl $0x0,0xfffffffc(%ebp)
;; n3(flags)
23: c7 45 fc ff ff ff ff movl $0xffffffff,0xfffffffc(%ebp)
2a: 8b 45 fc mov 0xfffffffc(%ebp),%eax
2d: 31 45 fc xor %eax,0xfffffffc(%ebp)
;; n4(flags)
30: c7 45 fc ff ff ff ff movl $0xffffffff,0xfffffffc(%ebp)
37: c7 45 fc 00 00 00 00 movl $0x0,0xfffffffc(%ebp)
;; n2(flags)
3e: dd 05 00 00 00 00 fldl 0x0
44: dd 5d f0 fstpl 0xfffffff0(%ebp)
47: c7 45 f0 00 00 00 00 movl $0x0,0xfffffff0(%ebp)
4e: c7 45 f4 00 00 00 00 movl $0x0,0xfffffff4(%ebp)
;; n4(flags)
55: dd 05 00 00 00 00 fldl 0x0
5b: dd 5d f0 fstpl 0xfffffff0(%ebp)
5e: dd 45 f0 fldl 0xfffffff0(%ebp)
61: dc 65 f0 fsubl 0xfffffff0(%ebp)
64: dd 5d f0 fstpl 0xfffffff0(%ebp)
;; epilogue, exit(0), etc.
67: 83 c4 f4 add $0xfffffff4,%esp
6a: 6a 00 push $0x0
6c: e8 fc ff ff ff call 6d <main+0x6d>
71: 83 c4 10 add $0x10,%esp
74: c9 leave
75: c3 ret
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