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* Re: c/8221: return value of a function is forgotten
@ 2002-10-14 2:16 Andrew Pinski
0 siblings, 0 replies; 4+ messages in thread
From: Andrew Pinski @ 2002-10-14 2:16 UTC (permalink / raw)
To: nobody; +Cc: gcc-prs
The following reply was made to PR c/8221; it has been noted by GNATS.
From: Andrew Pinski <pinskia@physics.uc.edu>
To: c.kaliszyk@zodiac.mimuw.edu.pl
Cc: gcc-gnats@gcc.gnu.org, cek@wp.pl
Subject: Re: c/8221: return value of a function is forgotten
Date: Mon, 14 Oct 2002 02:12:23 -0700
On Monday, Oct 14, 2002, at 01:44 US/Pacific,
c.kaliszyk@zodiac.mimuw.edu.pl wrote:
> I call "gcc -O2" on this:
> --cut--
> inline int getcputime(void) {
> int i;
> __asm__(
> "rdtsc
> movl %%eax, %0"
> : :"g" (i) :"ax", "dx");
> return i;
> }
>
> int main(void) {
> int x =3D getcputime();
> int y =3D getcputime();
> printf ("Time measure resolution: %i\n", y - x);
> return 0;
> }
I think you constraints are wrong, this works for me:
inline int getcputime(void) {
int i;
__asm__ volatile(
"rdtsc\n\
movl %%eax, %0"
: "=g" (i) : :"eax", "edx");
return i;
}
int main(void) {
int x = getcputime();
int y = getcputime();
printf ("Time measure resolution: %i\n", y - x);
return 0;
}
read
http://gcc.gnu.org/onlinedocs/gcc-3.2/gcc/Extended-Asm.html
for information, this helped me get the correct result.
Thanks,
Andrew Pinski
PS I think this bug is user error so can someone close it.
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: c/8221: return value of a function is forgotten
@ 2002-10-14 8:54 zack
0 siblings, 0 replies; 4+ messages in thread
From: zack @ 2002-10-14 8:54 UTC (permalink / raw)
To: c.kaliszyk, cek, gcc-bugs, gcc-prs, nobody
Synopsis: return value of a function is forgotten
State-Changed-From-To: open->closed
State-Changed-By: zack
State-Changed-When: Mon Oct 14 08:54:27 2002
State-Changed-Why:
Not a bug.
http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&pr=8221
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: c/8221: return value of a function is forgotten
@ 2002-10-14 8:46 Zack Weinberg
0 siblings, 0 replies; 4+ messages in thread
From: Zack Weinberg @ 2002-10-14 8:46 UTC (permalink / raw)
To: nobody; +Cc: gcc-prs
The following reply was made to PR c/8221; it has been noted by GNATS.
From: Zack Weinberg <zack@codesourcery.com>
To: c.kaliszyk@zodiac.mimuw.edu.pl
Cc: gcc-gnats@gcc.gnu.org, cek@wp.pl
Subject: Re: c/8221: return value of a function is forgotten
Date: Mon, 14 Oct 2002 08:43:14 -0700
On Mon, Oct 14, 2002 at 08:44:09AM -0000, c.kaliszyk@zodiac.mimuw.edu.pl wrote:
> inline int getcputime(void) {
> int i;
> __asm__(
> "rdtsc
> movl %%eax, %0"
> : :"g" (i) :"ax", "dx");
> return i;
> }
This asm statement claims that I is an input to the instruction, and
that there aren't any outputs. Thus, GCC assumes that I is not
modified by the asm.
The correct way to write getcputime() would be
static inline unsigned int
getcputime(void)
{
unsigned int i;
asm volatile ("rdtsc" : "=a" (i) : : "edx");
return i;
}
The key change is to put the I parameter in the _first_ block of asm
operands, and add a leading = to its constraint. That tells the
compiler that I contains an output.
Changing "g" to "a" in the constraint tells GCC that the output
appears in %eax, which obviates the need for the move instruction you
wrote. This also means you need not (must not) clobber edx.
All the other changes are also necessary. I leave as an exercise for
you to figure out why I made them.
zw
^ permalink raw reply [flat|nested] 4+ messages in thread
* c/8221: return value of a function is forgotten
@ 2002-10-14 1:46 c.kaliszyk
0 siblings, 0 replies; 4+ messages in thread
From: c.kaliszyk @ 2002-10-14 1:46 UTC (permalink / raw)
To: gcc-gnats; +Cc: cek
>Number: 8221
>Category: c
>Synopsis: return value of a function is forgotten
>Confidential: no
>Severity: serious
>Priority: medium
>Responsible: unassigned
>State: open
>Class: wrong-code
>Submitter-Id: net
>Arrival-Date: Mon Oct 14 01:46:00 PDT 2002
>Closed-Date:
>Last-Modified:
>Originator: c.kaliszyk@zodiac.mimuw.edu.pl
>Release: 3.2
>Organization:
>Environment:
Linux orka 2.5.40 #1 Thu Oct 3 07:30:38 CEST 2002 i686 unknown
configured with: /home/c/tmp/gcc-3.2/configure --prefix=3D/usr --enable-thr=
eads
>Description:
I tried to measure time twice, and substract these results.
When the function is inlined both results are stored in %ecx and after that
%ecx is substracted from itself, and I get zero.
Without inlining it works correct.
>How-To-Repeat:
I call "gcc -O2" on this:
--cut--
inline int getcputime(void) {
int i;
__asm__(
"rdtsc
movl %%eax, %0"
: :"g" (i) :"ax", "dx");
return i;
}
int main(void) {
int x =3D getcputime();
int y =3D getcputime();
printf ("Time measure resolution: %i\n", y - x);
return 0;
}
--cut--
>Fix:
Remove "inline" before the function definition or call gcc without -O
If the function is not inlined the return values of both calls are stored on
the stack in diffrent places and correctly substracted.
>Release-Note:
>Audit-Trail:
>Unformatted:
^ permalink raw reply [flat|nested] 4+ messages in thread
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2002-10-14 2:16 c/8221: return value of a function is forgotten Andrew Pinski
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2002-10-14 8:54 zack
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2002-10-14 1:46 c.kaliszyk
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