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* [Bug libc/15424] New: modf benchtest gives bogus results
@ 2013-05-01 22:00 ondra at iuuk dot mff.cuni.cz
2013-05-10 8:19 ` [Bug libc/15424] " siddhesh at redhat dot com
` (4 more replies)
0 siblings, 5 replies; 6+ messages in thread
From: ondra at iuuk dot mff.cuni.cz @ 2013-05-01 22:00 UTC (permalink / raw)
To: glibc-bugs
http://sourceware.org/bugzilla/show_bug.cgi?id=15424
Bug #: 15424
Summary: modf benchtest gives bogus results
Product: glibc
Version: unspecified
Status: NEW
Severity: normal
Priority: P2
Component: libc
AssignedTo: unassigned@sourceware.org
ReportedBy: ondra@iuuk.mff.cuni.cz
CC: drepper.fsp@gmail.com
Classification: Unclassified
When running benchtest for modf function I consistently get following results:
modf(): ITERS:2e+06: TOTAL:0.0150672s, MAX:1.32739e+08ns, MIN:9.88625e-321ns,
1.4822e-323 iter/s
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* [Bug libc/15424] modf benchtest gives bogus results
2013-05-01 22:00 [Bug libc/15424] New: modf benchtest gives bogus results ondra at iuuk dot mff.cuni.cz
@ 2013-05-10 8:19 ` siddhesh at redhat dot com
2013-05-10 9:16 ` neleai at seznam dot cz
` (3 subsequent siblings)
4 siblings, 0 replies; 6+ messages in thread
From: siddhesh at redhat dot com @ 2013-05-10 8:19 UTC (permalink / raw)
To: glibc-bugs
http://sourceware.org/bugzilla/show_bug.cgi?id=15424
Siddhesh Poyarekar <siddhesh at redhat dot com> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|NEW |WAITING
CC| |siddhesh at redhat dot com
AssignedTo|unassigned at sourceware |siddhesh at redhat dot com
|dot org |
--- Comment #1 from Siddhesh Poyarekar <siddhesh at redhat dot com> 2013-05-10 08:19:48 UTC ---
Does this happen with latest master? Changing from fixed iterations to fixed
time showed such spurious results with bench-modf once (likely because it was
not rebuilt), which went away when I did a bench-clean and rebuilt it.
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* [Bug libc/15424] modf benchtest gives bogus results
2013-05-01 22:00 [Bug libc/15424] New: modf benchtest gives bogus results ondra at iuuk dot mff.cuni.cz
2013-05-10 8:19 ` [Bug libc/15424] " siddhesh at redhat dot com
@ 2013-05-10 9:16 ` neleai at seznam dot cz
2013-05-17 12:01 ` siddhesh at redhat dot com
` (2 subsequent siblings)
4 siblings, 0 replies; 6+ messages in thread
From: neleai at seznam dot cz @ 2013-05-10 9:16 UTC (permalink / raw)
To: glibc-bugs
http://sourceware.org/bugzilla/show_bug.cgi?id=15424
--- Comment #2 from Ondrej Bilka <neleai at seznam dot cz> 2013-05-10 09:16:24 UTC ---
On Fri, May 10, 2013 at 08:19:48AM +0000, siddhesh at redhat dot com wrote:
> http://sourceware.org/bugzilla/show_bug.cgi?id=15424
>
> Siddhesh Poyarekar <siddhesh at redhat dot com> changed:
>
> What |Removed |Added
> ----------------------------------------------------------------------------
> Status|NEW |WAITING
> CC| |siddhesh at redhat dot com
> AssignedTo|unassigned at sourceware |siddhesh at redhat dot com
> |dot org |
>
> --- Comment #1 from Siddhesh Poyarekar <siddhesh at redhat dot com> 2013-05-10 08:19:48 UTC ---
> Does this happen with latest master? Changing from fixed iterations to fixed
> time showed such spurious results with bench-modf once (likely because it was
> not rebuilt), which went away when I did a bench-clean and rebuilt it.
>
Now it shows following results. According to them atan is ten times
faster than modf which is clearly wrong.
atan(): ITERS:4.48268e+09: TOTAL:7.79352s, MAX:166.672ns, MIN:0.638ns,
5.7518e+08 iter/s
atan(768bits): ITERS:4.8324e+08: TOTAL:9.83724s, MAX:38.805ns, MIN:17.654ns,
4.91235e+07 iter/s
modf(): ITERS:6.8975e+08: TOTAL:9.53964s, MAX:193.448ns, MIN:4.486ns,
7.23035e+07 iter/s
A following benchmark takes 8.9s which is order of magnitude more than
what benchtest tells.
extern double modf (double, double *);
int main(){ int i;
double x=0;
double ret=0;
double f;
for(i=0;i<1000000000;i++){
ret+=modf(x,&f);
x+=0.1;
}
return ret;
}
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* [Bug libc/15424] modf benchtest gives bogus results
2013-05-01 22:00 [Bug libc/15424] New: modf benchtest gives bogus results ondra at iuuk dot mff.cuni.cz
2013-05-10 8:19 ` [Bug libc/15424] " siddhesh at redhat dot com
2013-05-10 9:16 ` neleai at seznam dot cz
@ 2013-05-17 12:01 ` siddhesh at redhat dot com
2013-05-17 13:40 ` siddhesh at redhat dot com
2014-06-13 18:09 ` fweimer at redhat dot com
4 siblings, 0 replies; 6+ messages in thread
From: siddhesh at redhat dot com @ 2013-05-17 12:01 UTC (permalink / raw)
To: glibc-bugs
http://sourceware.org/bugzilla/show_bug.cgi?id=15424
--- Comment #3 from Siddhesh Poyarekar <siddhesh at redhat dot com> 2013-05-17 12:01:27 UTC ---
The problem is not modf, it is the code generated for atan. gcc figures out
that the atan call is needed only once and optimizes it out of the loop. It's
a problem with all trigonometrict functions - look for functions that take ~1-2
cycles minimum time. Patch coming up.
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* [Bug libc/15424] modf benchtest gives bogus results
2013-05-01 22:00 [Bug libc/15424] New: modf benchtest gives bogus results ondra at iuuk dot mff.cuni.cz
` (2 preceding siblings ...)
2013-05-17 12:01 ` siddhesh at redhat dot com
@ 2013-05-17 13:40 ` siddhesh at redhat dot com
2014-06-13 18:09 ` fweimer at redhat dot com
4 siblings, 0 replies; 6+ messages in thread
From: siddhesh at redhat dot com @ 2013-05-17 13:40 UTC (permalink / raw)
To: glibc-bugs
http://sourceware.org/bugzilla/show_bug.cgi?id=15424
Siddhesh Poyarekar <siddhesh at redhat dot com> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|WAITING |RESOLVED
Resolution| |FIXED
--- Comment #4 from Siddhesh Poyarekar <siddhesh at redhat dot com> 2013-05-17 13:40:34 UTC ---
Fixed in master.
commit 48a18de1e1e7dfeab6d034765caaf732b53cfcb7
Author: Siddhesh Poyarekar <siddhesh@redhat.com>
Date: Fri May 17 19:10:33 2013 +0530
Prevent optimizing out of benchmark function call
Resolves: #15424
The compiler would optimize the benchmark function call out of the
loop and call it only once, resulting in blazingly fast times for some
benchmarks (notably atan, sin and cos). Mark the inputs as volatile
so that the code is forced to read again from the input for each
iteration.
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* [Bug libc/15424] modf benchtest gives bogus results
2013-05-01 22:00 [Bug libc/15424] New: modf benchtest gives bogus results ondra at iuuk dot mff.cuni.cz
` (3 preceding siblings ...)
2013-05-17 13:40 ` siddhesh at redhat dot com
@ 2014-06-13 18:09 ` fweimer at redhat dot com
4 siblings, 0 replies; 6+ messages in thread
From: fweimer at redhat dot com @ 2014-06-13 18:09 UTC (permalink / raw)
To: glibc-bugs
https://sourceware.org/bugzilla/show_bug.cgi?id=15424
Florian Weimer <fweimer at redhat dot com> changed:
What |Removed |Added
----------------------------------------------------------------------------
Flags| |security-
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2013-05-01 22:00 [Bug libc/15424] New: modf benchtest gives bogus results ondra at iuuk dot mff.cuni.cz
2013-05-10 8:19 ` [Bug libc/15424] " siddhesh at redhat dot com
2013-05-10 9:16 ` neleai at seznam dot cz
2013-05-17 12:01 ` siddhesh at redhat dot com
2013-05-17 13:40 ` siddhesh at redhat dot com
2014-06-13 18:09 ` fweimer at redhat dot com
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