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* [Bug libstdc++/55047] New: operator() in std::exponential_distribution may call log(0)
@ 2012-10-24 2:25 hyounes at google dot com
2012-10-24 9:44 ` [Bug libstdc++/55047] " paolo.carlini at oracle dot com
` (4 more replies)
0 siblings, 5 replies; 6+ messages in thread
From: hyounes at google dot com @ 2012-10-24 2:25 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55047
Bug #: 55047
Summary: operator() in std::exponential_distribution may call
log(0)
Classification: Unclassified
Product: gcc
Version: 4.6.3
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: libstdc++
AssignedTo: unassigned@gcc.gnu.org
ReportedBy: hyounes@google.com
The implementation of operator() for std::exponential_distribution is:
template<typename _UniformRandomNumberGenerator>
result_type
operator()(_UniformRandomNumberGenerator& __urng,
const param_type& __p)
{
__detail::_Adaptor<_UniformRandomNumberGenerator, result_type>
__aurng(__urng);
return -std::log(__aurng()) / __p.lambda();
}
If I understand this correctly, __aurng() returns a value in [0,1). This
leaves the possibility of computing -log(0)/lambda, which I expect to be Inf.
On the other hand, -log(1)/lambda will never occur, so the resulting value can
never be 0. There are two problems with this implementation:
1. The actual range (0,Inf) U Inf is not consistent with the reported range of
[0,Inf) as computed by the min() and max() member functions.
2. -log(U01)/lambda is not the mathematically correct form for the inverse
transformation for the exponential distribution. -log(1 - U01)/lambda is the
correct form. This form also gives you the correct range of [0,Inf). It is an
incorrect optimization to change 1-U01 to just U01 when U01 is [0,1). It is
only correct if U01 is [0,1] or (0,1), but I do not believe that to be the case
here.
I believe the correct implementation should have the following return
statement:
return -std::log(result_type(1) - __aurng()) / __p.lambda();
The same problem appears in several other distributions. For example,
std::weibull_distribution.
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug libstdc++/55047] operator() in std::exponential_distribution may call log(0)
2012-10-24 2:25 [Bug libstdc++/55047] New: operator() in std::exponential_distribution may call log(0) hyounes at google dot com
@ 2012-10-24 9:44 ` paolo.carlini at oracle dot com
2012-10-24 12:20 ` paolo at gcc dot gnu.org
` (3 subsequent siblings)
4 siblings, 0 replies; 6+ messages in thread
From: paolo.carlini at oracle dot com @ 2012-10-24 9:44 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55047
Paolo Carlini <paolo.carlini at oracle dot com> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |ASSIGNED
Last reconfirmed| |2012-10-24
AssignedTo|unassigned at gcc dot |paolo.carlini at oracle dot
|gnu.org |com
Target Milestone|--- |4.7.3
Ever Confirmed|0 |1
--- Comment #1 from Paolo Carlini <paolo.carlini at oracle dot com> 2012-10-24 09:43:46 UTC ---
You are right, let's fix this asap in mainline and 4.7. I think it went that
the issue already existed many years ago when we integrated the contribution as
TR1 code and then we propagated it in more places :(
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug libstdc++/55047] operator() in std::exponential_distribution may call log(0)
2012-10-24 2:25 [Bug libstdc++/55047] New: operator() in std::exponential_distribution may call log(0) hyounes at google dot com
2012-10-24 9:44 ` [Bug libstdc++/55047] " paolo.carlini at oracle dot com
@ 2012-10-24 12:20 ` paolo at gcc dot gnu.org
2012-10-24 12:21 ` paolo.carlini at oracle dot com
` (2 subsequent siblings)
4 siblings, 0 replies; 6+ messages in thread
From: paolo at gcc dot gnu.org @ 2012-10-24 12:20 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55047
--- Comment #2 from paolo at gcc dot gnu.org <paolo at gcc dot gnu.org> 2012-10-24 12:20:32 UTC ---
Author: paolo
Date: Wed Oct 24 12:20:19 2012
New Revision: 192762
URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=192762
Log:
2012-10-24 Haakan Younes <hyounes@google.com>
Paolo Carlini <paolo.carlini@oracle.com>
PR libstdc++/55047
* include/bits/random.h (exponential_distribution<>::operator):
Fix formula to std::log(result_type(1) - __aurng()).
* include/bits/random.tcc: Likewise, everywhere.
Modified:
trunk/libstdc++-v3/ChangeLog
trunk/libstdc++-v3/include/bits/random.h
trunk/libstdc++-v3/include/bits/random.tcc
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug libstdc++/55047] operator() in std::exponential_distribution may call log(0)
2012-10-24 2:25 [Bug libstdc++/55047] New: operator() in std::exponential_distribution may call log(0) hyounes at google dot com
2012-10-24 9:44 ` [Bug libstdc++/55047] " paolo.carlini at oracle dot com
2012-10-24 12:20 ` paolo at gcc dot gnu.org
@ 2012-10-24 12:21 ` paolo.carlini at oracle dot com
2012-11-01 21:10 ` paolo at gcc dot gnu.org
2012-11-01 21:11 ` paolo.carlini at oracle dot com
4 siblings, 0 replies; 6+ messages in thread
From: paolo.carlini at oracle dot com @ 2012-10-24 12:21 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55047
--- Comment #3 from Paolo Carlini <paolo.carlini at oracle dot com> 2012-10-24 12:21:17 UTC ---
Fixed in mainline. A similar fix will go in 4_7-branch in a few days.
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug libstdc++/55047] operator() in std::exponential_distribution may call log(0)
2012-10-24 2:25 [Bug libstdc++/55047] New: operator() in std::exponential_distribution may call log(0) hyounes at google dot com
` (2 preceding siblings ...)
2012-10-24 12:21 ` paolo.carlini at oracle dot com
@ 2012-11-01 21:10 ` paolo at gcc dot gnu.org
2012-11-01 21:11 ` paolo.carlini at oracle dot com
4 siblings, 0 replies; 6+ messages in thread
From: paolo at gcc dot gnu.org @ 2012-11-01 21:10 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55047
--- Comment #4 from paolo at gcc dot gnu.org <paolo at gcc dot gnu.org> 2012-11-01 21:09:57 UTC ---
Author: paolo
Date: Thu Nov 1 21:09:51 2012
New Revision: 193070
URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=193070
Log:
2012-11-01 Haakan Younes <hyounes@google.com>
Paolo Carlini <paolo.carlini@oracle.com>
PR libstdc++/55047
* include/bits/random.h (exponential_distribution<>::operator):
Fix formula to std::log(result_type(1) - __aurng()).
* include/bits/random.tcc: Likewise, everywhere.
Modified:
branches/gcc-4_7-branch/libstdc++-v3/ChangeLog
branches/gcc-4_7-branch/libstdc++-v3/include/bits/random.h
branches/gcc-4_7-branch/libstdc++-v3/include/bits/random.tcc
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug libstdc++/55047] operator() in std::exponential_distribution may call log(0)
2012-10-24 2:25 [Bug libstdc++/55047] New: operator() in std::exponential_distribution may call log(0) hyounes at google dot com
` (3 preceding siblings ...)
2012-11-01 21:10 ` paolo at gcc dot gnu.org
@ 2012-11-01 21:11 ` paolo.carlini at oracle dot com
4 siblings, 0 replies; 6+ messages in thread
From: paolo.carlini at oracle dot com @ 2012-11-01 21:11 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55047
Paolo Carlini <paolo.carlini at oracle dot com> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|ASSIGNED |RESOLVED
Resolution| |FIXED
--- Comment #5 from Paolo Carlini <paolo.carlini at oracle dot com> 2012-11-01 21:11:02 UTC ---
Fixed mainline and 4.7.3.
^ permalink raw reply [flat|nested] 6+ messages in thread
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2012-10-24 2:25 [Bug libstdc++/55047] New: operator() in std::exponential_distribution may call log(0) hyounes at google dot com
2012-10-24 9:44 ` [Bug libstdc++/55047] " paolo.carlini at oracle dot com
2012-10-24 12:20 ` paolo at gcc dot gnu.org
2012-10-24 12:21 ` paolo.carlini at oracle dot com
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