https://bugs.llvm.org/show_bug.cgi?id=39344
Bug ID: 39344
Summary: uniform_real_distribution can't generate the full
range of finite floating point numbers
Product: libc++
Version: unspecified
Hardware: All
OS: All
Status: NEW
Severity: enhancement
Priority: P
Component: All Bugs
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected], [email protected]
Created attachment 21015
--> https://bugs.llvm.org/attachment.cgi?id=21015&action=edit
Example program demonstrating the problem.
std::uniform_real_distribution doesn't work if you try to use it to cover the
whole range of available finite floating point numbers:
...
double low = std::numeric_limits<double>::lowest();
double high = std::numeric_limits<double>::max();
std::uniform_real_distribution<> distribution(low, high);
... distribution(generator) ...
In that case, rather than returning a random finite float, it always returns
"inf".
The libc++ code for this function computes (high - low) which in this case ends
up being
std::numeric_limits<double>::max() - std::numeric_limits<double>::lowest()
and that expression overflows to "inf", which then ends up propagating to the
return value from std::uniform_real_distribution<>::operator().
See the attached example program uniform_real.cc for a test case.
Expected behaviour: program runs successfully with no output.
Observed behaviour: program fails with an assertion failure.
The specific code in the libc++ implementation that is at fault is this
function definition:
template<class _RealType>
template<class _URNG>
inline
typename uniform_real_distribution<_RealType>::result_type
uniform_real_distribution<_RealType>::operator()(_URNG& __g, const param_type&
__p)
{
return (__p.b() - __p.a())
* _VSTD::generate_canonical<_RealType,
numeric_limits<_RealType>::digits>(__g)
+ __p.a();
}
Note: a similar bug is also present in libstdc++ -- see
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87527
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