[Bug libstdc++/67011] division by zero in std::exponential_distribution

[Bug libstdc++/67011] division by zero in std::exponential_distribution

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67011

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
One is for std::exponential_distribution and one is for
std::tr1:exponential_distribution. The former is already fixed and the latter
is moribund and we don't really care about it any more.

[Bug libstdc++/67011] division by zero in std::exponential_distribution

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67011

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Although if you're still seeing a problem with std::exponential_distribution
then I guess it's not the same bug.

(Please ignore tr1/random.h unless you're actually using
std::tr1::exponential_distribution ... and if you are using that, you should
consider not using it :-)

[Bug libstdc++/67011] division by zero in std::exponential_distribution

[kretz at kde dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67011

Matthias Kretz <kretz at kde dot org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |kretz at kde dot org

--- Comment #3 from Matthias Kretz <kretz at kde dot org> ---
The problem is that (result_type(1) - __aurng()) must never be zero (because
std::log might raise FE_DIVBYZERO for std::log(0)). If __aurng() is a
distribution guaranteed to exclude result_type(1) then this works fine. AFAIK
std::generate_canoncial<float> sometimes generates a 1 (cf.
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63176).

Also, I'm wondering whether the 1 - rng wouldn't reduce the randomness.
Consider:
std::log(numeric_limits<float>::min()) -> -87.3365478515625
std::log(1.f - nextafter(1.f, 0.f))    -> -16.635532379150390625
IIUC, the effect of log(1 - rng) is that the distance between random values
from std::exponential_distribution is considerable larger than for log(rng).