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From: "rguenther at suse dot de" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/94087] std::random_device often fails when used from
 multiple threads
Date: Thu, 14 May 2020 11:51:58 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D94087

--- Comment #20 from rguenther at suse dot de <rguenther at suse dot de> ---
On Thu, 14 May 2020, redi at gcc dot gnu.org wrote:

> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D94087
>=20
> --- Comment #19 from Jonathan Wakely <redi at gcc dot gnu.org> ---
> If you mean the mersenne twister in the std::random_device object, that's=
 a
> union member and doesn't exist when a proper source (/dev/random, rdrand,
> rdseed etc) is available. So we'd need to add *another* mersenne twister =
object
> (which would double the size of std::random_device, changing ABI, or have=
 to be
> global and protected by a mutex, or thread-local) and we'd have to seed i=
t so
> it's not 100% deterministic, and MT has a state size of 19968 bits which =
needs
> a lot of seeding. It's not a good choice for many reasons.

Doh.  OK, guess I'd set up the twister in all cases and make it=20
programatically skip itself when rdrand/rdseed is available so we
could easily fall back to it.  Not sure what extra state there is
that warrants the union, but well ... I suppose simply calling
random() from the C library isn't an option ;)=