out-of-line and arch-specific random_device

[Marc Glisse]

On Mon, 27 Aug 2012, Ulrich Drepper wrote:

> This is done in the attached patch.  It's rather ugly because of the
> business with the TR1 support.  Is this really still needed?  Can't we
> remove that?  It really makes not much sense for a random_device to be
> predictable.

Er, I haven't read the context, but for simulations physicists usually 
require that random generators can be repeated, so 2 simulations with the 
same seed give the same results. But that may not be what you meant with 
predictable.

> Anyway, another change in the patch is support for a less expensive
> implementation on Ivy Bridge processors.  That processor has the
> rdrand instruction.  The code uses it if the instruction is usable.
> Has been tested on real hardware.  This is not the type of
> arch-specific code I meant earlier.  Will get to that tomorrow.
>
> There are other architectures with similar instructions.  Adding
> support for those will be simple.

I assume they are different enough that they can't all be abstracted
behind a nice common builtin (with default implementation in libgcc
and/or a macro advertising fast implementations of it) :-(


-- 
Marc Glisse