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From: "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libstdc++/87744] Some valid instantiations of
 linear_congruential_engine produce compiler errors when __int128 isn't
 available
Date: Wed, 07 Feb 2024 10:38:41 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D87744

--- Comment #13 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Jakub Jelinek from comment #11)
> Though, if it is common enough, one could try to optimize the __ll[0] =3D=
=3D 0
> && __xx[0] =3D=3D 0 case, one can do then either 32x32->64 or 64x64->64
> multiplication and be done with it.  But if it is rare in random's usage,=
 it
> would just make the code larger.

We will only use this new type when the calculation a*(m-1)+c doesn't fit i=
n 64
bits, and the input values should be uniformly distributed in [0,m) for a g=
ood
choice of parameters (and for a bad choice of parameters, you have bigger
problems than the calculation being slow!)

For a small value of a and large value of c we would use this new type but =
the
multiplication step would overflow infrequently, but I don't think that's a
good choice of parameters and not worth optimizing for.

So I think we should just use the full 64x64 multiplication unconditionally.

For operator% the if (__l._M_hi =3D=3D 0) branch is probably worth it, beca=
use the
general case is so much more expensive.=