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From: "jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug middle-end/79173] add-with-carry and subtract-with-borrow
 support (x86_64 and others)
Date: Sat, 24 Jun 2023 13:14:35 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D79173

--- Comment #31 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
(In reply to Vincent Lef=C3=A8vre from comment #30)
> (In reply to Jakub Jelinek from comment #29)
> > (In reply to Vincent Lef=C3=A8vre from comment #28)
> > > What do you mean by "the first additions will be less optimized"? (If=
 you
> > > don't know anything about the initial carryin and the arguments, you =
can't
> > > optimize at all, AFAIK.)
> >=20
> > I mean that if the compiler can't see it is in [0, 1], it will need to =
use 2
> > additions and or the 2 carry bits together.  But, because the ored carry
> > bits are in [0, 1] range, all the higher limbs could be done using addc.
>=20
> If the compiler can't see that carryin is in [0, 1], then it must not "or"
> the carry bits; it needs to add them, as carryout may be 2.

That is not how the clang builtin works, which is why I've implemented the |
and documented it that way, as it is a compatibility builtin.=