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Subject: Re: CFR: a new __unreachable() builtin
From: David Chisnall <David.Chisnall@cl.cam.ac.uk>
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Date: Wed, 13 May 2015 17:09:43 +0100
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To: Pedro Giffuni <pfg@FreeBSD.org>
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On 13 May 2015, at 17:05, Pedro Giffuni <pfg@FreeBSD.org> wrote:
>=20
> Hello;
>=20
> I am looking at the cdefs in other BSDs hoping to avoid adopting the
> same definitions with incompatible names and I noticed NetBSD is using
> a new __builtin_unreachable (void) function from gcc 4.6:
>=20
> https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html
>=20
> Apparently it was interesting enough that clang implemented it too so
> I created a code review differential for it.
>=20
> https://reviews.freebsd.org/D2536
>=20
> I don't want to add new C definitions unless they are going to be used
> so feel free to comment on the convenience or not of having it.

LLVM uses this quite heavily, in a macro that expands to something =
equivalent to assert(0 && "unreachable reached!=E2=80=9D) in debug mode =
and __builtin_unreachable() in release mode.  When you=E2=80=99re =
debugging, you get errors if you reach unreachable code and in =
deployment the compiler gets a useful hint for optimisation.

David