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Date: Sun, 17 May 2015 09:34:17 +0200
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Subject: Re: CFR: a new __unreachable() builtin
From: Ed Schouten <ed@nuxi.nl>
To: David Chisnall <David.Chisnall@cl.cam.ac.uk>
Cc: Pedro Giffuni <pfg@freebsd.org>,
 FreeBSD Current <freebsd-current@freebsd.org>
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2015-05-13 18:09 GMT+02:00 David Chisnall <David.Chisnall@cl.cam.ac.uk>:
> LLVM uses this quite heavily, in a macro that expands to something equiva=
lent to assert(0 && "unreachable reached!=E2=80=9D) in debug mode and __bui=
ltin_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.

Too bad we can't use this trick in our own assert(). You'd need to
define assert() like this:

#define assert(expr) do { \
  if (!(expr)) \
    __builtin_unreachable(); \
} while (0)

Unfortunately, this would cause the expression to be evaluated, which
is not allowed.

--=20
Ed Schouten <ed@nuxi.nl>