Date: Sun, 17 May 2015 09:34:17 +0200 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> Subject: Re: CFR: a new __unreachable() builtin Message-ID: <CABh_MKn=EHccc2S0E0iWmN6dZk1xRwUvY8Q6hK_6QxUwXvhxEQ@mail.gmail.com> In-Reply-To: <B452A3E8-DCC6-46B4-8066-17E8799072C3@cl.cam.ac.uk> References: <5553764A.9010202@FreeBSD.org> <B452A3E8-DCC6-46B4-8066-17E8799072C3@cl.cam.ac.uk>
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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 equivalent to assert(0 && "unreachable reached!”) in debug mode and __builtin_unreachable() in release mode. When you’re 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.
--
Ed Schouten <ed@nuxi.nl>
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