[PATCH B2/2] Use builtin_unreachable in assert

[Marek Polacek]

On Mon, Mar 02, 2015 at 09:18:31AM -0800, Richard Henderson wrote:
> On 03/01/2015 11:43 PM, Mike Frysinger wrote:
> > On 09 Jan 2015 10:16, Richard Henderson wrote:
> >> Finally, use __builtin_unreachable to tell the compiler about
> >> impossible paths.
> > 
> > i'm not sure this is the way to go.  the expectation (as required by POSIX)
> > is that you're guaranteed there are no side-effects when NDEBUG is applied
> > to assert.  by using __builtin_unreachable, that is no longer the case.
> 
> I wonder if, during the gcc 6 development cycle, we should experiment with a
> __builtin_side_effects_p, akin to __builtin_constant_p.  Then we could still
> follow POSIX re no side effects but even in the external context provide the
> information derivable from __builtin_unreachable.  I.e.
> 
> #define assert(X) \
>   (__builtin_side_effects_p (X) || !(X) ? 0 : __builtin_unreachable ())

Tom wrote a patch implementing something like that a while back (for C
only) <https://gcc.gnu.org/ml/gcc-patches/2014-04/msg01984.html>, but
the patch never made it in.  Maybe I could revive and adjust it during
gcc 6 stage 1 if it would help you.

	Marek