<div dir="ltr">On Sat, Aug 16, 2014 at 5:35 AM, David Herrmann <<a href="mailto:dh.herrmann@gmail.com" target="_blank">dh.herrmann@gmail.com</a>> wrote: <div class="gmail_extra"><div class="gmail_quote"> <blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">Hi <div class=""> On Fri, Aug 15, 2014 at 5:22 PM, Daniele Nicolodi <<a href="mailto:daniele@grinta.net">daniele@grinta.net</a>> wrote: > On 15/08/2014 16:30, David Herrmann wrote: >> Ok, took me a while, but I now figured out how to cause compilation to >> fail even in expressions that initialize types (_Static_assert is not >> allowed there): >> #define assert_const(expr) >> ((void)(__builtin_types_compatible_p(int[(expr) ? 1 : -1], int[1]))) >> >> Btw., I like that more than our current assert_cc() fallback. But I >> leave it up to you to decide. >> >> Anyhow, I found a way to make CONST_MAX work: >> #define CONST_MAX(_A, _B) >> (__builtin_choose_expr(__builtin_constant_p(_A) && >> __builtin_constant_p(_B), ((_A) > (_B)) ? (_A) : (_B), (void)0)) >> >> This will return (void) in case _A or _B is not constant. Works fine >> on LLVM, I now have to test it on gcc. If it works, I will commit it >> and fix resolvd. > > Hello, > > this may be completely stupid, but if the only use case you have for > CONST_MAX() is for computing the size of a data structure, I find > something like > > #define MAXSIZE(A, B) sizeof(union { __typeof(A) a; __typeof(B) b;}) > > a little more clear and less magic, and I believe it has the same > guarantees that the solution you found. </div>Your MAXSIZE macro might add padding: union A { int a; char b[5]; }; This union has size 8, not 5 (64bit). But CONST_MAX would return 5. Not sure whether that really matters, though. And we could probably add __packed__ to the definition. However, I noticed that GCC complains about using statement-expressions to initialize static-const structure members, even with my 'const' annotations added to MAX. *sigh* Thus, I think I'll keep CONST_MAX, as we'd require a 3rd macro otherwise. If you know a way to unify them all, please lemme know. </blockquote></div>Thanks for getting this in! I had given up on receiving constructive feedback on my attempt to solve this earlier [1]. Glad your patch garnered useful feedback that got to a committable result! </div><div class="gmail_extra">-Dan [1] <a href="http://lists.freedesktop.org/archives/systemd-devel/2014-August/021761.html">http://lists.freedesktop.org/archives/systemd-devel/2014-August/021761.html</a> </div> </div>