Re: [boost] BOOST_ASSERT and __assume on MSVC

From: Dave Harris (brangdon_at_[hidden])
Date: 2005-09-21 13:01:22

In-Reply-To: <dgmfeo$q1v$1_at_[hidden]>
kalita_at_[hidden] (Marcin Kalicinski) wrote (abridged):
> I think generally this is a good idea, but it has a small problem. I 
> would expect that BOOST_ASSUME should turn into __assume when compiled 
> in "release" mode of MSVC. As of above proposed solution this is not 
> the case - user additionally needs to define BOOST_DISABLE_ASSERTS to 
> get __assume. So when NDEBUG is defined and BOOST_DISABLE_ASSERTS is 
> not (I think this is 99% of cases, because few people actually use 
> BOOST_DISABLE_ASSERTS), BOOST_ASSUME will expand to BOOST_ASSERT and 
> the whole thing will be useless.

I did not use NDEBUG because BOOST_ASSERT doesn't, and I didn't want to 
revisit that decision. I imagine the policy is to use fine-grained flags 
and then add:

    #ifdef NDEBUG
        #define BOOST_DISABLE_ASSERTS
    #endif

in a user-editable header somewhere. Arguably there should be a 
BOOST_DISABLE_ASSUMES, too, but for now I'll use BOOST_DISABLE_ASSERTS.

> # define BOOST_ASSUME(e) { BOOST_ASSERT(e); if (e); else __assume(0); }

That evaluates e twice, and if e is false it is not clear whether the 
assert will actually do anything because it could be optimised away. I 
think:

   # define BOOST_ASSUME(e) __assume(e)

is simpler and better. 

> There's also a question what should BOOST_ENABLE_ASSERT_HANDLER do with 
> BOOST_ASSUME? Should it invoke the handler if condition is false?

Only if BOOST_DISABLE_ASSERTS is not defined. So the full code would be:

 #include <boost/config.hpp>
 #if defined(BOOST_MSVC) && defined(BOOST_DISABLE_ASSERTS)
 #    define BOOST_ASSUME(e) __assume(e)
 #else
 #    define BOOST_ASSUME(e) BOOST_ASSERT(e)
 #endif

-- Dave Harris, Nottingham, UK.