Re: [boost] BOOST_ASSERT and __assume on MSVC

From: Rob Stewart (stewart_at_[hidden])
Date: 2005-10-04 09:37:13

From: "Peter Dimov" <pdimov_at_[hidden]>
> Rob Stewart wrote:
> 
> > In the (elided ;-) example, ++x has a side effect so while it
> > doesn't affect whether __assume(false) is reached, the optimizer
> > shouldn't elide the increment unless it can prove that the new
> > value of x is not used.  Are you suggesting that __assume(false)
> > would override that?
> 
> Yes, I am. __assume( false ) asserts that this point is unreachable. Since 
> ++x always completes, it follows that ++x is unreachable as well, so it can 
> be elided. 

If that is the behavior of __assume(false), or at least given
that it is a reasonable interpretation of what it could do, then
including __assume() in BOOST_ASSERT() seems a little dangerous.

Its inclusion should only follow the examination of a large set
of extant uses of BOOST_ASSERT() (and perhaps even assert()) to
see what effect adding __assume() would have on the generated
code.  If there are reasonable cases in which the (possible)
effects of using __assume() would be deleterious, then there are
two possibilities:

 - Do not include __assume() in BOOST_ASSERT(), but consider
   adding BOOST_ASSUME() to enable explicit use.

 - Codify the counterexamples in another macro and document
   BOOST_ASSERT()'s __assume() functionality and point to the new
   macro for cases in which that isn't desirable.

-- 
Rob Stewart                           stewart_at_[hidden]
Software Engineer                     http://www.sig.com
Susquehanna International Group, LLP  using std::disclaimer;