SOURCE From: brangdon@[...].uk (Dave Harris)
>  In-Reply-To: <200509211851.j8LIp8VY019052@[...].com>
>  stewart@[...].com (Rob Stewart) wrote (abridged):
> 
>  > That misses what BOOST_ASSUME should do when
>  > BOOST_DISABLE_ASSERTS is not defined.  With BOOST_ASSERT enabled,
>  > BOOST_ASSUME should call both BOOST_ASSERT and __assume():
>  
>  Why?
>  
>  Consider the sequence:
>  
>           BOOST_ASSERT(e); __assume(e)
>  
>  Suppose (e) is true. Then BOOST_ASSERT does nothing, so why
>  invoke it? If

Because you don't know that it is true at compile time.

>  (e) is false, then the __assume tells us the sequence won't be executed at 
>  all, and both it and the BOOST_ASSERT can be optimised away. Whatever it 

If (e) is false, then BOOST_ASSERT takes some action, be it
invoking assert() or something else.  If you omit that call to
BOOST_ASSERT, then you simply get __assume(e) and that's not the
same.

>  is you want BOOST_ASSERT to do, you can't rely on it happening. It will 
>  depend on how clever the optimiser is, which will vary from platform to 
>  platform.

I don't understand your point.  Either (e) is true and
BOOST_ASSERT will do nothing (other than evaluate (e)), or it is
false and BOOST_ASSERT will fire.

The point is that *if* (e) is true, __assume(e) provides
information that the optimizer can use to generate better code.

>  In addition, it evaluates (e) twice. That's bad practice for a macro. It 

That's an assumption I'm not prepared to make.  My guess is that
the compiler uses the information to generate code but doesn't
consult that expression at run time (as with sizeof).

>  would be acceptable if it were necessary (and we certainly hope (e) has no 
>  side effects and that its value doesn't change between the assert and the 
>  assume) but here there is no need for it.

I do see a need for it and I doubt the side effects would occur.

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