Re: Simplification of relational expressions

[John Fine <johnsfine at verizon dot net>]

Andrew Haley wrote:
> Chris Thiel wrote
> > Is there a reason that the equality and inequality expressions (and only
> > those expressions) are simplified?  I understand that signed integer
> > overflow is undefined.
> >     
>
> I'm more than a little mystified by this question.  Unless the target is
> using something truly weird like saturating arithmetic we can always
> guarantee that a+1 != a, but we cannot guarantee that a+1 > a.
>
>   
The optimizer is always allowed to know the behaviors that result in 
THIS compiler from undefined behavior.  If the optimizer can predict the 
run time answer with complete certainty, it can use that answer.

You could create a compiler with saturating arithmetic, so the most 
positive integer plus one is still the most positive integer.  Then the 
optimizer of THAT compiler could safely assume a+1 >= a but couldn't 
assume a+1>a nor a+1!=a.

But for THIS compiler the most positive integer plus one is the most 
negative integer.  The compiler knows that even though the standard 
doesn't know it and a well written program shouldn't know it.  So THIS 
compiler can assume a+1!=a.