Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!usc!orion.oac.uci.edu!uci-ics!rfg
From: rfg@ics.uci.edu (Ronald Guilmette)
Newsgroups: comp.lang.c++
Subject: Re: subtle inlining/sequence points bug in CFront 2.0
Keywords: inline, CFront, bug
Message-ID: <26422837.21835@paris.ics.uci.edu>
Date: 5 May 90 01:11:19 GMT
References: <8066@goofy.Apple.COM>
Reply-To: rfg@ics.uci.edu (Ronald Guilmette)
Organization: UC Irvine Department of ICS
Lines: 57

In article <8066@goofy.Apple.COM> scott@goofy.apple.com (scott douglass) writes:
>I've found, actually I have been burned by, a subtle bug involving
>inlining and sequence points in CFront 2.0.  In some cases CFront
>accidentally eliminates the sequence point before the call of an
>inline function.

I believe that you have misunderstood the exact definition of the term
"sequence point" (at least as it is proscribed by the ANSI C standard).

Reading sections A.2 and 3.6 of the ANSI C standard may help.

>
>Example:
>
>    class A
>	{
>    public:
>	A* next;
>	int i;
>	A* get_next();
>	int f();
>	};
>
>    A* A::get_next() { return next; }
>
>    void g(A* a)
>	{
>	int b = ( a->get_next()->i == a->f() );
>	}
>
>Assume that A::f() may change A::next which CFront must since A::f()
>if not a const member.  Since order of evaluation is not defined for
>expressions either a->get_next() or a->f() will happen first, but
>the "->i" will definetly refer to one of two possible As.
>
>Now the problem.  If I make A::get_next() inline CFront generates
>code something like the following:
>
>	b = ( a->next->i == f(a) );
>
>The semantics of this expression are not defined.  Your C compiler
>may, and mine did, evaluate a->next into a temporary, evaluate a->f()
>which may change a->next, and finally evaluate temporary->i which
>may no longer be valid.

It seems that you may have desired that the entire left hand operand of ==
be evaluated before *any* of the right hand operand of the same == was
evaluated.  While this may indeed have been desirable in this case, I
don't see that this is required by either the ANSI C rules or the (virtually
identical?) C++ rules relating to sequence points.

I'd be perfectly happy to be proven wrong however.


// Ron Guilmette (rfg@ics.uci.edu)
// C++ Entomologist
// Motto:  If it sticks, force it.  If it breaks, it needed replacing anyway.