Re: [9fans] subtracting pointers on amd64 6c

[erik quanstrom <quanstro@quanstro.net>]

On Tue Jan  5 11:49:06 PST 2016, charles.forsyth@gmail.com wrote:

> On 5 January 2016 at 19:01, Devon H. O'Dell <devon.odell@gmail.com> wrote:
>
> > so given any of the examples in this thread, if you typedef'ed
> > ptrdiff_t to long, then the compiler technically isn't actually doing
> > anything wrong. whether it is doing something useful is a different
> > question.
> >
>
> Well, although I knew that was true, I didn't want to push the point because
> in practice, people have a right to expect certain reasonable behaviour,
> and it's quite reasonable to expect that p+(q-p) yields q if they both point
> into the same array that the system agreed to allocate somehow.
> It make sense to use 64 bits for the difference, and indeed the code
> block that adds the current cast has an if(1 && ...) suggesting an if(0 &&

yes!  this.  one thing i love about the plan 9 compilers is that my reasonable
expectations are not violated by some happy optimizer, or decision.

this gets us back to the op's pov

> i get that probably nobody cares about c standards here, but it might
> be useful to mention what c99 and c11 say about this issue, since the

which i think is wrong.  there is some current silliness with compilers that
conflates allowed with required and undefined with can be deleted.  i
see plan 9's decisions as rejecting this way of thinking.  however, this is
in no way anti-standard.

- erik