Re: US-CERT Vulnerability Note VU#162289

[Ian Lance Taylor <iant at google dot com>]

Robert Dewar <dewar@adacore.com> writes:

> An optimziation is dubious to me if
>
> a) it produces surprising changes in behavior (note the importance of
> the word surprising here)
>
> b) it does not provide significant performance gains (note the
> importance of the word significant here).
>
> I find this optimization qualifies as meeting both criteria a) and b),
> so that's why I consider it dubious.

To this particular optimization does not meet criteria a).  I never
write code that constructs a pointer which does not point to any
object, because I know that is invalid.  I think it is rather weird
that anybody would write code like this.  If I want to check whether
an index is out of bounds, I compare the index and the length of the
buffer.  I don't use a pointer comparison to check for an out of
bounds index.

I don't know about criteria b), I haven't measured.  It's easy to come
up with test cases where it gives a performance gain--many loops with
a pointer induction variable will benefit--but I don't know how
significant they are.

Ian