Efficient coding considered harmful?

[Knudsen]

In article <8775 at smoke.BRL.MIL>, gwyn at smoke.BRL.MIL (Doug Gwyn ) writes:
> (But don't write something like
> 	for (p = &a[j]; --p >= a; )
> which is nonportable.)

Why not portable?  I know there are some weird architectures that
may treat pointers strangely.  But assuming that p is declared apointer
to the same type as a[], don't the official semantics of C
guarantee that the above code is valid?
What am I missing?
-- 
Mike Knudsen  Bell Labs(AT&T)   att!ihlpl!knudsen
"Lawyers are like nuclear bombs and PClones.  Nobody likes them,
but the other guy's got one, so I better get one too."