On Tue, 2014-08-12 at 09:05 -0700, Steve Langasek wrote:
[...]
> Matthias, could you please have a look at the below test case? We have a
> regression in the latest nfs-kernel-server build, which appears to be caused
> by a gcc-4.9 bug.
>
> Should I work around this in nfs-utils, or is a quick fix possible in
> gcc-4.9?
>
> > char buf[100];
> >
> > void
> > add_name(char *old)
> > {
> > char *cp = old;
> >
> > while (cp && *cp) {
> > cp++;
> > }
> > __builtin_strncpy(buf, old, cp-old);
[...]
So far as I know (haven't checked the latest standard), pointer
subtraction has undefined behaviour unless both operands point into (or
one beyond) the same array. As this is not true of null pointers, the
compiler may infer that old can't be null, so cp can't be null, so there
is no need to check whether it is.
I.e. this is a bug in nfs-utils, not the compiler.
Ben.
--
Ben Hutchings
Humans are not rational beings; they are rationalising beings.
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