Re: gcc10's -Wreturn-local-addr gives FP warning about lib/careadlinkat

[Pádraig Brady]

On 06/02/2020 17:05, Jim Meyering wrote:
> On Thu, Feb 6, 2020 at 6:03 AM Pádraig Brady <address@hidden> wrote:
> > On 06/02/2020 00:27, Jim Meyering wrote:
> > > Building latest latest coreutils using latest-from-git gcc10 evokes
> > > this false positive:
> > >
> > > lib/careadlinkat.c: In function 'careadlinkat':
> > > cc1: error: function may return address of local variable
> > > [-Werror=return-local-addr]
> > > lib/careadlinkat.c:73:8: note: declared here
> > >      73 |   char stack_buf[1024];
> > >
> > > I'm guessing improved flow analysis will eventually suppress this. I
> > > hesitate to turn off the useful and normally-high-S/N
> > > -Wreturn-local-addr globally. Maybe just disable it in that one file,
> > > temporarily?
> >
> > The logic of the function looks fine.
> > Would an `assure (buf != stack_buf)` before the `return buf`
> > indicate that constraint to gcc with minimal runtime overhead?
>
> I would have preferred that, but it has no effect.

surprised.

> I then tried to suppress that warning in the affected file by adding
> these lines:
>
> /* Without this pragma, gcc 10.0.1 20200205 reports that
>     the "function may return address of local variable".  */
> # pragma GCC diagnostic ignored "-Wreturn-local-addr"
>
> But, surprisingly, that didn't help, either.

surprised++
That suggests something major is up with that error
which might be easier for gcc folks to find.

> Also tried Kaz Kylheku's return-early suggestion, to no avail.
>
> Hence, my last resort suggestion: disable that warning option for all
> gnulib compiles. Note that simply omitting the -W... option was
> insufficient (probably pulled in by some high-level one), so I
> resorted to adding -Wno-return-local-addr:

+1

This is a significant enough regression that it should be reported.
I didn't see a gcc bug report, only this meta bug:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90556

cheers,
Pádraig