Re: Test with an lto-build of libgfortran.

[Jakub Jelinek <jakub@redhat.com>]

On Thu, Sep 28, 2023 at 09:29:02AM +0200, Tobias Burnus wrote:
> the following works for me. I have only tried a normal build (where it
> does silence the same warning) and not an LTO build and I just believed
> the comment - see attached patch. Comments?
> 
> On 28.09.23 08:25, Richard Biener via Fortran wrote:
> 
> > This particular place in libgfortran has
> > 
> >    /* write_z, which calls xtoa_big, is called from transfer.c,
> >       formatted_transfer_scalar_write.  There it is passed the kind as
> >       argument, which means a maximum of 16.  The buffer is large
> >       enough, but the compiler does not know that, so shut up the
> >       warning here.  */
> > #pragma GCC diagnostic push
> > #pragma GCC diagnostic ignored "-Wstringop-overflow"
> >    *q = '\0';
> > #pragma GCC diagnostic pop
> > 
> > so obviously the #pragma doesn't survive through LTO.  Somehow I think
> > this is a known bug, but maybe I misremember (I think we are not streaming
> > any of the ad-hoc location parts).
> 
> I have replaced it now by the assert that "len <= 16", i.e.
> 
> +  if (len > 16)
> +    __builtin_unreachable ();
> 
> Build + tested on x86-64-gnu-linux
> Comment? OK for mainline?

Is it just that in correct programs len can't be > 16, or that it is really
impossible for it being > 16?  I mean, we have that artificial kind 17 for
powerpc which better should be turned into length of 16, but isn't e.g.
_gfortran_transfer_integer etc. just called with a kind argument?  Does
anything error earlier if it is larger?  I mean, say user calling
_gfortan_transfer_integer by hand with kind 1024?

Sure, we could still say it is UB to do that kind of thing and
__builtin_unreachable () would be a way to turn that UB into manifestly
reproducable UB.

	Jakub