Re: Test with an lto-build of libgfortran.

[Richard Biener <richard.guenther@gmail.com>]

On Wed, Sep 27, 2023 at 11:48 PM Jeff Law via Fortran
<fortran@gcc.gnu.org> wrote:
>
>
>
> On 9/27/23 12:21, Toon Moene wrote:
>
> >
> > The lto-ing of libgfortran did succeed, because I did get a new warning:
> >
> > gfortran -O3 -flto -flto-partition=none -static  -o xlintstrfz zchkrfp.o
> > zdrvrfp.o zdrvrf1.o zdrvrf2.o zdrvrf3.o zdrvrf4.o zerrrfp.o zlatb4.o
> > zlaipd.o zlarhs.o zsbmv.o zget04.o zpot01.o zpot03.o zpot02.o chkxer.o
> > xerbla.o alaerh.o aladhd.o alahd.o alasvm.o ../../libtmglib.a
> > ../../liblapack.a ../../librefblas.a
> > In function 'xtoa_big',
> >      inlined from 'write_z' at
> > /home/toon/compilers/gcc/libgfortran/io/write.c:1296:11,
> >      inlined from 'formatted_transfer_scalar_write' at
> > /home/toon/compilers/gcc/libgfortran/io/transfer.c:2136:4:
> > /home/toon/compilers/gcc/libgfortran/io/write.c:1222:6: warning: writing
> > 1 byte into a region of size 0 [-Wstringop-overflow=]
> >   1222 |   *q = '\0';
> >        |      ^
> > /home/toon/compilers/gcc/libgfortran/io/write.c: In function
> > 'formatted_transfer_scalar_write':
> > /home/toon/compilers/gcc/libgfortran/io/write.c:1291:8: note: at offset
> > [34, 4294967294] into destination object 'itoa_buf' of size 33
> >   1291 |   char itoa_buf[GFC_XTOA_BUF_SIZE];
> >        |        ^
> >
> > which was (of course) not given with a non-lto libgfortran.
> Yea.  This certainly can happen with LTO.  These warnings would
> definitely be something worth investigating.
>
> Essentially the inlining enabled by LTO can expose a different set of
> diagnostics.

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).

Richard.

>
> Jeff