Re: Test with an lto-build of libgfortran.

[Tobias Burnus <tobias@codesourcery.com>]

(replace gcc@ by gcc-patches@; see
https://gcc.gnu.org/pipermail/gcc/2023-September/242591.html
and other emails in that thread)

On 28.09.23 11:51, Jakub Jelinek wrote:
> On Thu, Sep 28, 2023 at 09:29:02AM +0200, Tobias Burnus wrote:
>> 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.  */
...
>> I have replaced it now by the assert that "len <= 16", i.e.
>> +  if (len > 16)
>> +    __builtin_unreachable ();
> 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.

My understanding is that kind=17 only pops up on PowerPC
for REAL variables as they represent __float128 in multiple ways.

Having said that, the current call tree is:

* xtoa_big: that's where the warning suppression
   was replaced by the unreachable.

* Only caller is 'write_z' with calls it by passing its
   last argument ('len') as last argument ('len')

* "internal_proto(write_z)" implies that it is not called from
   outside libgfortran. The internal only caller is:

*  formatted_transfer_scalar_write, which calls it as:

         case FMT_Z:
           ...
#ifdef HAVE_GFC_REAL_17
           if (type == BT_REAL && kind == 17)
             kind = 16;
#endif
           write_z (dtp, f, p, kind);

I am not aware of any logigal/integer/real(+comples)/character kind > 16,
except for this PPC one. And complex numbers are pairs of BT_REAL.

Thus, I think that patch should be fine - except:

> Does anything error earlier if it is larger?  I mean, say user calling
> _gfortan_transfer_integer by hand with kind 1024?

I think this will fail. We have various ways to deal with this in libgfortran;
I see some cases where the switch "default:" sets the length to 0; we have
other places where we use an "assert", I think we have other places were
we run into UB.

Thus, one option would be to either 'assert(len <= 16)' or
'assert((size_t)len < GFC_OTOA_BUF_SIZE - 1)' instead.

Or we could handle it as len=0 and silently ignore the output or ...

I am fine with either of the many options - except that I like something
explicit involving 'len' and a comparison (unreachable, assert, regarding as len = 0)
better than the existing warning suppression which is too indirect for
me. (Besides: it does not work for LTO.) Preferences? Tobias

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