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Date: Thu, 9 Dec 2021 21:37:02 +0100
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Subject: Re: [PATCH] PR libfortran/103634 - Runtime crash with PACK on
 zero-sized arrays
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To: Harald Anlauf <anlauf@gmx.de>, fortran <fortran@gcc.gnu.org>,
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From: Mikael Morin <morin-mikael@orange.fr>
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Hello,

On 09/12/2021 21:05, Harald Anlauf via Fortran wrote:
> Dear all,
> 
> I had thought that we had fixed this in the past (see PR31001),
> but it did fail for me with all gcc versions I have tried (7-12)
> for a slightly more elaborate case as in the old testcase.
> 
> The loop in pack_internal did try to access the first element of
> the array argument to PACK even if one (or more) extents were zero.
> This is not good.
> 
> Solution: check the extents and return early.  (We already do a
> related check for the vector argument if present).

If there is a vector argument, aren’t we supposed to copy it to the result ?
There is something else to pay attention for, the early return should 
come at least after the return array bounds have been set.  In the 
testcase an array with the correct bounds has been allocated beforehand 
to hold the return value, but it’s not always the case.

For what it’s worth, the non-generic variant in pack.m4 (or in 
pack_{i,f,c}{1,2,4,8,10,16}.c) has a zero extent check and it clears the 
source ptr in that case, which makes it setup the return array and then 
jump to the vector copy at the end of the function.