[committed] avoid assuming array indices are zero-based (PR 92952)

[Martin Sebor]

On 12/16/19 3:44 PM, Jakub Jelinek wrote:
> On Mon, Dec 16, 2019 at 03:28:29PM -0700, Martin Sebor wrote:
>> PR middle-end/92952 - gfortran.dg/lto/pr87689 FAILs
>>
>> gcc/ChangeLog:
>>
>> 	PR middle-end/92952
>> 	* builtins.c (compute_objsize): Adjust offset by the array low bound.
>>
>> Index: gcc/builtins.c
>> ===================================================================
>> --- gcc/builtins.c	(revision 279443)
>> +++ gcc/builtins.c	(working copy)
>> @@ -3999,6 +3999,16 @@ compute_objsize (tree dest, int ostype, tree *pdec
>>   	     above.  */
>>   	  if (TREE_CODE (dest) == ARRAY_REF)
>>   	    {
>> +	      tree lowbnd = array_ref_low_bound (dest);
>> +	      if (!integer_zerop (lowbnd) && tree_fits_uhwi_p (lowbnd))
>> +		{
>> +		  /* Adjust the offset by the low bound of the array
>> +		     domain (normally zero but 1 in Fortran).  */
>> +		  unsigned HOST_WIDE_INT lb = tree_to_uhwi (lowbnd);
>> +		  offrng[0] -= lb;
>> +		  offrng[1] -= lb;
>> +		}
> 
> I don't understand why uhwi is used.  offrng is an array of wide_int with
> precision of sizetype, so why not instead:
> 	      if (!integer_zerop (lowbnd) && TREE_CODE (lowbnd) == INTEGER_CST)
> 		{
> 		  /* Adjust the offset by the low bound of the array
> 		     domain (normally zero but 1 in Fortran).  */
> 		  wide_int lb = wi::to_wide (lowbnd, sizprec);
> 		  offrng[0] -= lb;
> 		  offrng[1] -= lb;
> 		}
> ?

I don't see what difference it makes.  They should both do the same
thing.  If you just prefer your alternative, feel free to change it.

Martin