On Wed, 18 Nov 2020, Jakub Jelinek wrote:
> Hi!
>
> The C and C++ FEs handle zero sized arrays differently, C uses
> NULL TYPE_MAX_VALUE on non-NULL TYPE_DOMAIN on complete ARRAY_TYPEs
> with bitsize_zero_node TYPE_SIZE, while C++ FE likes to set
> TYPE_MAX_VALUE to the largest value (and min to the lowest).
>
> Martin has used array_type_nelts in get_parm_array_spec where the
> function on the C form of [0] arrays returns error_mark_node and the code
> crashes soon afterwards. The following patch teaches array_type_nelts about
> this (e.g. dwarf2out already handles that as [0]). While it will change
> what is_empty_type returns for certain types (e.g. struct S { int a[0]; };),
> as those types occupy zero bits in C, it should make an ABI difference.
>
> So, the tree.c change makes the c-decl.c code handle the [0] arrays
> like any other constant extents, and the c-decl.c change just makes sure
> that if we'd run into error_mark_node e.g. from the VLA expressions, we
> don't crash on those.
>
> Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
Looks OK to me. I wonder if we want to change the C FE to follow the
C++ FE here. Note I also chickened out making the domain signed
(the C++ variant is to be interpreted as [0, -1] for zero-size arrays).
In principle [0,] (NULL TYPE_MAX_VALUE) is a flex-array domain but
I guess a COMPLETE_TYPE_P clearly says it isn't. Still it would be
nice to fix this inconsistency. (layout_type could even ICE on
complete array types with "incomplete" domain)
Thanks,
Richard.
> 2020-11-18 Jakub Jelinek <ja...@redhat.com>
>
> PR c/97860
> * tree.c (array_type_nelts): For complete arrays with zero min
> and NULL max and zero size return -1.
>
> * c-decl.c (get_parm_array_spec): Bail out of nelts is
> error_operand_p.
>
> * gcc.dg/pr97860.c: New test.
>
> --- gcc/tree.c.jj 2020-11-18 09:40:09.798660999 +0100
> +++ gcc/tree.c 2020-11-18 20:02:41.655398514 +0100
> @@ -3483,7 +3483,17 @@ array_type_nelts (const_tree type)
>
> /* TYPE_MAX_VALUE may not be set if the array has unknown length. */
> if (!max)
> - return error_mark_node;
> + {
> + /* zero sized arrays are represented from C FE as complete types with
> + NULL TYPE_MAX_VALUE and zero TYPE_SIZE, while C++ FE represents
> + them as min 0, max -1. */
> + if (COMPLETE_TYPE_P (type)
> + && integer_zerop (TYPE_SIZE (type))
> + && integer_zerop (min))
> + return build_int_cst (TREE_TYPE (min), -1);
> +
> + return error_mark_node;
> + }
>
> return (integer_zerop (min)
> ? max
> --- gcc/c/c-decl.c.jj 2020-11-11 01:46:03.245697697 +0100
> +++ gcc/c/c-decl.c 2020-11-18 20:03:53.053602265 +0100
> @@ -5775,6 +5775,8 @@ get_parm_array_spec (const struct c_parm
> type = TREE_TYPE (type))
> {
> tree nelts = array_type_nelts (type);
> + if (error_operand_p (nelts))
> + return attrs;
> if (TREE_CODE (nelts) != INTEGER_CST)
> {
> /* Each variable VLA bound is represented by the dollar
> --- gcc/testsuite/gcc.dg/pr97860.c.jj 2020-11-18 15:15:08.858931877 +0100
> +++ gcc/testsuite/gcc.dg/pr97860.c 2020-11-18 15:14:50.751135430 +0100
> @@ -0,0 +1,11 @@
> +/* PR c/97860 */
> +/* { dg-do compile } */
> +/* { dg-options "" } */
> +
> +void
> +foo (int n)
> +{
> + typedef int T[0];
> + typedef T V[n];
> + void bar (V);
> +}
>
> Jakub
>
>
--
Richard Biener <rguent...@suse.de>
SUSE Software Solutions Germany GmbH, Maxfeldstrasse 5, 90409 Nuernberg,
Germany; GF: Felix Imend
