c: Support C2x empty initializer braces

[Jakub Jelinek]

On Thu, Aug 25, 2022 at 05:33:02PM +0000, Joseph Myers wrote:
> 	* gimplify.cc (gimplify_modify_expr): Convert initialization from
> 	a variable-size CONSTRUCTOR to memset before call to
> 	gimplify_modify_expr_rhs.

> --- a/gcc/gimplify.cc
> +++ b/gcc/gimplify.cc
> @@ -6031,6 +6031,21 @@ gimplify_modify_expr (tree *expr_p, gimple_seq *pre_p, gimple_seq *post_p,
>        return GS_ALL_DONE;
>      }
>  
> +  /* Convert initialization from an empty variable-size CONSTRUCTOR to
> +     memset.  */
> +  if (TREE_TYPE (*from_p) != error_mark_node
> +      && TYPE_SIZE_UNIT (TREE_TYPE (*from_p))
> +      && !poly_int_tree_p (TYPE_SIZE_UNIT (TREE_TYPE (*from_p)))
> +      && TREE_CODE (*from_p) == CONSTRUCTOR
> +      && vec_safe_is_empty (CONSTRUCTOR_ELTS (*from_p)))

Perhaps
      && CONSTRUCTOR_NELTS (*from_p) == 0)
instead?

> +    {
> +        maybe_with_size_expr (from_p);

The indentation above is 8 spaces instead of tab.

> +	gcc_assert (TREE_CODE (*from_p) == WITH_SIZE_EXPR);
> +	return gimplify_modify_expr_to_memset (expr_p,
> +					       TREE_OPERAND (*from_p, 1),
> +					       want_value, pre_p);
> +    }
> +

Otherwise LGTM.

	Jakub