HELP: one issue during the implementation for counted_by attribute

HELP: one issue during the implementation for counted_by attribute

[Qing Zhao]

Hi, 

1. For the following source code (portion):

struct annotated {
  size_t foo;
  char b;
  char array[] __attribute__((counted_by (foo)));
};

static void noinline bar ()
{
  struct annotated *p2 = alloc_buf (10);
  p2->array[8] = 0;
  return;
}

2. I modified C FE to generate the following code for the routine “bar”:

;; Function bar (null)
;; enabled by -tree-original
{
  struct annotated * p2 = alloc_buf (10);

    struct annotated * p2 = alloc_buf (10);
  .ACCESS_WITH_SIZE ((char *) &p2->array, &p2->foo, 1, 8, -1)[8] = 0;
  return;
}

The gimpliflication asserted at:/home/opc/Install/latest-d/bin/gcc -O2 -fdump-tree-all ttt_1.c
ttt_1.c: In function ‘bar’:
ttt_1.c:29:5: internal compiler error: in create_tmp_var, at gimple-expr.cc:488
   29 |   p2->array[8] = 0;
      |   ~~^~~~~~~

3. The reason for this assertion failure is:  (in gcc/gimplify.cc)

16686         case CALL_EXPR:
16687           ret = gimplify_call_expr (expr_p, pre_p, fallback != fb_none);
16688 
16689           /* C99 code may assign to an array in a structure returned
16690              from a function, and this has undefined behavior only on
16691              execution, so create a temporary if an lvalue is
16692              required.  */
16693           if (fallback == fb_lvalue)
16694             {
16695               *expr_p = get_initialized_tmp_var (*expr_p, pre_p, post_p, false);
16696               mark_addressable (*expr_p);
16697               ret = GS_OK;
16698             }
16699           break;

At Line 16695, when gimplifier tried to create a temporary value for the .ACCESS_WITH_SIZE function as:
   tmp = .ACCESS_WITH_SIZE ((char *) &p2->array, &p2->foo, 1, 8, -1);

It asserted since the TYPE of the function .ACCESS_WITH_SIZE is an INCOMPLETE_TYPE (it’s the TYPE of p2->array, which is an incomplete type).

4. I am stuck on how to resolve this issue properly:
The first question is:

Where should  we generate
  tmp = .ACCESS_WITH_SIZE ((char *) &p2->array, &p2->foo, 1, 8, -1)

In C FE or in middle-end gimplification? 

Thanks a lot for your help.

Qing

Re: HELP: one issue during the implementation for counted_by attribute

[Qing Zhao]

Just an update on this issue.

Finally, I resolved this issue with the following solution:

For the source code (portion):

"
struct annotated {
  size_t foo;
  char array[] __attribute__((counted_by (foo)));
};

p2->array[8] = 0;
“

C FE will generate the following: (*.005t.original)

*(.ACCESS_WITH_SIZE (p2->array, &p2->foo, 1, 8, -1) + 8) = 0;

i.e, the RETURN type of the call to .ACCESS_WITH_SIZE should be a pointer type to char,  char *
(Previously, the RETURN type of the call is char [])"

This resolved the issue nicely. 

Let me know if you see any obvious issue with this solution. 

thanks.

Qing


> On Nov 30, 2023, at 11:07 AM, Qing Zhao <qing.zhao@oracle.com> wrote:
> 
> Hi, 
> 
> 1. For the following source code (portion):
> 
> struct annotated {
>  size_t foo;
>  char b;
>  char array[] __attribute__((counted_by (foo)));
> };
> 
> static void noinline bar ()
> {
>  struct annotated *p2 = alloc_buf (10);
>  p2->array[8] = 0;
>  return;
> }
> 
> 2. I modified C FE to generate the following code for the routine “bar”:
> 
> ;; Function bar (null)
> ;; enabled by -tree-original
> {
>  struct annotated * p2 = alloc_buf (10);
> 
>    struct annotated * p2 = alloc_buf (10);
>  .ACCESS_WITH_SIZE ((char *) &p2->array, &p2->foo, 1, 8, -1)[8] = 0;
>  return;
> }
> 
> The gimpliflication asserted at:/home/opc/Install/latest-d/bin/gcc -O2 -fdump-tree-all ttt_1.c
> ttt_1.c: In function ‘bar’:
> ttt_1.c:29:5: internal compiler error: in create_tmp_var, at gimple-expr.cc:488
>   29 |   p2->array[8] = 0;
>      |   ~~^~~~~~~
> 
> 3. The reason for this assertion failure is:  (in gcc/gimplify.cc)
> 
> 16686         case CALL_EXPR:
> 16687           ret = gimplify_call_expr (expr_p, pre_p, fallback != fb_none);
> 16688 
> 16689           /* C99 code may assign to an array in a structure returned
> 16690              from a function, and this has undefined behavior only on
> 16691              execution, so create a temporary if an lvalue is
> 16692              required.  */
> 16693           if (fallback == fb_lvalue)
> 16694             {
> 16695               *expr_p = get_initialized_tmp_var (*expr_p, pre_p, post_p, false);
> 16696               mark_addressable (*expr_p);
> 16697               ret = GS_OK;
> 16698             }
> 16699           break;
> 
> At Line 16695, when gimplifier tried to create a temporary value for the .ACCESS_WITH_SIZE function as:
>   tmp = .ACCESS_WITH_SIZE ((char *) &p2->array, &p2->foo, 1, 8, -1);
> 
> It asserted since the TYPE of the function .ACCESS_WITH_SIZE is an INCOMPLETE_TYPE (it’s the TYPE of p2->array, which is an incomplete type).
> 
> 4. I am stuck on how to resolve this issue properly:
> The first question is:
> 
> Where should  we generate
>  tmp = .ACCESS_WITH_SIZE ((char *) &p2->array, &p2->foo, 1, 8, -1)
> 
> In C FE or in middle-end gimplification? 
> 
> Thanks a lot for your help.
> 
> Qing
>