On Tue, Jan 14, 2014 at 09:42:37PM +0000, Joseph S. Myers wrote:
> On Mon, 13 Jan 2014, Marek Polacek wrote:
>
> > +/* Return true if T is a pointer to a zero-sized struct/union. */
> > +
> > +bool
> > +pointer_to_zero_sized_aggr_p (tree t)
> > +{
> > + t = strip_pointer_operator (t);
> > + if (RECORD_OR_UNION_TYPE_P (t)
> > + && TYPE_SIZE (t)
> > + && integer_zerop (TYPE_SIZE (t)))
> > + return true;
> > + return false;
>
> Given that GNU C also allows arrays of constant size 0, shouldn't the
> errors also apply in that case? (I don't know whether the original bug
> can appear for such arrays, but I'd think the errors should apply to
> anything with constant size 0 - not of course for VLAs where it just so
> happens that the compiler can tell at compile time that the size is always
> 0.)
Perhaps, but I don't think we can do it easily. Consider
int
foo (int *p, int *q)
{
return p - q;
}
int
bar (void)
{
int a[0], b[0];
return foo (a, b);
}
pointer_diff doesn't know that what p/q point to are zero-sized
arrays. We could do something about
int
foo (void)
{
int a[0], b[0];
return a - b;
}
but in the light of the beforementioned issue, I'm not sanguine about
that.
And no, the original bug doesn't trigger: we have
(((long int) p - (long int) q) /[ex] 4)
here and not
(((long int) p - (long int) q) /[ex] 0)
Here's patch with a change Florian suggested.
2014-01-15 Marek Polacek <pola...@redhat.com>
PR c/58346
c-family/
* c-common.c (pointer_to_zero_sized_aggr_p): New function.
* c-common.h: Declare it.
cp/
* typeck.c (pointer_diff): Give an error on arithmetic on pointer to
an empty aggregate.
c/
* c-typeck.c (pointer_diff): Give an error on arithmetic on pointer to
an empty aggregate.
testsuite/
* c-c++-common/pr58346.c: New test.
--- gcc/c-family/c-common.h.mp 2014-01-15 11:24:41.438608787 +0100
+++ gcc/c-family/c-common.h 2014-01-15 11:24:57.724675857 +0100
@@ -789,6 +789,7 @@ extern bool keyword_is_storage_class_spe
extern bool keyword_is_type_qualifier (enum rid);
extern bool keyword_is_decl_specifier (enum rid);
extern bool cxx_fundamental_alignment_p (unsigned);
+extern bool pointer_to_zero_sized_aggr_p (tree);
#define c_sizeof(LOC, T) c_sizeof_or_alignof_type (LOC, T, true, false, 1)
#define c_alignof(LOC, T) c_sizeof_or_alignof_type (LOC, T, false, false, 1)
--- gcc/c-family/c-common.c.mp 2014-01-15 11:24:43.117615687 +0100
+++ gcc/c-family/c-common.c 2014-01-15 11:24:57.723675852 +0100
@@ -11829,4 +11829,15 @@ cxx_fundamental_alignment_p (unsigned a
TYPE_ALIGN (long_double_type_node)));
}
+/* Return true if T is a pointer to a zero-sized struct/union. */
+
+bool
+pointer_to_zero_sized_aggr_p (tree t)
+{
+ t = strip_pointer_operator (t);
+ return (RECORD_OR_UNION_TYPE_P (t)
+ && TYPE_SIZE (t)
+ && integer_zerop (TYPE_SIZE (t)));
+}
+
#include "gt-c-family-c-common.h"
--- gcc/cp/typeck.c.mp 2014-01-15 11:24:48.940639724 +0100
+++ gcc/cp/typeck.c 2014-01-15 11:24:57.740675935 +0100
@@ -5043,6 +5043,14 @@ pointer_diff (tree op0, tree op1, tree p
return error_mark_node;
}
+ if (pointer_to_zero_sized_aggr_p (TREE_TYPE (op1)))
+ {
+ if (complain & tf_error)
+ error ("arithmetic on pointer to an empty aggregate");
+ else
+ return error_mark_node;
+ }
+
op1 = (TYPE_PTROB_P (ptrtype)
? size_in_bytes (target_type)
: integer_one_node);
--- gcc/c/c-typeck.c.mp 2014-01-15 11:24:53.699659333 +0100
+++ gcc/c/c-typeck.c 2014-01-15 11:24:57.732675897 +0100
@@ -3536,6 +3536,9 @@ pointer_diff (location_t loc, tree op0,
/* This generates an error if op0 is pointer to incomplete type. */
op1 = c_size_in_bytes (target_type);
+ if (pointer_to_zero_sized_aggr_p (TREE_TYPE (orig_op1)))
+ error_at (loc, "arithmetic on pointer to an empty aggregate");
+
/* Divide by the size, in easiest possible way. */
result = fold_build2_loc (loc, EXACT_DIV_EXPR, inttype,
op0, convert (inttype, op1));
--- gcc/testsuite/c-c++-common/pr58346.c.mp 2014-01-13 15:48:20.011420141
+0100
+++ gcc/testsuite/c-c++-common/pr58346.c 2014-01-13 20:25:17.544582444
+0100
@@ -0,0 +1,24 @@
+/* PR c/58346 */
+/* { dg-do compile } */
+
+struct U {
+#ifdef __cplusplus
+ char a[0];
+#endif
+};
+static struct U b[6];
+static struct U **u1, **u2;
+
+int
+foo (struct U *p, struct U *q)
+{
+ return q - p; /* { dg-error "arithmetic on pointer to an empty aggregate" }
*/
+}
+
+void
+bar (void)
+{
+ __PTRDIFF_TYPE__ d = u1 - u2; /* { dg-error "arithmetic on pointer to an
empty aggregate" } */
+ __asm volatile ("" : "+g" (d));
+ foo (&b[0], &b[4]);
+}
Marek
