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From: "palves at redhat dot com" <sourceware-bugzilla@sourceware.org>
To: gdb-prs@sourceware.org
Subject: [Bug c++/26550] New: Object of class with virtual base classes
 printed incorrectly (gdb.cp/ambiguous.exp)
Date: Fri, 28 Aug 2020 20:06:49 +0000
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https://sourceware.org/bugzilla/show_bug.cgi?id=3D26550

            Bug ID: 26550
           Summary: Object of class with virtual base classes printed
                    incorrectly (gdb.cp/ambiguous.exp)
           Product: gdb
           Version: unknown
            Status: NEW
          Severity: normal
          Priority: P2
         Component: c++
          Assignee: unassigned at sourceware dot org
          Reporter: palves at redhat dot com
  Target Milestone: ---

The gdb.cp/ambiguous.exp testcase ( as soon as these changes land in master:
https://sourceware.org/pipermail/gdb-patches/2020-August/171526.html )
runs into a GDB bug where GDB prints a value with virtual base classes
incorrectly.

The class in question inherits from A1 directly and also inherits
from two other distinct base classes that inherit virtually from A1 in
turn:

class A1 {
public:
  int x;
  int y;
};

class LV : public virtual A1 {
public:
  int z;
};

class KV : public virtual A1 {
public:
  int i;
};

class JVA1 : public KV, public LV, public A1 {
public:
  int jva1;
};

and then is initialized like this:

  JVA1 jva1;
  jva1.KV::x =3D 1;
  jva1.KV::y =3D 2;
  jva1.LV::x =3D 3;
  jva1.LV::y =3D 4;
  jva1.z =3D 5;
  jva1.i =3D 6;
  jva1.jva1 =3D 7;

The testcase shows:

 print jva1.KV::x
 $51 =3D 1431665544
 (gdb) FAIL: gdb.cp/ambiguous.exp: all fields: print jva1.KV::x
 print jva1.KV::y
 $52 =3D 21845
 (gdb) FAIL: gdb.cp/ambiguous.exp: all fields: print jva1.KV::y

 (gdb) print /x (KV)jva1
 $4 =3D {<A1> =3D <invalid address>, _vptr.KV =3D 0x555555557b88 <vtable for
JVA1+24>, i =3D 0x457}
 (gdb) print /x (A1)(KV)jva1
 Cannot access memory at address 0x0

And with the following we see how GDB is clearly confusing the offset of
_vptr.KV with the offset of A1:

 (gdb) print /x jva1
 $1 =3D {<KV> =3D {<A1> =3D {x =3D 0x2, y =3D 0x16}, _vptr.KV =3D 0x5555555=
57b88 <vtable
for JVA1+24>, i =3D 0x457}, ......}
                                                      ^^^^^^^^^^^^^^
 (gdb) print /x jva1.KV::x
 $2 =3D 0x55557b88
      ^^^^^^^^^^
 (gdb) print /x jva1.KV::y
 $3 =3D 0x5555
      ^^^^^^

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