[c++] C++ Frontend lays out superclasses like anonymous bitfields!

[lattner]

Bugzilla Link	104
Resolution	FIXED
Resolved on	Mar 06, 2010 13:58
Version	1.0
OS	All

Extended Description

In this test (test/Regression/C++Frontend/2003-11-09-ConstructorTypeSafety.cpp.tr):

---

struct contained {
unsigned X;
contained();
};

struct base {
unsigned A, B;
};

struct derived : public base {
contained _M_value_field;
};

int test() {
derived X;
}

There are lots of type safety problems. For example, the types are compiled to:

    %struct.base = type { uint, uint }
    %struct.contained = type { uint }
    %struct.derived = type { ubyte, ubyte, ubyte, ubyte, ubyte, ubyte,

ubyte, ubyte, %struct.contained }

instead of:
%struct.derived = type { %struct.base, %struct.contained }

Ugh. Of course, this is nuking all of my attempts at working on data-structure
based programs. :(

-Chris

[lattner]

assigned to @lattner

[lattner]

Whoa, this is scary. The C++ front-end would always lay out base classes as
anonymous bitfield references, causing no end of trouble. I would not be
suprised if this fixes some of the problems we are having with C++ codes: this
could cause miscompilations.

$ diff -u llvm-types.c~ llvm-types.c
--- llvm-types.c~ 2003-11-04 13:25:46.000000000 -0600
+++ llvm-types.c 2003-11-09 10:37:09.000000000 -0600
@@ -633,7 +633,8 @@
ElementAlignments[*Idx] = ByteAlignment;
*Size = StartOffset/8 + llvm_type_get_size(Ty);
++*Idx;

• } else if (DECL_NAME(field) == 0) { /* Is this an anonymous bitfield? */

• } else if (DECL_NAME(field) == 0 && /* Is this an anonymous bitfield? */

•         DECL_BIT_FIELD(field)) {
  

  unsigned NumPads;
  /* Is it attempting to align the current offset to some value? */
  if (GetDeclSize(field) == 0) {

---

With this patch, we now get:
%struct.derived = type { %struct.base, %struct.contained }

As expected.

-Chris