llvm-gcc is currently miscompiling the following code:

----
typedef union {
  int A; long long L;
} X;
typedef struct {
  X   x;
  int B;
} Y;
Y* foo(Y *l) { return l+1; }
---

In this case, the X union should have 8-byte alignment due to funny darwin rules (even though long 
long's are normally only 4-byte aligned).  When X is the first element of a struct, that increases its 
alignment requirements to 8-bytes, which increases the size of Y from 12 to 16 bytes.

llvm-gcc currently compiles this to:

---
        %struct.Y = type { %union.X, int }
        %union.X = type { long }

%struct.Y* %_Z3fooP1Y(%struct.Y* %l) {
        %tmp.2 = getelementptr %struct.Y* %l, int 1             ; <%struct.Y*> [#uses=1]
        ret %struct.Y* %tmp.2
}
---

... which is incorrect: Y is only 12 bytes in size.  This causes us to emit the following PPC code:

__Z3fooP1Y:
        addi r3, r3, 12
        blr

... instead of the correct code:

__Z3fooP1Y:
        addi r3, r3, 16
        blr

This problem was reduced from Bug 449.

-Chris