possible structure layout bug

[Stephen Kennedy <Stephen dot Kennedy at havok dot com>]

Normally, compilers place members from derived classes starting at
sizeof(base), but gcc breaks this rule if the class contains a simd type
and places the derived member in the padding of the base class.

Reproduced with gcc-3.3 and gcc-3.4 on debian linux x86.
See the attached case

%g++-3.4  layoutbug.cpp && a.out
__m128 size 16, align 16
Quad   size 16, align 16
Base1 32 Derived1 48 Offset 32 <- offset follows sizeof(base)
Base2 32 Derived2 32 Offset 20 <- offset of derived member is inside
Base3 64 Derived3 80 Offset 64 <- sizeof(base class)

Has anybody any comments? I know layouts vary from compiler to compiler,
but this seems to be inconsistent with itself.
Stephen.
-- 
Stephen Kennedy stephen.kennedy@havok.com http://www.havok.com/ 
Game Developer Frontline Award Winner 2002 & 2003
Computer Graphics World 2003 Innovation Award Winner

#include <stdio.h>

typedef int __m128 __attribute__ ((__mode__(__V4SF__)));

struct Quad
{
	float x __attribute__((aligned(16)));
	float y;
	float z;
	float w;
};

struct Base1
{
	Quad q;
	int i;
};

struct Derived1 : public Base1
{
	int i2;
};

struct Base2
{
	__m128 m;
	int i;
};

struct Derived2 : public Base2
{
	int i2;
};


struct Base3
{
	Quad q0;
	__m128 m;
	Quad q1;
	int i;
};

struct Derived3 : public Base3
{
	int i2;
};

#define offset(C,M) (int((&((C*)(16))->M))-16)

int main()
{
	printf("__m128 size %i, align %i\n", sizeof(__m128), __alignof__(__m128));
	printf("Quad   size %i, align %i\n", sizeof(Quad), __alignof__(Quad));
	printf("Base1 %i Derived1 %i Offset %i\n", sizeof(Base1), sizeof(Derived1), offset(Derived1, i2) );
	printf("Base2 %i Derived2 %i Offset %i\n", sizeof(Base2), sizeof(Derived2), offset(Derived2, i2) );
	printf("Base3 %i Derived3 %i Offset %i\n", sizeof(Base3), sizeof(Derived3), offset(Derived3, i2) );
	return 0;
}