Strange issue with triangle drawing

[AudioMonster]

I am drawing a simple triangle using the vertex :

struct Vertex2D
{
unsigned int color;
short x,y,z;
short _pad; // for padding, 32bits
};


rendering with :

sceGuDrawArray(GU_TRIANGLES,
GU_COLOR_8888 |
GU_VERTEX_16BIT |
GU_TRANSFORM_2D,
3,0,vertices);

If i am not explicitely initializing _pad with 0 then i have a very odd result ( the gouraud interpolation seems to fail too and do a crazy thing ).
What is the GU doing with that pad element ???

[Saotome]

I'm not really sure, but most triangle issues, where triangles don't show up as expected, are actualy cache issues.
Like here for example.

And _pad is for padding, so that all elements of the array are 32bit aligned.