Altivec + 16 byte alignment

[Richard Kenner]

    BIGGEST_ALIGNMENT is 128, but STACK_BOUNDARY is 64 unless you give 
    -mabi=altivec (in 3.2+). 

I must say that I find this confusing.  Although the documentation is
unclear, I've always thought of BIGGEST_ALIGNMENT as being the largest
alignment we can guarantee and I think there's code that depends on that.
But if STACK_BOUNDARY < BIGGEST_ALIGNMENT, that's clearly not true.  

Has the definition changed?  If so, how *do* you find the largest
alignment you can guarantee?