> The short answer is that AltiVec dictated this behavior. :-( Longer
> answers were given by Ian and Paul, and I agree with them.
OK, so: firstly, is this behaviour actually documented anywhere? In
AltiVec docs, maybe?
http://publib.boulder.ibm.com/infocenter/macxhelp/v6v81/index.jsp?topic=/com.ibm.vacpp6m.doc/language/ref/clrc03altivec_types.htm
"Vector types can be cast to other vector types. The cast does not
perform a conversion: it preserves the 128-bit pattern, but not
necessarily the value. A cast between a vector type and a scalar type is
not allowed."
Secondly, can we fix it?
We can do anything we want. Or we can do nothing -- it depends on the
point of view. :-) While looking at PR10735, one or two years ago I set
to implement some rules resembling the ones outlined by Ian, only to
find that I would break the Altivec tests in a horrible way.
For sure, we cannot fix the Altivec spec. Consider that Altivec does
not even have an instruction for a value-preserving V4SF->V4SI or
V4SI->V4SF conversion.
Paolo
