[vector] mulAll for byte and short

[John Rose]

On May 18, 2018, at 12:09 PM, Paul Sandoz <paul.sandoz at oracle.com> wrote:
> 
> Hi,
> 
> I cannot recall if this was discussed before and i did not find anything in prior emails. 
> 
> Out of all the reduction operations mulAll is an outlier and does not support byte and short? is than an oversight or deliberate? I suspect the latter but just wanted to double check.

A recurring theme is what to do about subrange types where overflow is more
likely because of the narrower range.  Applies to almost every operation, except
and/or/xor/not/neg.  Even add can overflow.

Here's a model that might let us make a little more progress with such operations,
if not now then later.

A vector type is a tuple of (Len, T, S :> T, Store: S->T).

1. a lane type T has a size and format (signed, unsigned, float, maybe also "just raw bits")
2. a lane type also has an associated scalar format S, which may be larger
3. arithmetic on lanes is defined as if performed in the associated scalar format
4. scalar result values are stored back into lanes using a type-specific Store function
5. when a vector returns an accumulated cross-lane result, it returns the scalar type S

If a result value S is not representable in the lane type T, the store function usually
performs truncation, as in Java.  However, special vector types may instead perform
saturation or throw overflow errors.

Adding this extra type S would allow us to model Java's rules for questionable operations
like logical shift on bytes and shorts.  Java promotes those guys to int (JLS 5.6.1) ,
so if T is byte then S is int.

Such a two-type framework might also help when dealing with small types which do not
correspond to Java primitives; I'm thinking of short float here, where T is 16 bits and S is
Java float.  Of course, if the CPU performs native 16-bit float arithmetic, the framework
wouldn't fully account for the numeric results produced, so it's not perfect.  But maybe
it helps.

Maybe we can hide the extra features (S, Store) in the species, or even the shape.

— John