performance issue with nullable value type

[John Rose]

On Jan 8, 2019, at 1:06 AM, forax at univ-mlv.fr wrote:
> 
> all boxes should have a scalarized representation for local variables independently 

That's OK as long as the scalarized representation can go dead (in the IR)
if it's not used.  That way, in code where a value is just passed from point A
to point B, there's no need to dereference it or check for null.

Likewise, if a scalarized representation *is* used (because of field access),
it's also desirable to keep around a "souvenir" of any known reference
from which the scalarized representation was originally loaded, if it
can be reloaded from there.  That way, in code which looks at a field
and then forwards the value unchanged to another place B, the code
can use the "souvenir" if needed (if the other place B needs a reference).
The "souvenir" might also be useful for reloading registers after a spill.

All this points to the IR design where we keep *both* a base pointer
(if available) *and* the field values (if needed) in a tuple, but we generate
code only for the parts we need.

— John