On Fri, Jan 20, 2017 at 12:35:32PM -0700, Jeff Law wrote:
> > This may be a naive question but why is it not possible to create
> > such an SSA name?
> Time and space complexity. To get the range information in this case we
> have to create new SSA_NAMEs and PHI nodes to merge them at BB3.
Not just that. Also lots of optimizations rely on the extra SSA_NAME copies
to be removed to be able to optimize well. If you'd have lots of SSA_NAMEs with
the same value, just differing with their range info, then unless passes
perform SCCVN or something similar, they wouldn't be able to figure equality
etc. And if in SCCVN those SSA_NAMEs with different range info, but same
value, compare the same, then they are going to be replaced just by one of
them anyway.
> Even if you create them, passes post VRP are going to blow them away :-)
Exactly.
Jakub
