Re: Exploiting dual mode operation

[Joern Rennecke <joern.rennecke@superh.com>]

> We see redundant sign extension removal as a partial 
> redundancy problem. This will give us not only the redundant 
> sign extensions, but also better placement for the 
> remaining ones. As a result sign extension instructions 
> may be moved and instructions that uses the highpart
> may be replaced with others that doesn't use it.

Why would you be using the highpart if you don't need to in the
first place?

Or are you saying that the port will have to duplicate patterns
that manipulate the highpart, to have a variant that uses and sets
only a smaller SUBREG, so that you can avoid tracking the lifeness
of the highpart separately?  That would be much more intrusive than
adding an attribute.

> This is not equivalent with live information for the highest 
> part of the registers. 
> 
> Our approach will not require changes in the description file 
> (which in our opinion is a major problem with your code) and will 
> limit the changes to much fewer files.

Without a special attribute in the machine description, or new code
that analyzes the md rtl to automatically generate this information,
you will have only scant information to feed into PRE computations.

And when you fix this problem, you will find that you need low-level
rtl to feed this pass, and then you don't want to push up the register
pressure with PRE.