Three old entries from PROBLEMS

[Joern Rennecke]

Quoting Joern Rennecke <amylaar@spamcop.net>:

> Quoting Paolo Bonzini <bonzini@gnu.org>:
...
>> That would be:
>>
>> clr r7
>> clr r8
>> ...
>> move strict-low-part(r7), r0
>> ...
>> move strict-low-part(r8), r1 --> could reuse r7
>>
>> This is not implemented but IMHO obsolete, most of the targets will
>> just use an AND to implement zero extension.
>
> If the target can actually write to partial registers and refer to the
> entire register without undue delay, you won't need and zero_extend or
> AND operation at all.  So this is obsolete only to the extent that such
> targets have become fewer / more arcane.
> FWIW the ARC mxp had that feature, but it doesn't look like that port
> will get into gcc mainline.

P.S.: more common in contemporary targets is the availability of vector
operations.  In order to use these to save zero extensions, instead of  
the result register you need the inputs to be prepared with a zero  
highpart.
E.g. to do a loop with an unsigned short biv, you zero-extend the initial
biv value and the loop increment value, then use a vector add for the biv
increment operation.