paradoxical subreg problem

[law@redhat.com]

In message <200201282037.PAA24126@makai.watson.ibm.com>, David Edelsohn writes:
 > >>>>> Richard Kenner writes:
 > 
 > Richard> Let me rephrase: why are we genering this if we don't know what tho
 > se
 > Richard> bits are?
 > 
 > 	I agree with Kenner.  There are two things we can discuss: should
 > this ever be generated and should this be generated in this particular
 > instance causing problems.  Regardless whether this compare result can be
 > determined at compile-time or run-time, if the bits are undefined, why is
 > GCC generating an expression that relies on undefined bits?
Kenner's initial claim was not that they were undefined, but that they were
bits we could pretend had any value that was interesting to us (don't care).
If you read the section on paradoxical subregs, this is what it implies.

I think we really need to clarify if the bits are "don't care" or "undefined";
once that's settled it shouldn't be terribly difficult to deal with combine
to make it follow those semantics.

I think the semantics should be "undefined".

jeff