llvm dev - Apr 2010 - [LLVMdev] On the transitivity of subregisterness

Woud it be reasonable to assert that TargetRegisterInfo::isSubRegister() is a
transitive relation?

In other words, if A is a subregister of B and B is a subregister of C then A is
a subregister of C.

With this assumption, we could teach TableGen to sort registers topologically
such that all the subregisters of R are numbered R+1, R+2, ... This produces
better locality of reference and allows other optimizations in the backend code
that deals in subregisters a lot.

/jakob

On Apr 21, 2010, at 11:19 AM, Jakob Stoklund Olesen wrote:
> Woud it be reasonable to assert that TargetRegisterInfo::isSubRegister() is
a transitive relation?
> 
> In other words, if A is a subregister of B and B is a subregister of C then
A is a subregister of C.
> 
This certainly makes sense to me. I suspect any backends that are doing things
that violate that are playing tricks with subregisters that would be (better?)
solvable another way.
> With this assumption, we could teach TableGen to sort registers
topologically such that all the subregisters of R are numbered R+1, R+2, ...
This produces better locality of reference and allows other optimizations in the
backend code that deals in subregisters a lot.
This sounds very useful.

-J

On Apr 21, 2010, at 11:19 AM, Jakob Stoklund Olesen wrote:
> Woud it be reasonable to assert that TargetRegisterInfo::isSubRegister() is
a transitive relation?
Yes.

Evan
> 
> In other words, if A is a subregister of B and B is a subregister of C then
A is a subregister of C.
> 
> With this assumption, we could teach TableGen to sort registers
topologically such that all the subregisters of R are numbered R+1, R+2, ...
This produces better locality of reference and allows other optimizations in the
backend code that deals in subregisters a lot.
> 
> /jakob
> 
> 
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