Eliminating copies between overlapping register classes

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#1

Hi,

I have two register classes A and B, where A contains a subset of the
registers in B:

A = [R0, R1, R2, ... R128]

B = [RO, R1, R2, ... R128,
     T0, T1, T2, ... T128]

I am using the Greedy Register Allocator, and I would expect the register
allocator to eliminate this copy:

%vreg0<def> = COPY %vreg1; B:%vreg0 A:%vreg1

but instead I end up with

%R0<def> = COPY %R1

Is there any way I can get the register allocator to eliminate these
kinds of copies?

Thanks,
Tom

#2

Such copies should be eliminated as aggressively as normal copies. Perhaps the debug output from RegisterCoalescer can tell you why they aren't.

/jakob

#3

Thanks for the debugging type, I caught my mistake. Class A wasn't
actually a subset of B. The definitions were actually like this:

A = [R0, R1, R2, ... R128, CC]

B = [RO, R1, R2, ... R128,
     T0, T1, T2, ... T128

Thanks,
Tom

#4

If you upgrade to trunk, TableGen will automatically synthesize a register class representing the intersection.

/jakob