[LLVMdev] Multi-class register allocatable only in one class

[Jakob Stoklund Olesen]

On Sep 13, 2010, at 6:59 AM, Carlos Sánchez de La Lama wrote:

> Hi people,
> 
> the LinearScan register allocator tries to use same register for both  
> live intervals, if the new interval is defined by a register copy  
> whose destination reg is compatible with the source register. This is  
> ok. However, this "check for compatibility" is wrongly done IMHO.
> 
> Say I have regclass1 with reg A, and regclass2 with regs {A, B}, but  
> regclass2 defines only "B" as allocatable by RA.

The register allocator assumes in many places that a register is either allocatable or reserved independently of the register class.

That is, if a register is allocatable in one register class, it is assumed to be allocatable from all register classes where it is present.

What are you trying to model?

/jakob