[LLVMdev] Eliminate PHI for non-copyable registers

[Chris Lattner]

On Feb 11, 2009, at 4:07 AM, Alex wrote:

> In my hardware there are two special registers cannot be copied but  
> can only be assigned and referenced (read) in the other instruction.  
> They are allocatable also.
>
> br i1 %if_cond, label %then, label %else
> then:
>   %x1 = fptosi float %y1 to i32
>   br label %endif
> else:
>   %x2 = fptosi float %y2 to i32
>   br label %endif
> endif:
>   %x3 = phi i32 [%x1, %then], [%x2, %else]
>
> PNE::LowerAtomiPHINode() fails because  
> TargetInstrInfo::copyRegToReg() doesn't support the copy of this  
> type of register.
>
> Most registers of this hardware are f32. These two special register  
> of type i32 are provided to relative index the other f32 registers.  
> The value of these i32 registers can only be written by a FP-to-INT  
> conversion instruction. But these two i32 registers are not designed  
> to be copied from one to the other.
>

This is a very interesting problem.  If you have registers like this,  
they should be non-allocatable (just like 'flags') which means that  
you don't have to define copy operations for them.

-Chris