[LLVMdev] [LLVMDev] [Question] The TargetRegisterClass has a confusing method.

[Bill Wendling]

The registers themselves are part of a "register class". They may not start at 0 and may not be in order. For instance, target classes for control registers, floating point registers, etc. Look at the "*.inc" files in one of the back-ends for examples of how the TargetRegisterClass objects are created. They are in the build directory under lib/Target/X86/X86Gen*.inc (replace X86 with your favorite architecture).

-bw

On Sep 1, 2010, at 3:46 AM, Jeff Kunkel wrote:

> Aye, I understand it's an index into an array, but what is the difference between the index i and the return value.
> 
> Do I have to worry that register values from a TargetRegisterClass could be either zero based or some other base? Are there other methods that rely on zero based in the TargetRegisterClass? 
> 
> I thought register values were "global," but this transformation makes a register value dependent on it's TargetRegisterClass.
> 
> Basically, I find it confusing to have two integer indexes for the same value. 
> 
> 
> On Tue, Aug 31, 2010 at 3:26 PM, Jeff Kunkel <jdkunk3 at gmail.com> wrote:
> The method below takes in a register and returns a register. To me this makes no sense because
> when I already have a register, why would I need to get it? Could someone please explain the
> transformation that is happening here?
> 
>   /// getRegister - Return the specified register in the class.
>   ///
>   unsigned getRegister(unsigned i) const {
>     assert(i < getNumRegs() && "Register number out of range!");
>     return RegsBegin[i];
>   }
> 
> 
> 
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