llvm dev - Sep 2010 - [LLVMdev] [LLVMDev] [Question] The TargetRegisterClass has a confusing method.

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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On Aug 31, 2010, at 12:26 PM, Jeff Kunkel 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];
>   }
> 
The "i" in here isn't the register but an index into the RegsBegin
array.

-bw

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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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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