[LLVMdev] Requiring LiveIntervals

[Tzu-Chien Chiu]

On 20/09/05, Chris Lattner <sabre at nondot.org> wrote:
> > because LiveVariables do not provide an interface to iterate through
> > all viritual registers.
> 
> Ok, you could add a method to LiveVariables that returns
> VirtRegInfo.size().  The virtual registers are defined by the range:
> [MRegisterInfo::FirstVirtualRegister,
>   MRegisterInfo::FirstVirtualRegister+VirtRegInfo.size()]
> 
> Alternatively, the same information can be obtained from the SSARegMap for
> the function: given a MachineFunction, use something like:
> 
> NumVRegs = MF.getSSARegMap()->getLastVirtReg()+1;
> 
> These also start from MRegisterInfo::FirstVirtualRegister.

I'm not sure if these two methods work because after the live
intervals are joined, some virtual registers are coalesced, and their
indices are invalidated. It's no longer that each register in:

  [MRegisterInfo::FirstVirtualRegister,
  MRegisterInfo::FirstVirtualRegister+VirtRegInfo.size()]

is valid.

> It was intended to be used as an internal interface, but if you really
> would like it, I would have no problem promoting it to be a public
> interface.  Let me know what you'd like.

A public interface. the interface of LiveIntervals is more convenient
than LiveVariables when building build the interference graph, which
is necessary for the family of Chaitin style register allocators.

Actually I am surprised that some common graph like the interference
graph and the data dependence graph (a.k.a. data precedence graph or
scheduling graph) is not available in CodeGen, except the control flow
graph. Though they are not difficult to construct, it's tedious that
every target back-end has to re-build them.

Perhaps:
1) All existing targets share the register allocators in CodeGen. There
is no need for ad hoc register allocator, and therefore no
interference graph analysis.
2) No target-independent instruction scheduling code, and therefore no
dependence graph.

Just curious.

-- 
Tzu-Chien Chiu,
3D Graphics Hardware Architect
<URL:http://www.csie.nctu.edu.tw/~jwchiu>