[Sparc] Mark the g0 register as constant

[arichardson]

Summary

This adds an implementation of isConstantPhysReg().

[arsenm]

Test would be nice

llvm/lib/Target/Sparc/SparcRegisterInfo.td

130

I think if you only want this to be G0, it's clearer to do:

def G0 ... {
  let isConstant = true
}

[arichardson]

I have zero interest and knowledge of sparc, so I'm not sure how to write a test.

I only opened this review because it was pointed out in the review that added isConstant to tablegen. Happy to let someone else pick this up.

llvm/lib/Target/Sparc/SparcRegisterInfo.td

 def CWP : SparcCtrlReg<9, "CWP">;
 def CANSAVE : SparcCtrlReg<10, "CANSAVE">;
 def CANRESTORE : SparcCtrlReg<11, "CANRESTORE">;
 def CLEANWIN : SparcCtrlReg<12, "CLEANWIN">;
 def OTHERWIN : SparcCtrlReg<13, "OTHERWIN">;
 def WSTATE : SparcCtrlReg<14, "WSTATE">;
 
 // Integer registers
+let isConstant = true in
 def G0 : Ri< 0, "G0">, DwarfRegNum<[0]>;
 def G1 : Ri< 1, "G1">, DwarfRegNum<[1]>;
 def G2 : Ri< 2, "G2">, DwarfRegNum<[2]>;
 def G3 : Ri< 3, "G3">, DwarfRegNum<[3]>;
 def G4 : Ri< 4, "G4">, DwarfRegNum<[4]>;
 def G5 : Ri< 5, "G5">, DwarfRegNum<[5]>;
 def G6 : Ri< 6, "G6">, DwarfRegNum<[6]>;
 def G7 : Ri< 7, "G7">, DwarfRegNum<[7]>;
 def O0 : Ri< 8, "O0">, DwarfRegNum<[8]>;