[RISCV] Simplify BP initialisation

[StephenFan]

Summary

We can re-use copyPhysReg rather than writing a specialised copy.

[craig.topper]

llvm/lib/Target/RISCV/RISCVFrameLowering.cpp

484

While they aren't necessary, leave the curly braces since the comment makes more than one line below the if.

[StephenFan]

Address craig.topper's comment.

[luismarques]

This is certainly cleaner, in the sense of the code operating at a higher level of abstraction. I wonder if it's worth the indirection cost.
I'll let others bikeshed this :-)

[jrtc27]

In D95227#2521189, @luismarques wrote:

This is certainly cleaner, in the sense of the code operating at a higher level of abstraction. I wonder if it's worth the indirection cost.
I'll let others bikeshed this :-)

Fewer places we have to teach to use a cmove cbp, csp dowstream in CHERI-LLVM :) (although we've already paid that cost, but still, diff reducing is good)

[asb]

LGTM.

llvm/lib/Target/RISCV/RISCVFrameLowering.cpp

if (RI->needsStackRealignment(MF)) {
             .addImm(ShiftAmount);
         BuildMI(MBB, MBBI, DL, TII->get(RISCV::SLLI), SPReg)
             .addReg(VR)
             .addImm(ShiftAmount);
       }
       // FP will be used to restore the frame in the epilogue, so we need
       // another base register BP to record SP after re-alignment. SP will
       // track the current stack after allocating variable sized objects.
-      if (hasBP(MF)) {
+      if (hasBP(MF))
         // move BP, SP
-        BuildMI(MBB, MBBI, DL, TII->get(RISCV::ADDI), BPReg)
-            .addReg(SPReg)
-            .addImm(0);
-      }
+        TII->copyPhysReg(MBB, MBBI, DL, BPReg, SPReg, false);
     }
   }
 }
 
 void RISCVFrameLowering::emitEpilogue(MachineFunction &MF,
                                       MachineBasicBlock &MBB) const {
   const RISCVRegisterInfo *RI = STI.getRegisterInfo();
   MachineFrameInfo &MFI = MF.getFrameInfo();