Guard against dereferencing a nullptr

[akshaykhadse]

Summary

In lib/CodeGen/PrologEpilogInserter.cpp file, RS is assigned via RS = TRI->requiresRegisterScavenging(MF) ? new RegScavenger() : nullptr;. This means that RS can be nullptr. While executing the TFI->processFunctionBeforeFrameFinalized(MF, RS);, the RS can be dereferenced in the call RS->enterBasicBlock(MBB); in file lib/Target/AMDGPU/SIFrameLowering.cpp

[skan]

llvm/lib/Target/AMDGPU/SIFrameLowering.cpp

1353

Error message?

[LuoYuanke]

@akshaykhadse, do you want me to help to land the patch?

[akshaykhadse]

In D146791#4270557, @LuoYuanke wrote:

@akshaykhadse, do you want me to help to land the patch?

Yes. That would be great!

llvm/lib/Target/AMDGPU/SIFrameLowering.cpp

for (MachineBasicBlock &MBB : MF) {
           // finalization.
           unsigned FIOp = AMDGPU::getNamedOperandIdx(MI.getOpcode(),
                                                      AMDGPU::OpName::vaddr);
           int FI = MI.getOperand(FIOp).getIndex();
           Register VReg =
             TII->getNamedOperand(MI, AMDGPU::OpName::vdata)->getReg();
           if (FuncInfo->allocateVGPRSpillToAGPR(MF, FI,
                                                 TRI->isAGPR(MRI, VReg))) {
+            assert(RS != nullptr);
             // FIXME: change to enterBasicBlockEnd()
             RS->enterBasicBlock(MBB);
             TRI->eliminateFrameIndex(MI, 0, FIOp, RS);
             SpillFIs.set(FI);
             continue;
           }
         } else if (TII->isStoreToStackSlot(MI, FrameIndex) ||
                    TII->isLoadFromStackSlot(MI, FrameIndex))