[PowerPC] Fix minor bug to avoid a failure in createTailCallBranchInstr

[stefanp]

Summary

It looks like dl = MBBI->getDebugLoc(); is correctly guarded by if (MBBI != MBB.end()) but later on in the same function unsigned RetOpcode = MBBI->getOpcode(); is not guarded so it is possible to reach unsigned RetOpcode = MBBI->getOpcode(); in a situation where MBBI is actually MBB.end().

If MBBI == MBB.end() then we don't need to execute the function at all. Added a guard to do that.

[hfinkel]

LGTM

[stefanp]

@hfinkel
Hi Hal,
Thank you for the approval. After doing a bit more investigation I realized that the condition should always be true. Before calling this function we always check that this is a return block. Therefore, I've change it slightly so that the guard is now an assert.

[hfinkel]

In D41358#961092, @stefanp wrote:

@hfinkel
Hi Hal,
Thank you for the approval. After doing a bit more investigation I realized that the condition should always be true. Before calling this function we always check that this is a return block. Therefore, I've change it slightly so that the guard is now an assert.

Sounds good; please proceed. The rest of the code clearly assumes that the iterator is valid.

lib/Target/PowerPC/PPCFrameLowering.cpp

if (IsReturnBlock) {
     } else {
       createTailCallBranchInstr(MBB);
     }
   }
 }
 
 void PPCFrameLowering::createTailCallBranchInstr(MachineBasicBlock &MBB) const {
   MachineBasicBlock::iterator MBBI = MBB.getFirstTerminator();
-  DebugLoc dl;
 
-  if (MBBI != MBB.end())
-    dl = MBBI->getDebugLoc();
+  // If there is no first terminator then we should not continue.
+  if (MBBI == MBB.end())
+    return;
 
+  DebugLoc dl = MBBI->getDebugLoc();
   const PPCInstrInfo &TII = *Subtarget.getInstrInfo();
 
   // Create branch instruction for pseudo tail call return instruction
   unsigned RetOpcode = MBBI->getOpcode();
   if (RetOpcode == PPC::TCRETURNdi) {
     MBBI = MBB.getLastNonDebugInstr();
     MachineOperand &JumpTarget = MBBI->getOperand(0);
     BuildMI(MBB, MBBI, dl, TII.get(PPC::TAILB)).