[GISel]: Fix more Undefined behavior in GlobalISel::IRTranslator

[aditya_nandakumar]

Summary

Turns out the MachineIRBuilder holds on references to TrackingMDRef (and IRTranslator has MachineIRBuilder objects). When the context gets destroyed, first all the MDRefs are deallocated and then the passes are destroyed. When the IRTranslator is about to get destroyed, it's destructor would end up trying to destroy the trackingMDRefs which the builders held on to leading to Undefined behavior.
Fix this by explicitly default constructing the MachineIRBuidlers at the end of runOnMachineFunction.

[dsanders]

Is it possible to add a test case for this? It seems that a test where the last instruction has a DILocation should trigger it.

lib/CodeGen/GlobalISel/IRTranslator.cpp

1132–1140

Based on our discussion off-list it was my understanding that ~IRTranslator() wasn't the only trigger for this. Wasn't the call to MachineIRBuilder::setMF() in IRTranslator::runOnMachineFunction() also triggering it?

If so, I'd suggest adapting this comment to talk about the MachineIRBuilder/DebugLoc outliving the DILocation it holds rather than going into detail about the LLVMContext case. That way it will cover both.

[aditya_nandakumar]

Updated comment based on Daniel's feedback.

[dsanders]

LGTM

lib/CodeGen/GlobalISel/IRTranslator.cpp

1132–1134

One tiny nit: Doxygen comments don't do anything here as far as I know so these should use //

[aditya_nandakumar]

Submitted in r303277

lib/CodeGen/GlobalISel/IRTranslator.cpp

 
 void IRTranslator::finalizeFunction() {
   // Release the memory used by the different maps we
   // needed during the translation.
   PendingPHIs.clear();
   ValToVReg.clear();
   FrameIndices.clear();
   MachinePreds.clear();
+  /// MachineIRBuilder::DebugLoc can outlive the DILocation it holds. Clear it
+  /// to avoid accessing free’d memory (in runOnMachineFunction) and to avoid
+  /// destroying it twice (in ~IRTranslator() and ~LLVMContext())
+  EntryBuilder = MachineIRBuilder();
+  CurBuilder = MachineIRBuilder();
 }
 
 bool IRTranslator::runOnMachineFunction(MachineFunction &CurMF) {
   MF = &CurMF;
   const Function &F = *MF->getFunction();
   if (F.empty())
     return false;
   CLI = MF->getSubtarget().getCallLowering();
   CurBuilder.setMF(*MF);
   EntryBuilder.setMF(*MF);
   MRI = &MF->getRegInfo();
   DL = &F.getParent()->getDataLayout();