ARM: Avoid dereferencing end() in ARMFrameLowering::emitEpilogue

This fixes the crash from PR29072, where the MachineBasicBlock::iterator
wasn't being properly checked against MachineBasicBlock::end() before
iterating.  This was another bug exposed by the new
ilist::iterator::operator*() assertion from r279314.

This testcase is poor quality.  bugpoint couldn't reduce any further,
and I haven't had time to dig into what's going on so I can't invent a
better one.  I didn't even get good CHECK lines in: this is just a
crasher.

I'm committing anyway since this is a real crash with an obvious fix,
but I'll leave PR29072 open and ask an ARM maintainer to help improve
the testcase.

llvm-svn: 279391

1 files changed, 2 insertions, 2 deletions

diff --git a/llvm/lib/Target/ARM/ARMFrameLowering.cpp b/llvm/lib/Target/ARM/ARMFrameLowering.cpp
index cfc0faaa..4c8646c 100644
--- a/llvm/lib/Target/ARM/ARMFrameLowering.cpp
+++ b/llvm/lib/Target/ARM/ARMFrameLowering.cpp
@@ -776,11 +776,11 @@ void ARMFrameLowering::emitEpilogue(MachineFunction &MF,
       emitSPUpdate(isARM, MBB, MBBI, dl, TII, NumBytes);
 
     // Increment past our save areas.
-    if (AFI->getDPRCalleeSavedAreaSize()) {
+    if (MBBI != MBB.end() && AFI->getDPRCalleeSavedAreaSize()) {
       MBBI++;
       // Since vpop register list cannot have gaps, there may be multiple vpop
       // instructions in the epilogue.
-      while (MBBI->getOpcode() == ARM::VLDMDIA_UPD)
+      while (MBBI != MBB.end() && MBBI->getOpcode() == ARM::VLDMDIA_UPD)
         MBBI++;
     }
     if (AFI->getDPRCalleeSavedGapSize()) {