diff --git a/llvm/lib/CodeGen/MachineBasicBlock.cpp b/llvm/lib/CodeGen/MachineBasicBlock.cpp
--- a/llvm/lib/CodeGen/MachineBasicBlock.cpp
+++ b/llvm/lib/CodeGen/MachineBasicBlock.cpp
@@ -1474,6 +1474,8 @@
 }
 
 DebugLoc MachineBasicBlock::rfindDebugLoc(reverse_instr_iterator MBBI) {
+  if (MBBI == instr_rend())
+    return findDebugLoc(instr_begin());
   // Skip debug declarations, we don't want a DebugLoc from them.
   MBBI = skipDebugInstructionsBackward(MBBI, instr_rbegin());
   if (!MBBI->isDebugInstr())
diff --git a/llvm/unittests/CodeGen/MachineBasicBlockTest.cpp b/llvm/unittests/CodeGen/MachineBasicBlockTest.cpp
--- a/llvm/unittests/CodeGen/MachineBasicBlockTest.cpp
+++ b/llvm/unittests/CodeGen/MachineBasicBlockTest.cpp
@@ -56,9 +56,8 @@
   EXPECT_EQ(DL0, MBB.findDebugLoc(MBB.instr_begin()));
   EXPECT_EQ(DL0, MBB.findDebugLoc(MBB.instr_end()));
 
-  // FIXME: This would crash (see https://reviews.llvm.org/D150577).
-  //EXPECT_EQ(DL0, MBB.rfindDebugLoc(MBB.instr_rbegin()));
-  //EXPECT_EQ(DL0, MBB.rfindDebugLoc(MBB.instr_rend()));
+  EXPECT_EQ(DL0, MBB.rfindDebugLoc(MBB.instr_rbegin()));
+  EXPECT_EQ(DL0, MBB.rfindDebugLoc(MBB.instr_rend()));
 
   EXPECT_EQ(DL0, MBB.findPrevDebugLoc(MBB.instr_begin()));
   EXPECT_EQ(DL0, MBB.findPrevDebugLoc(MBB.instr_end()));
@@ -84,8 +83,7 @@
   EXPECT_EQ(DL3, MBB.findDebugLoc(MI3));
   EXPECT_EQ(DL0, MBB.findDebugLoc(MBB.instr_end()));
 
-  // FIXME: This would crash (see https://reviews.llvm.org/D150577).
-  //EXPECT_EQ(DL1, MBB.rfindDebugLoc(MBB.instr_rend()));
+  EXPECT_EQ(DL1, MBB.rfindDebugLoc(MBB.instr_rend()));
   EXPECT_EQ(DL1, MBB.rfindDebugLoc(MI1));
   EXPECT_EQ(DL3, MBB.rfindDebugLoc(MI2));
   EXPECT_EQ(DL3, MBB.rfindDebugLoc(MI3));