Index: lib/Target/Mips/MipsInstrInfo.cpp
===================================================================
--- lib/Target/Mips/MipsInstrInfo.cpp
+++ lib/Target/Mips/MipsInstrInfo.cpp
@@ -157,24 +157,23 @@
   assert(!BytesRemoved && "code size not handled");
 
   MachineBasicBlock::reverse_iterator I = MBB.rbegin(), REnd = MBB.rend();
-  unsigned removed;
-
-  // Skip all the debug instructions.
-  while (I != REnd && I->isDebugValue())
-    ++I;
-
-  if (I == REnd)
-    return 0;
-
-  MachineBasicBlock::iterator FirstBr = ++I.getReverse();
+  unsigned removed = 0;
 
   // Up to 2 branches are removed.
   // Note that indirect branches are not removed.
-  for (removed = 0; I != REnd && removed < 2; ++I, ++removed)
+  while (I != REnd && removed < 2) {
+    // Skip past debug instructions.
+    if (I->isDebugValue()) {
+      ++I;
+      continue;
+    }
     if (!getAnalyzableBrOpc(I->getOpcode()))
       break;
-
-  MBB.erase((--I).getReverse(), FirstBr);
+    // Remove the branch.
+    I->eraseFromParent();
+    I = MBB.rbegin();
+    ++removed;
+  }
 
   return removed;
 }
Index: test/CodeGen/Mips/pr35071.ll
===================================================================
--- /dev/null
+++ test/CodeGen/Mips/pr35071.ll
@@ -0,0 +1,73 @@
+; RUN: llc -mtriple mips64-unknown-freebsd12.0 -relocation-model pic -mcpu=mips4 -target-abi n64 -O2 -o - %s
+
+; Test that the long branch pass does not crash due to the control flow
+; optimizer producing malformed basic block operands due to the backend
+; failing to handle debug information around branch instructions.
+
+define void @f() !dbg !5 {
+entry:
+  %cmp = icmp eq i32 undef, 0, !dbg !16
+  %conv = zext i1 %cmp to i32, !dbg !16
+  tail call void @llvm.dbg.value(metadata i32 %conv, metadata !11, metadata !DIExpression()), !dbg !17
+  %tobool = icmp eq i32 undef, 0, !dbg !18
+  br i1 %tobool, label %if.end, label %cleanup7.critedge, !dbg !21
+
+if.end:                                           ; preds = %entry
+  %call6 = call i32 bitcast (i32 (...)* @j to i32 (i32)*)(i32 signext %conv)
+#4, !dbg !22
+  br label %cleanup7, !dbg !23
+
+cleanup7.critedge:                                ; preds = %entry
+  call void @llvm.lifetime.end.p0i8(i64 4, i8* nonnull undef) #4, !dbg !24
+  br label %cleanup7
+
+cleanup7:                                         ; preds = %cleanup7.critedge,
+  ret void
+}
+
+declare void @llvm.lifetime.end.p0i8(i64, i8* nocapture) #1
+
+declare i32 @j(...)
+
+declare void @llvm.dbg.value(metadata, metadata, metadata) #3
+attributes #1 = { argmemonly nounwind }
+attributes #3 = { nounwind readnone speculatable }
+attributes #4 = { nounwind }
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!3, !4}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "clang
+version 6.0.0", isOptimized: true, runtimeVersion:
+0, emissionKind: FullDebug, enums: !2)
+!1 = !DIFile(filename:
+"/tmp//<stdin>", directory:
+"/tmp/")
+!2 = !{}
+!3 = !{i32 2, !"Debug Info Version", i32 3}
+!4 = !{i32 7, !"PIC Level", i32 2}
+!5 = distinct !DISubprogram(name: "f", scope: !6, file: !6, line: 8, type: !7,
+isLocal: false, isDefinition: true, scopeLine: 8, isOptimized: true, unit: !0,
+variables: !10)
+!6 = !DIFile(filename:
+"/tmp/test.c",
+directory: "/tmp")
+!7 = !DISubroutineType(types: !8)
+!8 = !{!9}
+!9 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!10 = !{!11, !12, !14}
+!11 = !DILocalVariable(name: "e", scope: !5, file: !6, line: 9, type: !9)
+!12 = !DILocalVariable(name: "g", scope: !13, file: !6, line: 11, type: !9)
+!13 = distinct !DILexicalBlock(scope: !5, file: !6, line: 10, column: 3)
+!14 = !DILocalVariable(name: "d", scope: !13, file: !6, line: 12, type: !15)
+!15 = !DIDerivedType(tag: DW_TAG_typedef, name: "a", file: !6, line: 2,
+baseType: !9)
+!16 = !DILocation(line: 9, column: 15, scope: !5)
+!17 = !DILocation(line: 9, column: 7, scope: !5)
+!18 = !DILocation(line: 12, column: 5, scope: !19)
+!19 = distinct !DILexicalBlock(scope: !20, file: !6, line: 12, column: 5)
+!20 = distinct !DILexicalBlock(scope: !5, file: !6, line: 10, column: 3)
+!21 = !DILocation(line: 12, column: 5, scope: !20)
+!22 = !DILocation(line: 16, column: 3, scope: !5)
+!23 = !DILocation(line: 17, column: 1, scope: !5)
+!24 = !DILocation(line: 15, column: 3, scope: !5)