diff --git a/llvm/lib/Transforms/Utils/FixIrreducible.cpp b/llvm/lib/Transforms/Utils/FixIrreducible.cpp
--- a/llvm/lib/Transforms/Utils/FixIrreducible.cpp
+++ b/llvm/lib/Transforms/Utils/FixIrreducible.cpp
@@ -281,6 +281,9 @@
     LLVM_DEBUG(dbgs() << "Found headers:");
     for (auto BB : reverse(Blocks)) {
       for (const auto P : predecessors(BB)) {
+        // Skip unreachable predecessors.
+        if (!DT.isReachableFromEntry(P))
+          continue;
         if (!Blocks.count(P)) {
           LLVM_DEBUG(dbgs() << " " << BB->getName());
           Headers.insert(BB);
diff --git a/llvm/test/Transforms/FixIrreducible/unreachable.ll b/llvm/test/Transforms/FixIrreducible/unreachable.ll
new file mode 100644
--- /dev/null
+++ b/llvm/test/Transforms/FixIrreducible/unreachable.ll
@@ -0,0 +1,24 @@
+; RUN: opt %s -fix-irreducible -S -o - | FileCheck %s
+
+; CHECK-LABEL: @unreachable(
+; CHECK: entry:
+; CHECK-NOT: irr.guard:
+define void @unreachable(i32 %n) {
+entry:
+  br label %loop.body
+
+loop.body:
+  br label %inner.block
+
+unreachable.block:
+  br label %inner.block
+
+inner.block:
+  br i1 undef, label %loop.exit, label %loop.latch
+
+loop.latch:
+  br label %loop.body
+
+loop.exit:
+  ret void
+}