Index: lib/Analysis/DivergenceAnalysis.cpp
===================================================================
--- lib/Analysis/DivergenceAnalysis.cpp
+++ lib/Analysis/DivergenceAnalysis.cpp
@@ -138,6 +138,11 @@
   //   a2 = 2;
   // a = phi(a1, a2); // sync dependent on (tid < 5)
   BasicBlock *ThisBB = TI->getParent();
+
+  // Unreachable blocks may not be in the dominator tree.
+  if (!DT.isReachableFromEntry(ThisBB))
+    return;
+
   BasicBlock *IPostDom = PDT.getNode(ThisBB)->getIDom()->getBlock();
   if (IPostDom == nullptr)
     return;
Index: test/Analysis/DivergenceAnalysis/AMDGPU/unreachable-loop-block.ll
===================================================================
--- /dev/null
+++ test/Analysis/DivergenceAnalysis/AMDGPU/unreachable-loop-block.ll
@@ -0,0 +1,17 @@
+; RUN: opt %s -mtriple amdgcn-- -analyze -divergence | FileCheck %s
+
+; CHECK: DIVERGENT:  %tmp = cmpxchg volatile
+define void @unreachable_loop(i32 %tidx) #0 {
+entry:
+  unreachable
+
+unreachable_loop:                                        ; preds = %do.body.i, %if.then11
+  %tmp = cmpxchg volatile i32 addrspace(1)* null, i32 0, i32 0 seq_cst seq_cst
+  %cmp.i = extractvalue { i32, i1 } %tmp, 1
+  br i1 %cmp.i, label %unreachable_loop, label %end
+
+end:                                      ; preds = %do.body.i51, %atomicAdd_g_f.exit
+  unreachable
+}
+
+attributes #0 = { norecurse nounwind }