Index: lib/Target/AMDGPU/AMDGPUTargetMachine.cpp
===================================================================
--- lib/Target/AMDGPU/AMDGPUTargetMachine.cpp
+++ lib/Target/AMDGPU/AMDGPUTargetMachine.cpp
@@ -45,6 +45,7 @@
 #include "llvm/Transforms/IPO/PassManagerBuilder.h"
 #include "llvm/Transforms/Scalar.h"
 #include "llvm/Transforms/Scalar/GVN.h"
+#include "llvm/Transforms/Utils.h"
 #include "llvm/Transforms/Vectorize.h"
 #include <memory>
 
@@ -678,6 +679,7 @@
 }
 
 bool AMDGPUPassConfig::addPreISel() {
+  addPass(&LowerSwitchID);
   addPass(createFlattenCFGPass());
   return false;
 }
Index: lib/Transforms/Scalar/StructurizeCFG.cpp
===================================================================
--- lib/Transforms/Scalar/StructurizeCFG.cpp
+++ lib/Transforms/Scalar/StructurizeCFG.cpp
@@ -275,6 +275,7 @@
     AU.addRequired<LoopInfoWrapperPass>();
 
     AU.addPreserved<DominatorTreeWrapperPass>();
+    AU.addPreservedID(LowerSwitchID);
     RegionPass::getAnalysisUsage(AU);
   }
 };
Index: test/CodeGen/AMDGPU/diverge-switch-default.ll
===================================================================
--- /dev/null
+++ test/CodeGen/AMDGPU/diverge-switch-default.ll
@@ -0,0 +1,66 @@
+; RUN: llc -march=amdgcn -mcpu=gfx900 -print-after=si-annotate-control-flow %s -o /dev/null 2>&1 | FileCheck %s
+
+target triple = "amdgcn--"
+
+; CHECK-LABEL: @switch_unreachable_default
+
+define amdgpu_kernel void @switch_unreachable_default(i32 addrspace(1)* %out, i8 addrspace(1)* %in0, i8 addrspace(1)* %in1) #0 {
+centry:
+  %tid = call i32 @llvm.amdgcn.workitem.id.x()
+  switch i32 %tid, label %sw.default [
+    i32 0, label %sw.bb0
+    i32 1, label %sw.bb1
+  ]
+
+sw.bb0:
+  br label %sw.epilog
+
+sw.bb1:
+  br label %sw.epilog
+
+sw.default:
+  unreachable
+
+sw.epilog:
+  %ptr = phi i8 addrspace(1)* [%in0, %sw.bb0], [%in1, %sw.bb1]
+  %gep_in = getelementptr inbounds i8, i8 addrspace(1)* %ptr, i64 0
+  br label %sw.while
+
+; The loop below is necessary to preserve the effect of the
+; unreachable default on divergence analysis in the presence of other
+; optimizations. The loop consists of a single block where the loop
+; exit is divergent because it depends on the divergent phi at the
+; start of the block. The checks below ensure that the loop exit is
+; handled correctly as divergent. But the data-flow within the block
+; is sensitive to optimizations; so we just ensure that the relevant
+; operations in the block body are indeed in the same block.
+
+; CHECK: [[PHI:%[a-zA-Z0-9._]+]]  = phi i64
+; CHECK-NOT: {{ br }}
+; CHECK: load i8
+; CHECK-NOT: {{ br }}
+; CHECK: [[ICMP:%[a-zA-Z0-9._]+]] = icmp eq
+; CHECK: [[IF:%[a-zA-Z0-9._]+]]   = call i64 @llvm.amdgcn.if.break(i1 [[ICMP]], i64 [[PHI]])
+; CHECK: [[LOOP:%[a-zA-Z0-9._]+]] = call i1 @llvm.amdgcn.loop(i64 [[IF]])
+; CHECK: br i1 [[LOOP]]
+
+sw.while:
+  %p = phi i8 addrspace(1)* [ %gep_in, %sw.epilog ], [ %incdec.ptr, %sw.while ]
+  %count = phi i32 [ 0, %sw.epilog ], [ %count.inc, %sw.while ]
+  %char = load i8, i8 addrspace(1)* %p, align 1
+  %tobool = icmp eq i8 %char, 0
+  %incdec.ptr = getelementptr inbounds i8, i8 addrspace(1)* %p, i64 1
+  %count.inc = add i32 %count, 1
+  br i1 %tobool, label %sw.exit, label %sw.while
+
+sw.exit:
+  %tid64 = zext i32 %tid to i64
+  %gep_out = getelementptr inbounds i32, i32 addrspace(1)* %out, i64 %tid64
+  store i32 %count, i32 addrspace(1)* %gep_out, align 4
+  ret void
+}
+
+declare i32 @llvm.amdgcn.workitem.id.x() #0
+
+attributes #0 = { nounwind readnone }
+attributes #1 = { convergent noinline optnone }