Index: lib/Transforms/Scalar/MemCpyOptimizer.cpp
===================================================================
--- lib/Transforms/Scalar/MemCpyOptimizer.cpp
+++ lib/Transforms/Scalar/MemCpyOptimizer.cpp
@@ -379,6 +379,11 @@
                                              Value *StartPtr, Value *ByteVal) {
   if (!DL) return nullptr;
 
+  // FIXME: memcpy / memset for address spaces >= 256 are not supported in the
+  // code generator
+  if (cast<PointerType>(StartPtr->getType())->getAddressSpace() >= 256)
+    return nullptr;
+
   // Okay, so we now have a single store that can be splatable.  Scan to find
   // all subsequent stores of the same value to offset from the same pointer.
   // Join these together into ranges, so we can decide whether contiguous blocks
Index: test/Transforms/MemCpyOpt/addrspace-256.ll
===================================================================
--- /dev/null
+++ test/Transforms/MemCpyOpt/addrspace-256.ll
@@ -0,0 +1,23 @@
+; RUN: opt < %s -memcpyopt -S | FileCheck %s
+
+; Memcpy / memset for address spaces >= 256 are not supported in the code
+; generator.  This test makes sure this pass doesn't create new memsets (memcpys
+; are always formed from existing ones).
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+
+%test = type { i8*, i8*, i8*, i8* }
+
+define void @store_nulls(%test addrspace(256)* %x) {
+  %1 = getelementptr inbounds %test addrspace(256)* %x, i32 0, i32 0
+  store i8* null, i8* addrspace(256)* %1
+  %2 = getelementptr inbounds %test addrspace(256)* %x, i32 0, i32 1
+  store i8* null, i8* addrspace(256)* %2
+  %3 = getelementptr inbounds %test addrspace(256)* %x, i32 0, i32 2
+  store i8* null, i8* addrspace(256)* %3
+  %4 = getelementptr inbounds %test addrspace(256)* %x, i32 0, i32 3
+  store i8* null, i8* addrspace(256)* %4
+; CHECK-LABEL: @store_nulls(
+; CHECK-NOT: memset
+  ret void
+}