Index: clang/include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -2991,6 +2991,9 @@
   "parameter may not be qualified with an address space">;
 def err_field_with_address_space : Error<
   "field may not be qualified with an address space">;
+def err_qualifier_with_address_space : Error<
+  "qualifier '%select{const|volatile}0' %select{needed|invalid}1 "
+  "for variables in address space '%2'">;
 def err_compound_literal_with_address_space : Error<
   "compound literal in function scope may not be qualified with an address space">;
 def err_address_space_mismatch_templ_inst : Error<
Index: clang/lib/Basic/Targets/AVR.cpp
===================================================================
--- clang/lib/Basic/Targets/AVR.cpp
+++ clang/lib/Basic/Targets/AVR.cpp
@@ -308,6 +308,7 @@
   Builder.defineMacro("__AVR");
   Builder.defineMacro("__AVR__");
   Builder.defineMacro("__ELF__");
+  Builder.defineMacro("__flash", "__attribute__((address_space(1)))");
 
   if (!this->CPU.empty()) {
     auto It = llvm::find_if(
Index: clang/lib/CodeGen/TargetInfo.cpp
===================================================================
--- clang/lib/CodeGen/TargetInfo.cpp
+++ clang/lib/CodeGen/TargetInfo.cpp
@@ -21,6 +21,7 @@
 #include "clang/AST/RecordLayout.h"
 #include "clang/Basic/CodeGenOptions.h"
 #include "clang/Basic/DiagnosticFrontend.h"
+#include "clang/Basic/DiagnosticSema.h"
 #include "clang/CodeGen/CGFunctionInfo.h"
 #include "clang/CodeGen/SwiftCallingConv.h"
 #include "llvm/ADT/SmallBitVector.h"
@@ -8032,6 +8033,17 @@
   AVRTargetCodeGenInfo(CodeGenTypes &CGT)
       : TargetCodeGenInfo(std::make_unique<DefaultABIInfo>(CGT)) {}
 
+  LangAS getGlobalVarAddressSpace(CodeGenModule &CGM, const VarDecl *D) const {
+    // Check if a global/static variable is defined within address space 1
+    // but not constant.
+    if (toTargetAddressSpace(D->getType().getAddressSpace()) == 1 &&
+        !D->getType().isConstQualified())
+      CGM.getDiags().Report(D->getLocation(),
+                            diag::err_qualifier_with_address_space)
+          << 0 << 0 << "__flash";
+    return TargetCodeGenInfo::getGlobalVarAddressSpace(CGM, D);
+  }
+
   void setTargetAttributes(const Decl *D, llvm::GlobalValue *GV,
                            CodeGen::CodeGenModule &CGM) const override {
     if (GV->isDeclaration())
Index: clang/test/CodeGen/address-space-avr.c
===================================================================
--- clang/test/CodeGen/address-space-avr.c
+++ clang/test/CodeGen/address-space-avr.c
@@ -3,10 +3,16 @@
 // Test that function declarations in nonzero address spaces without prototype
 // are called correctly.
 
+// CHECK: @var0 {{.*}} addrspace(1) constant i16 333
+// CHECK: @bar.var1 {{.*}} addrspace(1) constant i16 555
 // CHECK: define{{.*}} void @bar() addrspace(1)
 // CHECK: call addrspace(1) void bitcast (void (...) addrspace(1)* @foo to void (i16) addrspace(1)*)(i16 3)
 // CHECK: declare void @foo(...) addrspace(1)
+
+__flash const int var0 = 333;
+
 void foo();
 void bar(void) {
+	static __flash const int var1 = 555;
 	foo(3);
 }
Index: clang/test/CodeGen/avr-flash.c
===================================================================
--- /dev/null
+++ clang/test/CodeGen/avr-flash.c
@@ -0,0 +1,8 @@
+// RUN: %clang_cc1 -triple avr -emit-llvm-only -verify %s
+
+static __flash int arr[] = {4, 6, 1, 2, 5}; // expected-error {{qualifier 'const' needed for variables in address space '__flash'}}
+
+int foo(int i, int j) {
+  static __flash int b[] = {4, 6, 1, 2, 5}; // expected-error {{qualifier 'const' needed for variables in address space '__flash'}}
+  return arr[i] + b[j];
+}