diff --git a/clang/test/CodeGen/asan-globals-alias.cpp b/clang/test/CodeGen/asan-globals-alias.cpp
--- a/clang/test/CodeGen/asan-globals-alias.cpp
+++ b/clang/test/CodeGen/asan-globals-alias.cpp
@@ -36,8 +36,8 @@
 // CHECK: @__mod_joydev_ids_device_table ={{.*}} alias
 
 // CHECK-LABEL: define internal void @asan.module_ctor
-// ASAN: call void @__asan_register_globals({{.*}}, i{{32|64}} 4)
-// KASAN: call void @__asan_register_globals({{.*}}, i{{32|64}} 1)
+// ASAN: call void @__asan_register_globals({{.*}}, i{{32|64}} 5)
+// KASAN: call void @__asan_register_globals({{.*}}, i{{32|64}} 2)
 // CHECK-NEXT: ret void
 
 // CHECK-LABEL: define internal void @asan.module_dtor
diff --git a/clang/test/CodeGen/asan-globals-odr.cpp b/clang/test/CodeGen/asan-globals-odr.cpp
--- a/clang/test/CodeGen/asan-globals-odr.cpp
+++ b/clang/test/CodeGen/asan-globals-odr.cpp
@@ -24,7 +24,7 @@
 // GLOB_ALIAS_INDICATOR: @0 = internal global {{.*}} @1 to i64), {{.*}}, i64 ptrtoint (i8* [[ODR]] to i64) }]
 
 // ALIAS0-NOT: private alias
-// ALIAS1: @1 = private alias {{.*}} [[VAR]]
+// ALIAS1: @2 = private alias {{.*}} [[VAR]]
 
-// CHECK: call void @__asan_register_globals(i64 ptrtoint ([1 x { i64, i64, i64, i64, i64, i64, i64, i64 }]* @0 to i64), i64 1)
-// CHECK: call void @__asan_unregister_globals(i64 ptrtoint ([1 x { i64, i64, i64, i64, i64, i64, i64, i64 }]* @0 to i64), i64 1)
+// CHECK: call void @__asan_register_globals(i64 ptrtoint ([2 x { i64, i64, i64, i64, i64, i64, i64, i64 }]* @0 to i64), i64 2)
+// CHECK: call void @__asan_unregister_globals(i64 ptrtoint ([2 x { i64, i64, i64, i64, i64, i64, i64, i64 }]* @0 to i64), i64 2)
diff --git a/clang/test/CodeGen/asan-globals.cpp b/clang/test/CodeGen/asan-globals.cpp
--- a/clang/test/CodeGen/asan-globals.cpp
+++ b/clang/test/CodeGen/asan-globals.cpp
@@ -33,8 +33,8 @@
 // ASAN-NEXT: call void @__asan_version_mismatch_check
 // KASAN-NOT: call void @__asan_init
 // KASAN-NOT: call void @__asan_version_mismatch_check
-// ASAN-NEXT: call void @__asan_register_globals({{.*}}, i{{32|64}} 7)
-// KASAN-NEXT: call void @__asan_register_globals({{.*}}, i{{32|64}} 5)
+// ASAN-NEXT: call void @__asan_register_globals({{.*}}, i{{32|64}} 8)
+// KASAN-NEXT: call void @__asan_register_globals({{.*}}, i{{32|64}} 6)
 // CHECK-NEXT: ret void
 
 // CHECK:      define internal void @asan.module_dtor() #[[#ATTR]] {
diff --git a/clang/test/CodeGen/asan-static-odr.cpp b/clang/test/CodeGen/asan-static-odr.cpp
--- a/clang/test/CodeGen/asan-static-odr.cpp
+++ b/clang/test/CodeGen/asan-static-odr.cpp
@@ -13,5 +13,5 @@
 // CHECK-NOT: private alias
 // CHECK: [[VAR:@.*global.*]] ={{.*}} global { i32, [60 x i8] } zeroinitializer, align 32
 // CHECK: @0 = internal global {{.*}} [[VAR]] to i64), {{.*}}, i64 -1 }]
-// CHECK: call void @__asan_register_globals(i64 ptrtoint ([1 x { i64, i64, i64, i64, i64, i64, i64, i64 }]* @0 to i64), i64 1)
-// CHECK: call void @__asan_unregister_globals(i64 ptrtoint ([1 x { i64, i64, i64, i64, i64, i64, i64, i64 }]* @0 to i64), i64 1)
+// CHECK: call void @__asan_register_globals(i64 ptrtoint ([2 x { i64, i64, i64, i64, i64, i64, i64, i64 }]* @0 to i64), i64 2)
+// CHECK: call void @__asan_unregister_globals(i64 ptrtoint ([2 x { i64, i64, i64, i64, i64, i64, i64, i64 }]* @0 to i64), i64 2)
diff --git a/llvm/lib/Transforms/Instrumentation/AddressSanitizer.cpp b/llvm/lib/Transforms/Instrumentation/AddressSanitizer.cpp
--- a/llvm/lib/Transforms/Instrumentation/AddressSanitizer.cpp
+++ b/llvm/lib/Transforms/Instrumentation/AddressSanitizer.cpp
@@ -2372,6 +2372,19 @@
     return false;
   }
 
+  // Create a global zero-sized array before GlobalsToChange[0] so that the
+  // underflow of it would be observable.
+  ArrayType *ZeroSizedArrayTy = ArrayType::get(IRB.getInt8Ty(), 0);
+  GlobalVariable *UnderflowObserverGV = new GlobalVariable(
+      M, ZeroSizedArrayTy, /*isConstant*/ false, GlobalVariable::PrivateLinkage,
+      ConstantArray::get(ZeroSizedArrayTy, {}),
+      Twine(kAsanGenPrefix) + "underflow_check_placeholder",
+      GlobalsToChange[0]);
+  GlobalsToChange.insert(GlobalsToChange.begin(), UnderflowObserverGV);
+
+  // Update size as we insert a dummy one.
+  n = GlobalsToChange.size();
+
   auto &DL = M.getDataLayout();
 
   // A global is described by a structure
diff --git a/llvm/test/Instrumentation/AddressSanitizer/global_metadata.ll b/llvm/test/Instrumentation/AddressSanitizer/global_metadata.ll
--- a/llvm/test/Instrumentation/AddressSanitizer/global_metadata.ll
+++ b/llvm/test/Instrumentation/AddressSanitizer/global_metadata.ll
@@ -28,8 +28,8 @@
 ; CHECK: [[LOCDESCR:@___asan_gen_.[0-9]+]] = private unnamed_addr constant { [22 x i8]*, i32, i32 } { [22 x i8]* [[FILENAME]], i32 5, i32 5 }
 ; NOALIAS: @__asan_global_global = {{.*}}i64 ptrtoint ({ i32, [60 x i8] }* @global to i64){{.*}} section "asan_globals"{{.*}}, !associated
 ; NOALIAS: @__asan_global_.str = {{.*}}i64 ptrtoint ({ [14 x i8], [50 x i8] }* @{{.str|1}} to i64){{.*}} section "asan_globals"{{.*}}, !associated
-; ALIAS: @__asan_global_global = {{.*}}i64 ptrtoint ({ i32, [60 x i8] }* @0 to i64){{.*}} section "asan_globals"{{.*}}, !associated
-; ALIAS: @__asan_global_.str = {{.*}}i64 ptrtoint ({ [14 x i8], [50 x i8] }* @3 to i64){{.*}} section "asan_globals"{{.*}}, !associated
+; ALIAS: @__asan_global_global = {{.*}}i64 ptrtoint ({ i32, [60 x i8] }* @1 to i64){{.*}} section "asan_globals"{{.*}}, !associated
+; ALIAS: @__asan_global_.str = {{.*}}i64 ptrtoint ({ [14 x i8], [50 x i8] }* @4 to i64){{.*}} section "asan_globals"{{.*}}, !associated
 
 ; The metadata has to be inserted to llvm.compiler.used to avoid being stripped
 ; during LTO.
diff --git a/llvm/test/Instrumentation/AddressSanitizer/global_metadata_darwin.ll b/llvm/test/Instrumentation/AddressSanitizer/global_metadata_darwin.ll
--- a/llvm/test/Instrumentation/AddressSanitizer/global_metadata_darwin.ll
+++ b/llvm/test/Instrumentation/AddressSanitizer/global_metadata_darwin.ll
@@ -20,11 +20,11 @@
 
 ; Find the metadata for @global:
 ; NOALIAS: [[METADATA:@.+]] = internal global {{.*}} @global {{.*}} section "__DATA,__asan_globals,regular"
-; ALIAS: [[METADATA:@.+]] = internal global {{.*}} @0 {{.*}} section "__DATA,__asan_globals,regular"
+; ALIAS: [[METADATA:@.+]] = internal global {{.*}} @1 {{.*}} section "__DATA,__asan_globals,regular"
 
 ; Find the liveness binder for @global and its metadata:
 ; NOALIAS: @__asan_binder_global = internal global {{.*}} @global {{.*}} [[METADATA]] {{.*}} section "__DATA,__asan_liveness,regular,live_support"
-; ALIAS: @__asan_binder_global = internal global {{.*}} @0 {{.*}} [[METADATA]] {{.*}} section "__DATA,__asan_liveness,regular,live_support"
+; ALIAS: @__asan_binder_global = internal global {{.*}} @1 {{.*}} [[METADATA]] {{.*}} section "__DATA,__asan_liveness,regular,live_support"
 
 ; The binder has to be inserted to llvm.compiler.used to avoid being stripped
 ; during LTO.
@@ -33,7 +33,7 @@
 ; Test that there is the flag global variable:
 ; CHECK: @___asan_globals_registered = common hidden global i64 0
 
-; ALIAS: @0 = private alias {{.*}} @global
+; ALIAS: @1 = private alias {{.*}} @global
 
 ; Test that __asan_register_image_globals is invoked from the constructor:
 ; CHECK-LABEL: define internal void @asan.module_ctor
diff --git a/llvm/test/Instrumentation/AddressSanitizer/local_alias.ll b/llvm/test/Instrumentation/AddressSanitizer/local_alias.ll
--- a/llvm/test/Instrumentation/AddressSanitizer/local_alias.ll
+++ b/llvm/test/Instrumentation/AddressSanitizer/local_alias.ll
@@ -24,11 +24,10 @@
 ; CHECK-NOINDICATOR-NOT: __odr_asan_gen_a
 ; CHECK-NOALIAS-NOT: private alias
 ; CHECK-INDICATOR: @__odr_asan_gen_a = global i8 0, align 1
-; CHECK-ALIAS: @0 = private alias { [2 x i32], [56 x i8] }, { [2 x i32], [56 x i8] }* @a
-
-; CHECK-ALIAS: @1 = private alias { [2 x i32], [56 x i8] }, { [2 x i32], [56 x i8] }* @b
-; CHECK-ALIAS: @2 = private alias { [2 x i32], [56 x i8] }, { [2 x i32], [56 x i8] }* @c
-; CHECK-ALIAS: @3 = private alias { [2 x i32], [56 x i8] }, { [2 x i32], [56 x i8] }* @d
+; CHECK-ALIAS: @1 = private alias { [2 x i32], [56 x i8] }, { [2 x i32], [56 x i8] }* @a
+; CHECK-ALIAS: @2 = private alias { [2 x i32], [56 x i8] }, { [2 x i32], [56 x i8] }* @b
+; CHECK-ALIAS: @3 = private alias { [2 x i32], [56 x i8] }, { [2 x i32], [56 x i8] }* @c
+; CHECK-ALIAS: @4 = private alias { [2 x i32], [56 x i8] }, { [2 x i32], [56 x i8] }* @d
 
 ; Function Attrs: nounwind sanitize_address uwtable
 define i32 @foo(i32 %M) #0 {
diff --git a/llvm/test/Instrumentation/AddressSanitizer/odr-check-ignore.ll b/llvm/test/Instrumentation/AddressSanitizer/odr-check-ignore.ll
--- a/llvm/test/Instrumentation/AddressSanitizer/odr-check-ignore.ll
+++ b/llvm/test/Instrumentation/AddressSanitizer/odr-check-ignore.ll
@@ -11,16 +11,16 @@
 @c = internal global [2 x i32] zeroinitializer, align 4
 @d = unnamed_addr global [2 x i32] zeroinitializer, align 4
 
-; NOALIAS:      @__asan_global_a = private global { i64, i64, i64, i64, i64, i64, i64, i64 } { i64 ptrtoint ({ [2 x i32], [56 x i8] }* @a to i64), i64 8, i64 64, i64 ptrtoint ([2 x i8]* @___asan_gen_.1 to i64), i64 ptrtoint ([8 x i8]* @___asan_gen_ to i64), i64 0, i64 0, i64 0 }
-; NOALIAS-NEXT: @__asan_global_b = private global { i64, i64, i64, i64, i64, i64, i64, i64 } { i64 ptrtoint ({ [2 x i32], [56 x i8] }* @b to i64), i64 8, i64 64, i64 ptrtoint ([2 x i8]* @___asan_gen_.2 to i64), i64 ptrtoint ([8 x i8]* @___asan_gen_ to i64), i64 0, i64 0, i64 -1 }
-; NOALIAS-NEXT: @__asan_global_c = private global { i64, i64, i64, i64, i64, i64, i64, i64 } { i64 ptrtoint ({ [2 x i32], [56 x i8] }* @c to i64), i64 8, i64 64, i64 ptrtoint ([2 x i8]* @___asan_gen_.3 to i64), i64 ptrtoint ([8 x i8]* @___asan_gen_ to i64), i64 0, i64 0, i64 -1 }
-; NOALIAS-NEXT: @__asan_global_d = private global { i64, i64, i64, i64, i64, i64, i64, i64 } { i64 ptrtoint ({ [2 x i32], [56 x i8] }* @d to i64), i64 8, i64 64, i64 ptrtoint ([2 x i8]* @___asan_gen_.4 to i64), i64 ptrtoint ([8 x i8]* @___asan_gen_ to i64), i64 0, i64 0, i64 0 }
+; NOALIAS:      @__asan_global_a = private global { i64, i64, i64, i64, i64, i64, i64, i64 } { i64 ptrtoint ({ [2 x i32], [56 x i8] }* @a to i64), i64 8, i64 64, i64 ptrtoint ([2 x i8]* @___asan_gen_.2 to i64), i64 ptrtoint ([8 x i8]* @___asan_gen_ to i64), i64 0, i64 0, i64 0 }
+; NOALIAS-NEXT: @__asan_global_b = private global { i64, i64, i64, i64, i64, i64, i64, i64 } { i64 ptrtoint ({ [2 x i32], [56 x i8] }* @b to i64), i64 8, i64 64, i64 ptrtoint ([2 x i8]* @___asan_gen_.3 to i64), i64 ptrtoint ([8 x i8]* @___asan_gen_ to i64), i64 0, i64 0, i64 -1 }
+; NOALIAS-NEXT: @__asan_global_c = private global { i64, i64, i64, i64, i64, i64, i64, i64 } { i64 ptrtoint ({ [2 x i32], [56 x i8] }* @c to i64), i64 8, i64 64, i64 ptrtoint ([2 x i8]* @___asan_gen_.4 to i64), i64 ptrtoint ([8 x i8]* @___asan_gen_ to i64), i64 0, i64 0, i64 -1 }
+; NOALIAS-NEXT: @__asan_global_d = private global { i64, i64, i64, i64, i64, i64, i64, i64 } { i64 ptrtoint ({ [2 x i32], [56 x i8] }* @d to i64), i64 8, i64 64, i64 ptrtoint ([2 x i8]* @___asan_gen_.5 to i64), i64 ptrtoint ([8 x i8]* @___asan_gen_ to i64), i64 0, i64 0, i64 0 }
 
-; ALIAS:      @__asan_global_a = private global { i64, i64, i64, i64, i64, i64, i64, i64 } { i64 ptrtoint ({ [2 x i32], [56 x i8] }* @0 to i64), i64 8, i64 64, i64 ptrtoint ([2 x i8]* @___asan_gen_.1 to i64), i64 ptrtoint ([8 x i8]* @___asan_gen_ to i64), i64 0, i64 0, i64 0 }
-; ALIAS-NEXT: @__asan_global_b = private global { i64, i64, i64, i64, i64, i64, i64, i64 } { i64 ptrtoint ({ [2 x i32], [56 x i8] }* @1 to i64), i64 8, i64 64, i64 ptrtoint ([2 x i8]* @___asan_gen_.2 to i64), i64 ptrtoint ([8 x i8]* @___asan_gen_ to i64), i64 0, i64 0, i64 -1 }
-; ALIAS-NEXT: @__asan_global_c = private global { i64, i64, i64, i64, i64, i64, i64, i64 } { i64 ptrtoint ({ [2 x i32], [56 x i8] }* @2 to i64), i64 8, i64 64, i64 ptrtoint ([2 x i8]* @___asan_gen_.3 to i64), i64 ptrtoint ([8 x i8]* @___asan_gen_ to i64), i64 0, i64 0, i64 -1 }
-; ALIAS-NEXT: @__asan_global_d = private global { i64, i64, i64, i64, i64, i64, i64, i64 } { i64 ptrtoint ({ [2 x i32], [56 x i8] }* @3 to i64), i64 8, i64 64, i64 ptrtoint ([2 x i8]* @___asan_gen_.4 to i64), i64 ptrtoint ([8 x i8]* @___asan_gen_ to i64), i64 0, i64 0, i64 0 }
-; ALIAS:      @0 = private alias {{.*}} @a
-; ALIAS-NEXT: @1 = private alias {{.*}} @b
-; ALIAS-NEXT: @2 = private alias {{.*}} @c
-; ALIAS-NEXT: @3 = private alias {{.*}} @d
+; ALIAS:      @__asan_global_a = private global { i64, i64, i64, i64, i64, i64, i64, i64 } { i64 ptrtoint ({ [2 x i32], [56 x i8] }* @1 to i64), i64 8, i64 64, i64 ptrtoint ([2 x i8]* @___asan_gen_.2 to i64), i64 ptrtoint ([8 x i8]* @___asan_gen_ to i64), i64 0, i64 0, i64 0 }
+; ALIAS-NEXT: @__asan_global_b = private global { i64, i64, i64, i64, i64, i64, i64, i64 } { i64 ptrtoint ({ [2 x i32], [56 x i8] }* @2 to i64), i64 8, i64 64, i64 ptrtoint ([2 x i8]* @___asan_gen_.3 to i64), i64 ptrtoint ([8 x i8]* @___asan_gen_ to i64), i64 0, i64 0, i64 -1 }
+; ALIAS-NEXT: @__asan_global_c = private global { i64, i64, i64, i64, i64, i64, i64, i64 } { i64 ptrtoint ({ [2 x i32], [56 x i8] }* @3 to i64), i64 8, i64 64, i64 ptrtoint ([2 x i8]* @___asan_gen_.4 to i64), i64 ptrtoint ([8 x i8]* @___asan_gen_ to i64), i64 0, i64 0, i64 -1 }
+; ALIAS-NEXT: @__asan_global_d = private global { i64, i64, i64, i64, i64, i64, i64, i64 } { i64 ptrtoint ({ [2 x i32], [56 x i8] }* @4 to i64), i64 8, i64 64, i64 ptrtoint ([2 x i8]* @___asan_gen_.5 to i64), i64 ptrtoint ([8 x i8]* @___asan_gen_ to i64), i64 0, i64 0, i64 0 }
+; ALIAS:      @1 = private alias {{.*}} @a
+; ALIAS-NEXT: @2 = private alias {{.*}} @b
+; ALIAS-NEXT: @3 = private alias {{.*}} @c
+; ALIAS-NEXT: @4 = private alias {{.*}} @d
diff --git a/llvm/test/Instrumentation/AddressSanitizer/win-string-literal.ll b/llvm/test/Instrumentation/AddressSanitizer/win-string-literal.ll
--- a/llvm/test/Instrumentation/AddressSanitizer/win-string-literal.ll
+++ b/llvm/test/Instrumentation/AddressSanitizer/win-string-literal.ll
@@ -14,9 +14,9 @@
 ; CHECK: @"__asan_global_??_C@_04JIHMPGLA@asdf?$AA@" =
 ; CHECK-SAME: private global { i64, i64, i64, i64, i64, i64, i64, i64 }
 ; CHECK-SAME: { i64 ptrtoint ({ [5 x i8], [59 x i8] }* @"??_C@_04JIHMPGLA@asdf?$AA@" to i64),
-; CHECK-SAME:   i64 5, i64 64, i64 ptrtoint ([17 x i8]* @___asan_gen_.1 to i64),
+; CHECK-SAME:   i64 5, i64 64, i64 ptrtoint ([17 x i8]* @___asan_gen_.2 to i64),
 ; CHECK-SAME:   i64 ptrtoint ([8 x i8]* @___asan_gen_ to i64), i64 0,
-; CHECK-SAME:   i64 ptrtoint ({ [6 x i8]*, i32, i32 }* @___asan_gen_.3 to i64), i64 0 },
+; CHECK-SAME:   i64 ptrtoint ({ [6 x i8]*, i32, i32 }* @___asan_gen_.4 to i64), i64 0 },
 ; CHECK-SAME:   section ".ASAN$GL", comdat($"??_C@_04JIHMPGLA@asdf?$AA@"), align 64
 
 ; ModuleID = 't.cpp'