Index: lib/Transforms/Instrumentation/AddressSanitizer.cpp
===================================================================
--- lib/Transforms/Instrumentation/AddressSanitizer.cpp
+++ lib/Transforms/Instrumentation/AddressSanitizer.cpp
@@ -2191,7 +2191,16 @@
   return Version;
 }
 
+static bool isAlreadyAsanInstrumented(const Module &M) {
+  // Looks for a function inserted by the second (module) pass of
+  // ASAN, to detect modules fully ASAN instrumented.
+  return M.getFunction(kAsanPoisonGlobalsName) != nullptr;
+}
+
 bool AddressSanitizerModule::runOnModule(Module &M) {
+  // Skip modules which already have ASan instrumentation.
+  if (isAlreadyAsanInstrumented(M))
+    return false;
   C = &(M.getContext());
   int LongSize = M.getDataLayout().getPointerSizeInBits();
   IntptrTy = Type::getIntNTy(*C, LongSize);
@@ -2314,6 +2323,10 @@
 
 // virtual
 bool AddressSanitizer::doInitialization(Module &M) {
+  // Skip modules which already have ASan instrumentation.
+  if (isAlreadyAsanInstrumented(M))
+    return false;
+
   // Initialize the private fields. No one has accessed them before.
   GlobalsMD.init(M);
 
@@ -2405,6 +2418,9 @@
 }
 
 bool AddressSanitizer::runOnFunction(Function &F) {
+  // Skip modules which already have ASan instrumentation.
+  if (isAlreadyAsanInstrumented(*F.getParent()))
+    return false;
   if (F.getLinkage() == GlobalValue::AvailableExternallyLinkage) return false;
   if (!ClDebugFunc.empty() && ClDebugFunc == F.getName()) return false;
   if (F.getName().startswith("__asan_")) return false;
Index: test/Instrumentation/AddressSanitizer/skip-previously-instrumented.ll
===================================================================
--- /dev/null
+++ test/Instrumentation/AddressSanitizer/skip-previously-instrumented.ll
@@ -0,0 +1,177 @@
+; Test to ensure previously-instrumented IR is handled gracefully
+; (ASAN skipped and not aborted).
+; RUN: opt < %s -asan -asan-module -S | FileCheck %s
+source_filename = "hello.c"
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+; Make sure that it compiled successfully
+; CHECK: %call = call i32 (i8*, ...) @printf
+
+@.str = internal unnamed_addr constant { [13 x i8], [51 x i8] } { [13 x i8] c"hello world\0A\00", [51 x i8] zeroinitializer }, align 32
+@__asan_gen_ = private constant [8 x i8] c"hello.c\00", align 1
+@__asan_gen_.1 = private unnamed_addr constant [17 x i8] c"<string literal>\00", align 1
+@__asan_gen_.2 = private unnamed_addr constant [8 x i8] c"hello.c\00", align 1
+@__asan_gen_.3 = private unnamed_addr constant { [8 x i8]*, i32, i32 } { [8 x i8]* @__asan_gen_.2, i32 5, i32 12 }
+@0 = internal global [1 x { i64, i64, i64, i64, i64, i64, i64, i64 }] [{ i64, i64, i64, i64, i64, i64, i64, i64 } { i64 ptrtoint ({ [13 x i8], [51 x i8] }* @.str to i64), i64 13, i64 64, i64 ptrtoint ([17 x i8]* @__asan_gen_.1 to i64), i64 ptrtoint ([8 x i8]* @__asan_gen_ to i64), i64 0, i64 ptrtoint ({ [8 x i8]*, i32, i32 }* @__asan_gen_.3 to i64), i64 0 }]
+@llvm.global_ctors = appending global [1 x { i32, void ()*, i8* }] [{ i32, void ()*, i8* } { i32 1, void ()* @asan.module_ctor, i8* null }]
+@llvm.global_dtors = appending global [1 x { i32, void ()*, i8* }] [{ i32, void ()*, i8* } { i32 1, void ()* @asan.module_dtor, i8* null }]
+
+; Function Attrs: noinline nounwind optnone sanitize_address uwtable
+define i32 @main() #0 {
+entry:
+  %retval = alloca i32, align 4
+  store i32 0, i32* %retval, align 4
+  %call = call i32 (i8*, ...) @printf(i8* getelementptr inbounds ({ [13 x i8], [51 x i8] }, { [13 x i8], [51 x i8] }* @.str, i32 0, i32 0, i32 0))
+  ret i32 0
+}
+
+declare i32 @printf(i8*, ...) #1
+
+declare void @__asan_report_load_n(i64, i64)
+
+declare void @__asan_loadN(i64, i64)
+
+declare void @__asan_report_load1(i64)
+
+declare void @__asan_load1(i64)
+
+declare void @__asan_report_load2(i64)
+
+declare void @__asan_load2(i64)
+
+declare void @__asan_report_load4(i64)
+
+declare void @__asan_load4(i64)
+
+declare void @__asan_report_load8(i64)
+
+declare void @__asan_load8(i64)
+
+declare void @__asan_report_load16(i64)
+
+declare void @__asan_load16(i64)
+
+declare void @__asan_report_store_n(i64, i64)
+
+declare void @__asan_storeN(i64, i64)
+
+declare void @__asan_report_store1(i64)
+
+declare void @__asan_store1(i64)
+
+declare void @__asan_report_store2(i64)
+
+declare void @__asan_store2(i64)
+
+declare void @__asan_report_store4(i64)
+
+declare void @__asan_store4(i64)
+
+declare void @__asan_report_store8(i64)
+
+declare void @__asan_store8(i64)
+
+declare void @__asan_report_store16(i64)
+
+declare void @__asan_store16(i64)
+
+declare void @__asan_report_exp_load_n(i64, i64, i32)
+
+declare void @__asan_exp_loadN(i64, i64, i32)
+
+declare void @__asan_report_exp_load1(i64, i32)
+
+declare void @__asan_exp_load1(i64, i32)
+
+declare void @__asan_report_exp_load2(i64, i32)
+
+declare void @__asan_exp_load2(i64, i32)
+
+declare void @__asan_report_exp_load4(i64, i32)
+
+declare void @__asan_exp_load4(i64, i32)
+
+declare void @__asan_report_exp_load8(i64, i32)
+
+declare void @__asan_exp_load8(i64, i32)
+
+declare void @__asan_report_exp_load16(i64, i32)
+
+declare void @__asan_exp_load16(i64, i32)
+
+declare void @__asan_report_exp_store_n(i64, i64, i32)
+
+declare void @__asan_exp_storeN(i64, i64, i32)
+
+declare void @__asan_report_exp_store1(i64, i32)
+
+declare void @__asan_exp_store1(i64, i32)
+
+declare void @__asan_report_exp_store2(i64, i32)
+
+declare void @__asan_exp_store2(i64, i32)
+
+declare void @__asan_report_exp_store4(i64, i32)
+
+declare void @__asan_exp_store4(i64, i32)
+
+declare void @__asan_report_exp_store8(i64, i32)
+
+declare void @__asan_exp_store8(i64, i32)
+
+declare void @__asan_report_exp_store16(i64, i32)
+
+declare void @__asan_exp_store16(i64, i32)
+
+declare i8* @__asan_memmove(i8*, i8*, i64)
+
+declare i8* @__asan_memcpy(i8*, i8*, i64)
+
+declare i8* @__asan_memset(i8*, i32, i64)
+
+declare void @__asan_handle_no_return()
+
+declare void @__sanitizer_ptr_cmp(i64, i64)
+
+declare void @__sanitizer_ptr_sub(i64, i64)
+
+declare void @__asan_before_dynamic_init(i64)
+
+declare void @__asan_after_dynamic_init()
+
+declare void @__asan_register_globals(i64, i64)
+
+declare void @__asan_unregister_globals(i64, i64)
+
+declare void @__asan_register_image_globals(i64)
+
+declare void @__asan_unregister_image_globals(i64)
+
+declare void @__asan_register_elf_globals(i64, i64, i64)
+
+declare void @__asan_unregister_elf_globals(i64, i64, i64)
+
+declare void @__asan_init()
+
+define internal void @asan.module_ctor() {
+  call void @__asan_init()
+  call void @__asan_version_mismatch_check_v8()
+  call void @__asan_register_globals(i64 ptrtoint ([1 x { i64, i64, i64, i64, i64, i64, i64, i64 }]* @0 to i64), i64 1)
+  ret void
+}
+
+declare void @__asan_version_mismatch_check_v8()
+
+define internal void @asan.module_dtor() {
+  call void @__asan_unregister_globals(i64 ptrtoint ([1 x { i64, i64, i64, i64, i64, i64, i64, i64 }]* @0 to i64), i64 1)
+  ret void
+}
+
+!llvm.asan.globals = !{!0}
+!llvm.module.flags = !{!2}
+!llvm.ident = !{!3}
+
+!0 = !{[13 x i8]* getelementptr inbounds ({ [13 x i8], [51 x i8] }, { [13 x i8], [51 x i8] }* @.str, i32 0, i32 0), !1, !"<string literal>", i1 false, i1 false}
+!1 = !{!"hello.c", i32 5, i32 12}
+!2 = !{i32 1, !"wchar_size", i32 4}
+!3 = !{!"clang version 6.0.0 (trunk 320030) (llvm/trunk 320068)"}