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 +1,15 @@
+; Test to ensure previously-instrumented IR is handled gracefully
+; (ASAN skipped and not aborted).
+; RUN: opt < %s -asan -asan-module -S -o %t.ll
+; RUN: opt < %t.ll -asan -asan-module -S -o %t2.ll
+; RUN: diff %t.ll %t2.ll
+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"
+
+@.str = private unnamed_addr constant [13 x i8] c"hello world\0A\00", align 1
+
+!llvm.asan.globals = !{!0}
+
+!0 = !{[13 x i8]* @.str, !1, !"<string literal>", i1 false, i1 false}
+!1 = !{!"hello.c", i32 5, i32 12}