diff --git a/llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp b/llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp
--- a/llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp
@@ -2821,6 +2821,16 @@
 }
 
 Value *LibCallSimplifier::optimizeBCopy(CallInst *CI, IRBuilderBase &B) {
+  // Do not transform a bcopy call into llvm.memmove if it calls into another
+  // function.  This ensures that the fortified implementation with
+  // __builtin___memmove_chk is not overriden.
+  Function *Callee = CI->getCalledFunction();
+  if (!Callee->empty()) {
+    BasicBlock &BB = Callee->getEntryBlock();
+    if (isa<CallInst>(BB.getFirstNonPHIOrDbgOrLifetime()))
+      return nullptr;
+  }
+
   // bcopy(src, dst, n) -> llvm.memmove(dst, src, n)
   return B.CreateMemMove(CI->getArgOperand(1), Align(1), CI->getArgOperand(0),
                          Align(1), CI->getArgOperand(2));
diff --git a/llvm/test/Transforms/InstCombine/bcopy-chk.ll b/llvm/test/Transforms/InstCombine/bcopy-chk.ll
new file mode 100644
--- /dev/null
+++ b/llvm/test/Transforms/InstCombine/bcopy-chk.ll
@@ -0,0 +1,27 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt < %s -O1 -S | FileCheck %s
+
+declare i8* @__memmove_chk(i8*, i8*, i64, i64)
+declare i64 @llvm.objectsize.i64.p0i8(i8*, i1, i1, i1)
+
+define available_externally dso_local void @bcopy(i8* nocapture readonly %src, i8* nocapture %dst, i64 %len) #0 {
+  %size = call i64 @llvm.objectsize.i64.p0i8(i8* %dst, i1 false, i1 true, i1 false)
+  call i8* @__memmove_chk(i8* %dst, i8* %src, i64 %len, i64 %size)
+  ret void
+}
+
+@buf = dso_local global [4 x i8] zeroinitializer, align 1
+
+define dso_local void @fortified_bcopy(i64 %argc) {
+; CHECK-LABEL: @fortified_bcopy(
+; CHECK-NEXT:    [[ADD:%.*]] = add i64 [[ARGC:%.*]], 1
+; CHECK-NEXT:    [[TMP1:%.*]] = call i8* @__memmove_chk(i8* getelementptr inbounds ([4 x i8], [4 x i8]* @buf, i64 0, i64 2), i8* getelementptr inbounds ([4 x i8], [4 x i8]* @buf, i64 0, i64 1), i64 [[ADD]], i64 2)
+; CHECK-NEXT:    ret void
+;
+  %add = add i64 %argc, 1
+  tail call void @bcopy(i8* getelementptr inbounds ([4 x i8], [4 x i8]* @buf, i64 0, i64 1), i8* getelementptr inbounds ([4 x i8], [4 x i8]* @buf, i64 0, i64 2), i64 %add) #1
+  ret void
+}
+
+attributes #0 = { alwaysinline }
+attributes #1 = { builtin }