Index: llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp
===================================================================
--- llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp
+++ llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp
@@ -3073,6 +3073,11 @@
     }
   }
 
+  // Simplification is for external library calls, so bail out if the callee
+  // has an implementation.
+  if (!Callee->empty())
+    return nullptr;
+
   // Also try to simplify calls to fortified library functions.
   if (Value *SimplifiedFortifiedCI =
           FortifiedSimplifier.optimizeCall(CI, Builder)) {
Index: llvm/test/Transforms/InstCombine/bcopy-chk.ll
===================================================================
--- /dev/null
+++ 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 }