diff --git a/llvm/lib/Analysis/MemoryLocation.cpp b/llvm/lib/Analysis/MemoryLocation.cpp
--- a/llvm/lib/Analysis/MemoryLocation.cpp
+++ b/llvm/lib/Analysis/MemoryLocation.cpp
@@ -253,12 +253,17 @@
       assert((ArgIdx == 0 || ArgIdx == 1) && "Invalid argument index for str function");
       return MemoryLocation::getAfter(Arg, AATags);
 
-    case LibFunc_memset_chk: {
+    case LibFunc_memset_chk:
       assert(ArgIdx == 0 && "Invalid argument index for memset_chk");
+      LLVM_FALLTHROUGH;
+    case LibFunc_memcpy_chk: {
+      assert((ArgIdx == 0 || ArgIdx == 1) &&
+             "Invalid argument index for memcpy_chk");
       LocationSize Size = LocationSize::afterPointer();
       if (const auto *Len = dyn_cast<ConstantInt>(Call->getArgOperand(2))) {
-        // memset_chk writes at most Len bytes. It may write less, if Len
-        // exceeds the specified max size and aborts.
+        // memset_chk writes at most Len bytes, memcpy_chk reads/writes at most
+        // Len bytes. They may read/write less, if Len exceeds the specified max
+        // size and aborts.
         Size = LocationSize::upperBound(Len->getZExtValue());
       }
       return MemoryLocation(Arg, Size, AATags);
diff --git a/llvm/test/Analysis/BasicAA/libfuncs.ll b/llvm/test/Analysis/BasicAA/libfuncs.ll
--- a/llvm/test/Analysis/BasicAA/libfuncs.ll
+++ b/llvm/test/Analysis/BasicAA/libfuncs.ll
@@ -323,9 +323,9 @@
 ; CHECK:       Just Mod:  Ptr: i8* %a	<->  %res = tail call i8* @__memcpy_chk(i8* %a, i8* %b, i64 4, i64 %n)
 ; CHECK-NEXT:  Just Mod:  Ptr: i8* %res	<->  %res = tail call i8* @__memcpy_chk(i8* %a, i8* %b, i64 4, i64 %n)
 ; CHECK-NEXT:  Just Mod:  Ptr: i8* %a.gep.1	<->  %res = tail call i8* @__memcpy_chk(i8* %a, i8* %b, i64 4, i64 %n)
-; CHECK-NEXT:  Just Mod:  Ptr: i8* %a.gep.5	<->  %res = tail call i8* @__memcpy_chk(i8* %a, i8* %b, i64 4, i64 %n)
+; CHECK-NEXT:  NoModRef:  Ptr: i8* %a.gep.5	<->  %res = tail call i8* @__memcpy_chk(i8* %a, i8* %b, i64 4, i64 %n)
 ; CHECK-NEXT:  Just Ref:  Ptr: i8* %b.gep.1	<->  %res = tail call i8* @__memcpy_chk(i8* %a, i8* %b, i64 4, i64 %n)
-; CHECK-NEXT:  Just Ref:  Ptr: i8* %b.gep.5	<->  %res = tail call i8* @__memcpy_chk(i8* %a, i8* %b, i64 4, i64 %n)
+; CHECK-NEXT:  NoModRef:  Ptr: i8* %b.gep.5	<->  %res = tail call i8* @__memcpy_chk(i8* %a, i8* %b, i64 4, i64 %n)
 ;
 entry:
   load i8, i8* %a