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
@@ -223,6 +223,18 @@
       }
       return MemoryLocation(Arg, Size, AATags);
     }
+    case LibFunc_strncpy: {
+      assert((ArgIdx == 0 || ArgIdx == 1) &&
+             "Invalid argument index for strncpy");
+      LocationSize Size = LocationSize::afterPointer();
+      if (const auto *Len = dyn_cast<ConstantInt>(Call->getArgOperand(2))) {
+        // strncpy is guaranteed to write Len bytes, but only reads up to Len
+        // bytes.
+        Size = ArgIdx == 0 ? LocationSize::precise(Len->getZExtValue())
+                           : LocationSize::upperBound(Len->getZExtValue());
+      }
+      return MemoryLocation(Arg, Size, AATags);
+    }
     case LibFunc_memset_pattern16:
       assert((ArgIdx == 0 || ArgIdx == 1) &&
              "Invalid argument index for memset_pattern16");
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
@@ -136,9 +136,9 @@
 ; CHECK-NEXT:  Just Ref:  Ptr: i8* %b	<->  %res = tail call i8* @strncpy(i8* %a, i8* %b, i64 4)
 ; CHECK-NEXT:  Just Mod:  Ptr: i8* %res	<->  %res = tail call i8* @strncpy(i8* %a, i8* %b, i64 4)
 ; CHECK-NEXT:  Just Mod:  Ptr: i8* %a.gep.1	<->  %res = tail call i8* @strncpy(i8* %a, i8* %b, i64 4)
-; CHECK-NEXT:  Just Mod:  Ptr: i8* %a.gep.5	<->  %res = tail call i8* @strncpy(i8* %a, i8* %b, i64 4)
+; CHECK-NEXT:  NoModRef:  Ptr: i8* %a.gep.5	<->  %res = tail call i8* @strncpy(i8* %a, i8* %b, i64 4)
 ; CHECK-NEXT:  Just Ref:  Ptr: i8* %b.gep.1	<->  %res = tail call i8* @strncpy(i8* %a, i8* %b, i64 4)
-; CHECK-NEXT:  Just Ref:  Ptr: i8* %b.gep.5	<->  %res = tail call i8* @strncpy(i8* %a, i8* %b, i64 4)
+; CHECK-NEXT:  NoModRef:  Ptr: i8* %b.gep.5	<->  %res = tail call i8* @strncpy(i8* %a, i8* %b, i64 4)
 ;
 entry:
   %res = tail call i8* @strncpy(i8* %a, i8* %b, i64 4)