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
@@ -213,6 +213,16 @@
   LibFunc F;
   if (TLI && TLI->getLibFunc(*Call, F) && TLI->has(F)) {
     switch (F) {
+    case LibFunc_memset_chk: {
+      assert(ArgIdx == 0 && "Invalid argument index for memset_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.
+        Size = 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
@@ -183,7 +183,7 @@
 ; CHECK:       Just Mod (MustAlias):  Ptr: i8* %a	<->  %res = tail call i8* @__memset_chk(i8* %a, i32 0, i64 4, i64 %n)
 ; CHECK-NEXT:  Just Mod:  Ptr: i8* %res	<->  %res = tail call i8* @__memset_chk(i8* %a, i32 0, i64 4, i64 %n)
 ; CHECK-NEXT:  Just Mod:  Ptr: i8* %a.gep.1	<->  %res = tail call i8* @__memset_chk(i8* %a, i32 0, i64 4, i64 %n)
-; CHECK-NEXT:  Just Mod:  Ptr: i8* %a.gep.5	<->  %res = tail call i8* @__memset_chk(i8* %a, i32 0, i64 4, i64 %n)
+; CHECK-NEXT:  NoModRef:  Ptr: i8* %a.gep.5	<->  %res = tail call i8* @__memset_chk(i8* %a, i32 0, i64 4, i64 %n)
 ;
 entry:
   %res = tail call i8* @__memset_chk(i8* %a, i32 0, i64 4, i64 %n)