diff --git a/llvm/include/llvm/Analysis/MemoryLocation.h b/llvm/include/llvm/Analysis/MemoryLocation.h
--- a/llvm/include/llvm/Analysis/MemoryLocation.h
+++ b/llvm/include/llvm/Analysis/MemoryLocation.h
@@ -253,10 +253,14 @@
   static MemoryLocation getForDest(const MemIntrinsic *MI);
   static MemoryLocation getForDest(const AtomicMemIntrinsic *MI);
   static MemoryLocation getForDest(const AnyMemIntrinsic *MI);
+  /// The result returned is only correct in contexts where \p CI is guaranteed
+  /// to execute.
   static Optional<MemoryLocation> getForDest(const CallBase *CI,
                                              const TargetLibraryInfo &TLI);
 
   /// Return a location representing a particular argument of a call.
+  /// The result returned is only correct in contexts where \p Call is
+  /// guaranteed to execute.
   static MemoryLocation getForArgument(const CallBase *Call, unsigned ArgIdx,
                                        const TargetLibraryInfo *TLI);
   static MemoryLocation getForArgument(const CallBase *Call, unsigned ArgIdx,
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
@@ -137,6 +137,7 @@
   LibFunc LF;
   if (TLI.getLibFunc(*CB, LF) && TLI.has(LF)) {
     switch (LF) {
+    case LibFunc_memset_chk:
     case LibFunc_strncpy:
     case LibFunc_strcpy:
     case LibFunc_strcat:
@@ -254,7 +255,7 @@
       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());
+        Size = LocationSize::precise(Len->getZExtValue());
       }
       return MemoryLocation(Arg, Size, AATags);
     }
diff --git a/llvm/test/Transforms/DeadStoreElimination/libcalls.ll b/llvm/test/Transforms/DeadStoreElimination/libcalls.ll
--- a/llvm/test/Transforms/DeadStoreElimination/libcalls.ll
+++ b/llvm/test/Transforms/DeadStoreElimination/libcalls.ll
@@ -468,8 +468,7 @@
 ; strncpy -> __memset_chk, full overwrite
 define void @dse_strncpy_memset_chk_test1(i8* noalias %out, i8* noalias %in, i64 %n) {
 ; CHECK-LABEL: @dse_strncpy_memset_chk_test1(
-; CHECK-NEXT:    [[CALL:%.*]] = tail call i8* @strncpy(i8* [[OUT:%.*]], i8* [[IN:%.*]], i64 100)
-; CHECK-NEXT:    [[CALL_2:%.*]] = tail call i8* @__memset_chk(i8* [[OUT]], i32 42, i64 100, i64 [[N:%.*]])
+; CHECK-NEXT:    [[CALL_2:%.*]] = tail call i8* @__memset_chk(i8* [[OUT:%.*]], i32 42, i64 100, i64 [[N:%.*]])
 ; CHECK-NEXT:    ret void
 ;
   %call = tail call i8* @strncpy(i8* %out, i8* %in, i64 100)
@@ -481,8 +480,7 @@
 
 define void @dse_memset_chk_cannot_eliminates_store(i8* %out, i64 %n) {
 ; CHECK-LABEL: @dse_memset_chk_cannot_eliminates_store(
-; CHECK-NEXT:    store i8 10, i8* [[OUT:%.*]], align 1
-; CHECK-NEXT:    [[CALL_2:%.*]] = tail call i8* @__memset_chk(i8* [[OUT]], i32 42, i64 100, i64 [[N:%.*]])
+; CHECK-NEXT:    [[CALL_2:%.*]] = tail call i8* @__memset_chk(i8* [[OUT:%.*]], i32 42, i64 100, i64 [[N:%.*]])
 ; CHECK-NEXT:    ret void
 ;
   store i8 10, i8* %out
@@ -494,7 +492,6 @@
 ; CHECK-LABEL: @dse_memset_chk_eliminates_store_local_object_escapes_after(
 ; CHECK-NEXT:    [[A:%.*]] = alloca [200 x i8], align 1
 ; CHECK-NEXT:    [[OUT:%.*]] = bitcast [200 x i8]* [[A]] to i8*
-; CHECK-NEXT:    store i8 10, i8* [[OUT]], align 1
 ; CHECK-NEXT:    [[OUT_100:%.*]] = getelementptr i8, i8* [[OUT]], i64 100
 ; CHECK-NEXT:    store i8 10, i8* [[OUT_100]], align 1
 ; CHECK-NEXT:    [[CALL_2:%.*]] = tail call i8* @__memset_chk(i8* [[OUT]], i32 42, i64 100, i64 [[N:%.*]])
@@ -516,7 +513,6 @@
 ; CHECK-NEXT:    [[A:%.*]] = alloca [200 x i8], align 1
 ; CHECK-NEXT:    [[OUT:%.*]] = bitcast [200 x i8]* [[A]] to i8*
 ; CHECK-NEXT:    call void @use(i8* [[OUT]])
-; CHECK-NEXT:    store i8 10, i8* [[OUT]], align 1
 ; CHECK-NEXT:    [[OUT_100:%.*]] = getelementptr i8, i8* [[OUT]], i64 100
 ; CHECK-NEXT:    store i8 0, i8* [[OUT_100]], align 1
 ; CHECK-NEXT:    [[CALL_2:%.*]] = tail call i8* @__memset_chk(i8* [[OUT]], i32 42, i64 100, i64 [[N:%.*]])