diff --git a/llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp b/llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp
--- a/llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp
+++ b/llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp
@@ -1022,7 +1022,7 @@
     return I.first->second;
   }
 
-  Optional<MemoryLocation> getLocForWriteEx(Instruction *I) const {
+  Optional<MemoryLocation> getLocForWriteEx(Instruction *I) {
     if (!I->mayWriteToMemory())
       return None;
 
@@ -1032,6 +1032,22 @@
           !CB->onlyAccessesInaccessibleMemOrArgMem())
         return None;
 
+      LibFunc F;
+      if (TLI.getLibFunc(*CB, F) && TLI.has(F) && F == LibFunc_memset_chk) {
+        // Use the precise location size specified by the 3rd argument, if the
+        // accessed object is not visible to the caller at all or does not
+        // escape before the call. Even if memset_chk terminates, no-one will be
+        // able to read the accessed memory.
+        if (const auto *Len = dyn_cast<ConstantInt>(CB->getArgOperand(2))) {
+          Value *Ptr = CB->getArgOperand(0);
+          const Value *UO = getUnderlyingObject(Ptr);
+          if (isInvisibleToCallerAfterRet(UO) ||
+              EI.isNotCapturedBeforeOrAt(UO, CB))
+            return MemoryLocation(Ptr,
+                                  LocationSize::precise(Len->getZExtValue()),
+                                  CB->getAAMetadata());
+        }
+      }
       return MemoryLocation::getForDest(CB, TLI);
     }
 
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
@@ -494,7 +494,6 @@
 ; 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 +515,6 @@
 ; 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:%.*]])