Index: clang-tools-extra/clang-tidy/bugprone/BugproneTidyModule.cpp
===================================================================
--- clang-tools-extra/clang-tidy/bugprone/BugproneTidyModule.cpp
+++ clang-tools-extra/clang-tidy/bugprone/BugproneTidyModule.cpp
@@ -17,6 +17,7 @@
 #include "BranchCloneCheck.h"
 #include "CopyConstructorInitCheck.h"
 #include "DanglingHandleCheck.h"
+#include "DoNotReferAtomicTwiceCheck.h"
 #include "DynamicStaticInitializersCheck.h"
 #include "ExceptionEscapeCheck.h"
 #include "FoldInitTypeCheck.h"
@@ -84,6 +85,8 @@
         "bugprone-copy-constructor-init");
     CheckFactories.registerCheck<DanglingHandleCheck>(
         "bugprone-dangling-handle");
+    CheckFactories.registerCheck<DoNotReferAtomicTwiceCheck>(
+        "bugprone-do-not-refer-atomic-twice");
     CheckFactories.registerCheck<DynamicStaticInitializersCheck>(
         "bugprone-dynamic-static-initializers");
     CheckFactories.registerCheck<ExceptionEscapeCheck>(
Index: clang-tools-extra/clang-tidy/bugprone/CMakeLists.txt
===================================================================
--- clang-tools-extra/clang-tidy/bugprone/CMakeLists.txt
+++ clang-tools-extra/clang-tidy/bugprone/CMakeLists.txt
@@ -9,6 +9,7 @@
   BugproneTidyModule.cpp
   CopyConstructorInitCheck.cpp
   DanglingHandleCheck.cpp
+  DoNotReferAtomicTwiceCheck.cpp
   DynamicStaticInitializersCheck.cpp
   ExceptionEscapeCheck.cpp
   FoldInitTypeCheck.cpp
Index: clang-tools-extra/clang-tidy/bugprone/DoNotReferAtomicTwiceCheck.h
===================================================================
--- /dev/null
+++ clang-tools-extra/clang-tidy/bugprone/DoNotReferAtomicTwiceCheck.h
@@ -0,0 +1,34 @@
+//===--- DoNotReferAtomicTwiceCheck.h - clang-tidy --------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_BUGPRONE_DONOTREFERATOMICTWICECHECK_H
+#define LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_BUGPRONE_DONOTREFERATOMICTWICECHECK_H
+
+#include "../ClangTidyCheck.h"
+
+namespace clang {
+namespace tidy {
+namespace bugprone {
+
+/// Finds atomic variable which is referred twice in an expression.
+///
+/// For the user-facing documentation see:
+/// http://clang.llvm.org/extra/clang-tidy/checks/bugprone-do-not-refer-atomic-twice.html
+class DoNotReferAtomicTwiceCheck : public ClangTidyCheck {
+public:
+  DoNotReferAtomicTwiceCheck(StringRef Name, ClangTidyContext *Context)
+      : ClangTidyCheck(Name, Context) {}
+  void registerMatchers(ast_matchers::MatchFinder *Finder) override;
+  void check(const ast_matchers::MatchFinder::MatchResult &Result) override;
+};
+
+} // namespace bugprone
+} // namespace tidy
+} // namespace clang
+
+#endif // LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_BUGPRONE_DONOTREFERATOMICTWICECHECK_H
Index: clang-tools-extra/clang-tidy/bugprone/DoNotReferAtomicTwiceCheck.cpp
===================================================================
--- /dev/null
+++ clang-tools-extra/clang-tidy/bugprone/DoNotReferAtomicTwiceCheck.cpp
@@ -0,0 +1,44 @@
+//===--- DoNotReferAtomicTwiceCheck.cpp - clang-tidy ----------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "DoNotReferAtomicTwiceCheck.h"
+#include "clang/AST/ASTContext.h"
+#include "clang/ASTMatchers/ASTMatchFinder.h"
+
+using namespace clang::ast_matchers;
+
+namespace clang {
+namespace tidy {
+namespace bugprone {
+
+void DoNotReferAtomicTwiceCheck::registerMatchers(MatchFinder *Finder) {
+  Finder->addMatcher(
+      declRefExpr(hasType(hasUnqualifiedDesugaredType(atomicType())),
+                  to(varDecl().bind("atomic")),
+                  hasAncestor(binaryOperator(
+                      unless(hasDescendant(atomicExpr())),
+                      hasRHS(hasDescendant(
+                          declRefExpr(to(varDecl(equalsBoundNode("atomic"))))
+                              .bind("rhs"))))),
+                  unless(equalsBoundNode("rhs"))),
+      this);
+}
+
+void DoNotReferAtomicTwiceCheck::check(const MatchFinder::MatchResult &Result) {
+  const auto *MatchedVar = Result.Nodes.getNodeAs<VarDecl>("atomic");
+  const auto *MatchedRef = Result.Nodes.getNodeAs<DeclRefExpr>("rhs");
+  if (!MatchedRef || !MatchedVar)
+    return;
+  diag(MatchedRef->getExprLoc(),
+       "Do not refer to '%0' atomic variable twice in an expression")
+      << MatchedVar->getName();
+}
+
+} // namespace bugprone
+} // namespace tidy
+} // namespace clang
Index: clang-tools-extra/clang-tidy/cert/CERTTidyModule.cpp
===================================================================
--- clang-tools-extra/clang-tidy/cert/CERTTidyModule.cpp
+++ clang-tools-extra/clang-tidy/cert/CERTTidyModule.cpp
@@ -10,6 +10,7 @@
 #include "../ClangTidyModule.h"
 #include "../ClangTidyModuleRegistry.h"
 #include "../bugprone/BadSignalToKillThreadCheck.h"
+#include "../bugprone/DoNotReferAtomicTwiceCheck.h"
 #include "../bugprone/ReservedIdentifierCheck.h"
 #include "../bugprone/SpuriouslyWakeUpFunctionsCheck.h"
 #include "../bugprone/UnhandledSelfAssignmentCheck.h"
@@ -85,6 +86,8 @@
 
     // C checkers
     // CON
+    CheckFactories.registerCheck<bugprone::DoNotReferAtomicTwiceCheck>(
+        "cert-con40-c");
     CheckFactories.registerCheck<bugprone::SpuriouslyWakeUpFunctionsCheck>(
         "cert-con36-c");
     // DCL
Index: clang-tools-extra/docs/ReleaseNotes.rst
===================================================================
--- clang-tools-extra/docs/ReleaseNotes.rst
+++ clang-tools-extra/docs/ReleaseNotes.rst
@@ -75,6 +75,11 @@
 
 New checks
 ^^^^^^^^^^
+- New :doc:`bugprone-do-not-refer-atomic-twice
+  <clang-tidy/checks/bugprone-do-not-refer-atomic-twice>` check.
+
+  Finds atomic variable which is referred twice in an expression.
+
 - New :doc:`cppcoreguidelines-avoid-non-const-global-variables
   <clang-tidy/checks/cppcoreguidelines-avoid-non-const-global-variables>` check.
   Finds non-const global variables as described in check I.2 of C++ Core
@@ -137,6 +142,11 @@
   :doc:`bugprone-spuriously-wake-up-functions
   <clang-tidy/checks/bugprone-spuriously-wake-up-functions>` was added.
 
+- New alias :doc:`cert-con40-c
+  <clang-tidy/checks/cert-con40-c>` to
+  :doc:`bugprone-do-not-refer-atomic-twice
+  <clang-tidy/checks/bugprone-do-not-refer-atomic-twice>` was added.
+
 - New alias :doc:`cert-con54-cpp
   <clang-tidy/checks/cert-con54-cpp>` to
   :doc:`bugprone-spuriously-wake-up-functions
Index: clang-tools-extra/docs/clang-tidy/checks/bugprone-do-not-refer-atomic-twice.rst
===================================================================
--- /dev/null
+++ clang-tools-extra/docs/clang-tidy/checks/bugprone-do-not-refer-atomic-twice.rst
@@ -0,0 +1,17 @@
+.. title:: clang-tidy - bugprone-do-not-refer-atomic-twice
+
+bugprone-do-not-refer-atomic-twice
+==================================
+
+Finds atomic variable which is referred twice in an expression.
+
+.. code-block: c
+
+    atomic_int n = ATOMIC_VAR_INIT(0);
+    int compute_sum(void) {
+        return n * (n + 1) / 2;
+    }
+
+This check corresponds to the CERT C Coding Standard rule
+`CON40-C. Do not refer to an atomic variable twice in an expression
+<https://wiki.sei.cmu.edu/confluence/display/c/CON40-C.+Do+not+refer+to+an+atomic+variable+twice+in+an+expression>`_.
Index: clang-tools-extra/docs/clang-tidy/checks/cert-con40-c.rst
===================================================================
--- /dev/null
+++ clang-tools-extra/docs/clang-tidy/checks/cert-con40-c.rst
@@ -0,0 +1,10 @@
+.. title:: clang-tidy - cert-con40-c
+.. meta::
+   :http-equiv=refresh: 5;URL=bugprone-do-not-refer-atomic-twice.html
+
+cert-con40-c
+============
+
+The cert-con40-c check is an alias, please see
+`bugprone-do-not-refer-atomic-twice <bugprone-do-not-refer-atomic-twice>`_
+for more information.
Index: clang-tools-extra/docs/clang-tidy/checks/list.rst
===================================================================
--- clang-tools-extra/docs/clang-tidy/checks/list.rst
+++ clang-tools-extra/docs/clang-tidy/checks/list.rst
@@ -52,6 +52,7 @@
    `bugprone-branch-clone <bugprone-branch-clone.html>`_,
    `bugprone-copy-constructor-init <bugprone-copy-constructor-init.html>`_, "Yes"
    `bugprone-dangling-handle <bugprone-dangling-handle.html>`_,
+   `bugprone-do-not-refer-atomic-twice <bugprone-do-not-refer-atomic-twice.html>`_,
    `bugprone-dynamic-static-initializers <bugprone-dynamic-static-initializers.html>`_,
    `bugprone-exception-escape <bugprone-exception-escape.html>`_,
    `bugprone-fold-init-type <bugprone-fold-init-type.html>`_,
@@ -97,6 +98,7 @@
    `bugprone-unused-return-value <bugprone-unused-return-value.html>`_,
    `bugprone-use-after-move <bugprone-use-after-move.html>`_,
    `bugprone-virtual-near-miss <bugprone-virtual-near-miss.html>`_, "Yes"
+   `cert-con40-c <cert-con40-c.html>`_,
    `cert-dcl21-cpp <cert-dcl21-cpp.html>`_,
    `cert-dcl50-cpp <cert-dcl50-cpp.html>`_,
    `cert-dcl58-cpp <cert-dcl58-cpp.html>`_,
@@ -132,6 +134,7 @@
    `clang-analyzer-valist.Uninitialized <clang-analyzer-valist.Uninitialized.html>`_,
    `clang-analyzer-valist.Unterminated <clang-analyzer-valist.Unterminated.html>`_,
    `cppcoreguidelines-avoid-goto <cppcoreguidelines-avoid-goto.html>`_,
+   `cppcoreguidelines-avoid-non-const-global-variables <cppcoreguidelines-avoid-non-const-global-variables.html>`_,
    `cppcoreguidelines-init-variables <cppcoreguidelines-init-variables.html>`_, "Yes"
    `cppcoreguidelines-interfaces-global-init <cppcoreguidelines-interfaces-global-init.html>`_,
    `cppcoreguidelines-macro-usage <cppcoreguidelines-macro-usage.html>`_,
@@ -188,7 +191,7 @@
    `llvm-prefer-isa-or-dyn-cast-in-conditionals <llvm-prefer-isa-or-dyn-cast-in-conditionals.html>`_, "Yes"
    `llvm-prefer-register-over-unsigned <llvm-prefer-register-over-unsigned.html>`_, "Yes"
    `llvm-twine-local <llvm-twine-local.html>`_, "Yes"
-   `llvmlibc-restrict-system-libc-headers <llvmlibc-restrict-system-libc-headers.html>`_, "Yes"
+   `llvmlibc-restrict-system-libc-headers <llvmlibc-restrict-system-libc-headers.html>`_,
    `misc-definitions-in-headers <misc-definitions-in-headers.html>`_, "Yes"
    `misc-misplaced-const <misc-misplaced-const.html>`_,
    `misc-new-delete-overloads <misc-new-delete-overloads.html>`_,
@@ -209,7 +212,7 @@
    `modernize-deprecated-headers <modernize-deprecated-headers.html>`_, "Yes"
    `modernize-deprecated-ios-base-aliases <modernize-deprecated-ios-base-aliases.html>`_, "Yes"
    `modernize-loop-convert <modernize-loop-convert.html>`_, "Yes"
-   `modernize-make-shared <modernize-make-shared.html>`_, "Yes"
+   `modernize-make-shared <modernize-make-shared.html>`_,
    `modernize-make-unique <modernize-make-unique.html>`_, "Yes"
    `modernize-pass-by-value <modernize-pass-by-value.html>`_, "Yes"
    `modernize-raw-string-literal <modernize-raw-string-literal.html>`_, "Yes"
@@ -383,7 +386,6 @@
    `clang-analyzer-unix.cstring.NullArg <clang-analyzer-unix.cstring.NullArg.html>`_, `Clang Static Analyzer <https://clang.llvm.org/docs/analyzer/checkers.html>`_,
    `cppcoreguidelines-avoid-c-arrays <cppcoreguidelines-avoid-c-arrays.html>`_, `modernize-avoid-c-arrays <modernize-avoid-c-arrays.html>`_,
    `cppcoreguidelines-avoid-magic-numbers <cppcoreguidelines-avoid-magic-numbers.html>`_, `readability-magic-numbers <readability-magic-numbers.html>`_,
-   `cppcoreguidelines-avoid-non-const-global-variables <cppcoreguidelines-avoid-non-const-global-variables.html>`_, , , ""
    `cppcoreguidelines-c-copy-assignment-signature <cppcoreguidelines-c-copy-assignment-signature.html>`_, `misc-unconventional-assign-operator <misc-unconventional-assign-operator.html>`_,
    `cppcoreguidelines-explicit-virtual-functions <cppcoreguidelines-explicit-virtual-functions.html>`_, `modernize-use-override <modernize-use-override.html>`_, "Yes"
    `cppcoreguidelines-non-private-member-variables-in-classes <cppcoreguidelines-non-private-member-variables-in-classes.html>`_, `misc-non-private-member-variables-in-classes <misc-non-private-member-variables-in-classes.html>`_,
@@ -417,4 +419,3 @@
    `hicpp-use-override <hicpp-use-override.html>`_, `modernize-use-override <modernize-use-override.html>`_, "Yes"
    `hicpp-vararg <hicpp-vararg.html>`_, `cppcoreguidelines-pro-type-vararg <cppcoreguidelines-pro-type-vararg.html>`_,
    `llvm-qualified-auto <llvm-qualified-auto.html>`_, `readability-qualified-auto <readability-qualified-auto.html>`_, "Yes"
-
Index: clang-tools-extra/test/clang-tidy/checkers/bugprone-do-not-refer-atomic-twice.cpp
===================================================================
--- /dev/null
+++ clang-tools-extra/test/clang-tidy/checkers/bugprone-do-not-refer-atomic-twice.cpp
@@ -0,0 +1,79 @@
+// RUN: %check_clang_tidy %s bugprone-do-not-refer-atomic-twice %t
+#define _Bool bool
+typedef _Atomic _Bool atomic_bool;
+typedef _Atomic int atomic_int;
+#define ATOMIC_VAR_INIT(VALUE) (VALUE)
+
+atomic_bool b = ATOMIC_VAR_INIT(false);
+atomic_int n = ATOMIC_VAR_INIT(0);
+_Atomic int n2 = ATOMIC_VAR_INIT(0);
+_Atomic(int) n3 = ATOMIC_VAR_INIT(0);
+
+void warn1() {
+  n = (n + 1) / 2;
+  // CHECK-MESSAGES: :[[@LINE-1]]:8: warning: Do not refer to 'n' atomic variable twice in an expression [bugprone-do-not-refer-atomic-twice]
+  n2 = (n2 + 1) / 2;
+  // CHECK-MESSAGES: :[[@LINE-1]]:9: warning: Do not refer to 'n2' atomic variable twice in an expression [bugprone-do-not-refer-atomic-twice]
+  n3 = (n3 + 1) / 2;
+  // CHECK-MESSAGES: :[[@LINE-1]]:9: warning: Do not refer to 'n3' atomic variable twice in an expression [bugprone-do-not-refer-atomic-twice]
+  b = b && true;
+  // CHECK-MESSAGES: :[[@LINE-1]]:7: warning: Do not refer to 'b' atomic variable twice in an expression [bugprone-do-not-refer-atomic-twice]
+}
+
+int warn2_1() {
+  return n * (n + 1) / 2;
+  // CHECK-MESSAGES: :[[@LINE-1]]:15: warning: Do not refer to 'n' atomic variable twice in an expression [bugprone-do-not-refer-atomic-twice]
+}
+
+int warn2_2() {
+  return n2 * (n2 + 1) / 2;
+  // CHECK-MESSAGES: :[[@LINE-1]]:16: warning: Do not refer to 'n2' atomic variable twice in an expression [bugprone-do-not-refer-atomic-twice]
+}
+
+int warn2_3() {
+  return n3 * (n3 + 1) / 2;
+  // CHECK-MESSAGES: :[[@LINE-1]]:16: warning: Do not refer to 'n3' atomic variable twice in an expression [bugprone-do-not-refer-atomic-twice]
+}
+
+int warn2_4() {
+  return (b && true) || b;
+  // CHECK-MESSAGES: :[[@LINE-1]]:25: warning: Do not refer to 'b' atomic variable twice in an expression [bugprone-do-not-refer-atomic-twice]
+}
+
+void good1() {
+  n = 12;
+  n2 = 12;
+  n3 = 12;
+  b = true;
+}
+
+int good2_1() {
+  return n;
+}
+
+int good2_2() {
+  return n2;
+}
+
+int good2_3() {
+  return n3;
+}
+
+bool good2_4() {
+  return b;
+}
+
+void good3() {
+  n += 12;
+  n2 += 12;
+  n3 += 12;
+  b ^= 1;
+}
+
+void good4() {
+  n++;
+  n2++;
+  n3++;
+}
+
+int main() {}