diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -10526,7 +10526,8 @@
 def note_omp_atomic_compare: Note<
   "%select{expected compound statement|expected exactly one expression statement|expected assignment statement|expected conditional operator|expect result value to be at false expression|"
   "expect binary operator in conditional expression|expect '<', '>' or '==' as order operator|expect comparison in a form of 'x == e', 'e == x', 'x ordop expr', or 'expr ordop x'|"
-  "expect lvalue for result value|expect scalar value|expect integer value}0">;
+  "expect lvalue for result value|expect scalar value|expect integer value|expect '==' operator|expect an assignment statement|expect a 'if' statement|expect no more than two statements|"
+  "expect a compound statement}0">;
 def err_omp_atomic_several_clauses : Error<
   "directive '#pragma omp atomic' cannot contain more than one 'read', 'write', 'update', 'capture', or 'compare' clause">;
 def err_omp_several_mem_order_clauses : Error<
diff --git a/clang/lib/Sema/SemaOpenMP.cpp b/clang/lib/Sema/SemaOpenMP.cpp
--- a/clang/lib/Sema/SemaOpenMP.cpp
+++ b/clang/lib/Sema/SemaOpenMP.cpp
@@ -10974,6 +10974,16 @@
     NotScalar,
     /// Not an integer.
     NotInteger,
+    /// Not an equality operator.
+    NotEQ,
+    /// Invalid assignment (not v == x).
+    InvalidAssignment,
+    /// Not if statement
+    NotIfStmt,
+    /// More than two statements in a compund statement.
+    MoreThanTwoStmts,
+    /// Not a compound statement.
+    NotCompoundStmt,
     /// No error.
     NoError,
   };
@@ -10997,7 +11007,7 @@
   Expr *getCond() const { return C; }
   bool isXBinopExpr() const { return IsXBinopExpr; }
 
-private:
+protected:
   /// Reference to ASTContext
   ASTContext &ContextRef;
   /// 'x' lvalue part of the source atomic expression.
@@ -11024,6 +11034,35 @@
 
   /// Check if all captured values have right type.
   bool checkType(ErrorInfoTy &ErrorInfo) const;
+
+  static bool CheckValue(const Expr *E, ErrorInfoTy &ErrorInfo,
+                         bool ShouldBeLValue) {
+    if (ShouldBeLValue && !E->isLValue()) {
+      ErrorInfo.Error = ErrorTy::XNotLValue;
+      ErrorInfo.ErrorLoc = ErrorInfo.NoteLoc = E->getExprLoc();
+      ErrorInfo.ErrorRange = ErrorInfo.NoteRange = E->getSourceRange();
+      return false;
+    }
+
+    if (!E->isInstantiationDependent()) {
+      QualType QTy = E->getType();
+      if (!QTy->isScalarType()) {
+        ErrorInfo.Error = ErrorTy::NotScalar;
+        ErrorInfo.ErrorLoc = ErrorInfo.NoteLoc = E->getExprLoc();
+        ErrorInfo.ErrorRange = ErrorInfo.NoteRange = E->getSourceRange();
+        return false;
+      }
+
+      if (!QTy->isIntegerType()) {
+        ErrorInfo.Error = ErrorTy::NotInteger;
+        ErrorInfo.ErrorLoc = ErrorInfo.NoteLoc = E->getExprLoc();
+        ErrorInfo.ErrorRange = ErrorInfo.NoteRange = E->getSourceRange();
+        return false;
+      }
+    }
+
+    return true;
+  }
 };
 
 bool OpenMPAtomicCompareChecker::checkCondUpdateStmt(IfStmt *S,
@@ -11206,41 +11245,13 @@
   // 'x' and 'e' cannot be nullptr
   assert(X && E && "X and E cannot be nullptr");
 
-  auto CheckValue = [&ErrorInfo](const Expr *E, bool ShouldBeLValue) {
-    if (ShouldBeLValue && !E->isLValue()) {
-      ErrorInfo.Error = ErrorTy::XNotLValue;
-      ErrorInfo.ErrorLoc = ErrorInfo.NoteLoc = E->getExprLoc();
-      ErrorInfo.ErrorRange = ErrorInfo.NoteRange = E->getSourceRange();
-      return false;
-    }
-
-    if (!E->isInstantiationDependent()) {
-      QualType QTy = E->getType();
-      if (!QTy->isScalarType()) {
-        ErrorInfo.Error = ErrorTy::NotScalar;
-        ErrorInfo.ErrorLoc = ErrorInfo.NoteLoc = E->getExprLoc();
-        ErrorInfo.ErrorRange = ErrorInfo.NoteRange = E->getSourceRange();
-        return false;
-      }
-
-      if (!QTy->isIntegerType()) {
-        ErrorInfo.Error = ErrorTy::NotInteger;
-        ErrorInfo.ErrorLoc = ErrorInfo.NoteLoc = E->getExprLoc();
-        ErrorInfo.ErrorRange = ErrorInfo.NoteRange = E->getSourceRange();
-        return false;
-      }
-    }
-
-    return true;
-  };
-
-  if (!CheckValue(X, true))
+  if (!CheckValue(X, ErrorInfo, true))
     return false;
 
-  if (!CheckValue(E, false))
+  if (!CheckValue(E, ErrorInfo, false))
     return false;
 
-  if (D && !CheckValue(D, false))
+  if (D && !CheckValue(D, ErrorInfo, false))
     return false;
 
   return true;
@@ -11288,6 +11299,375 @@
 
   return checkType(ErrorInfo);
 }
+
+class OpenMPAtomicCompareCaptureChecker final
+    : public OpenMPAtomicCompareChecker {
+public:
+  OpenMPAtomicCompareCaptureChecker(Sema &S) : OpenMPAtomicCompareChecker(S) {}
+
+  Expr *getV() const { return V; }
+  Expr *getR() const { return R; }
+  bool isFailOnly() const { return IsFailOnly; }
+
+  /// Check if statement \a S is valid for <tt>atomic compare capture</tt>.
+  bool checkStmt(Stmt *S, ErrorInfoTy &ErrorInfo);
+
+private:
+  bool checkType(ErrorInfoTy &ErrorInfo);
+
+  // NOTE: Form 3, 4, 5 in the following comments mean the 3rd, 4th, and 5th
+  // form of 'conditional-update-capture-atomic' structured block on the v5.2
+  // spec p.p. 82:
+  // (1) { v = x; cond-update-stmt }
+  // (2) { cond-update-stmt v = x; }
+  // (3) if(x == e) { x = d; } else { v = x; }
+  // (4) { r = x == e; if(r) { x = d; } }
+  // (5) { r = x == e; if(r) { x = d; } else { v = x; } }
+
+  /// Check if it is valid 'if(x == e) { x = d; } else { v = x; }' (form 3)
+  bool checkForm3(IfStmt *S, ErrorInfoTy &ErrorInfo);
+
+  /// Check if it is valid '{ r = x == e; if(r) { x = d; } }',
+  /// or '{ r = x == e; if(r) { x = d; } else { v = x; } }' (form 4 and 5)
+  bool checkForm45(Stmt *S, ErrorInfoTy &ErrorInfo);
+
+  /// 'v' lvalue part of the source atomic expression.
+  Expr *V = nullptr;
+  /// 'r' lvalue part of the source atomic expression.
+  Expr *R = nullptr;
+  /// If 'v' is only updated when the comparison fails.
+  bool IsFailOnly = false;
+};
+
+bool OpenMPAtomicCompareCaptureChecker::checkType(ErrorInfoTy &ErrorInfo) {
+  if (!OpenMPAtomicCompareChecker::checkType(ErrorInfo))
+    return false;
+
+  assert(V && "V cannot be nullptr");
+
+  if (!CheckValue(V, ErrorInfo, true))
+    return false;
+
+  if (R && !CheckValue(R, ErrorInfo, true))
+    return false;
+
+  return true;
+}
+
+bool OpenMPAtomicCompareCaptureChecker::checkForm3(IfStmt *S,
+                                                   ErrorInfoTy &ErrorInfo) {
+  IsFailOnly = true;
+
+  auto *Then = S->getThen();
+  if (auto *CS = dyn_cast<CompoundStmt>(Then)) {
+    if (CS->body_empty()) {
+      ErrorInfo.Error = ErrorTy::NoStmt;
+      ErrorInfo.ErrorLoc = ErrorInfo.NoteLoc = CS->getBeginLoc();
+      ErrorInfo.ErrorRange = ErrorInfo.NoteRange = CS->getSourceRange();
+      return false;
+    }
+    if (CS->size() > 1) {
+      ErrorInfo.Error = ErrorTy::MoreThanOneStmt;
+      ErrorInfo.ErrorLoc = ErrorInfo.NoteLoc = CS->getBeginLoc();
+      ErrorInfo.ErrorRange = ErrorInfo.NoteRange = S->getSourceRange();
+      return false;
+    }
+    Then = CS->body_front();
+  }
+
+  auto *BO = dyn_cast<BinaryOperator>(Then);
+  if (!BO) {
+    ErrorInfo.Error = ErrorTy::NotAnAssignment;
+    ErrorInfo.ErrorLoc = ErrorInfo.NoteLoc = Then->getBeginLoc();
+    ErrorInfo.ErrorRange = ErrorInfo.NoteRange = Then->getSourceRange();
+    return false;
+  }
+  if (BO->getOpcode() != BO_Assign) {
+    ErrorInfo.Error = ErrorTy::NotAnAssignment;
+    ErrorInfo.ErrorLoc = BO->getExprLoc();
+    ErrorInfo.NoteLoc = BO->getOperatorLoc();
+    ErrorInfo.ErrorRange = ErrorInfo.NoteRange = BO->getSourceRange();
+    return false;
+  }
+
+  X = BO->getLHS();
+  D = BO->getRHS();
+
+  auto *Cond = dyn_cast<BinaryOperator>(S->getCond());
+  if (!Cond) {
+    ErrorInfo.Error = ErrorTy::NotABinaryOp;
+    ErrorInfo.ErrorLoc = ErrorInfo.NoteLoc = Cond->getExprLoc();
+    ErrorInfo.ErrorRange = ErrorInfo.NoteRange = Cond->getSourceRange();
+    return false;
+  }
+
+  if (Cond->getOpcode() != BO_EQ) {
+    ErrorInfo.Error = ErrorTy::NotEQ;
+    ErrorInfo.ErrorLoc = ErrorInfo.NoteLoc = Cond->getExprLoc();
+    ErrorInfo.ErrorRange = ErrorInfo.NoteRange = Cond->getSourceRange();
+    return false;
+  }
+
+  if (checkIfTwoExprsAreSame(ContextRef, X, Cond->getLHS())) {
+    E = Cond->getRHS();
+  } else if (checkIfTwoExprsAreSame(ContextRef, X, Cond->getRHS())) {
+    E = Cond->getLHS();
+  } else {
+    ErrorInfo.Error = ErrorTy::InvalidComparison;
+    ErrorInfo.ErrorLoc = ErrorInfo.NoteLoc = Cond->getExprLoc();
+    ErrorInfo.ErrorRange = ErrorInfo.NoteRange = Cond->getSourceRange();
+    return false;
+  }
+
+  auto *Else = S->getElse();
+  if (auto *CS = dyn_cast<CompoundStmt>(Else)) {
+    if (CS->body_empty()) {
+      ErrorInfo.Error = ErrorTy::NoStmt;
+      ErrorInfo.ErrorLoc = ErrorInfo.NoteLoc = CS->getBeginLoc();
+      ErrorInfo.ErrorRange = ErrorInfo.NoteRange = CS->getSourceRange();
+      return false;
+    }
+    if (CS->size() > 1) {
+      ErrorInfo.Error = ErrorTy::MoreThanOneStmt;
+      ErrorInfo.ErrorLoc = ErrorInfo.NoteLoc = CS->getBeginLoc();
+      ErrorInfo.ErrorRange = ErrorInfo.NoteRange = S->getSourceRange();
+      return false;
+    }
+    Else = CS->body_front();
+  }
+
+  auto *ElseBO = dyn_cast<BinaryOperator>(Else);
+  if (!ElseBO) {
+    ErrorInfo.Error = ErrorTy::NotAnAssignment;
+    ErrorInfo.ErrorLoc = ErrorInfo.NoteLoc = Else->getBeginLoc();
+    ErrorInfo.ErrorRange = ErrorInfo.NoteRange = Else->getSourceRange();
+    return false;
+  }
+  if (ElseBO->getOpcode() != BO_Assign) {
+    ErrorInfo.Error = ErrorTy::NotAnAssignment;
+    ErrorInfo.ErrorLoc = ElseBO->getExprLoc();
+    ErrorInfo.NoteLoc = ElseBO->getOperatorLoc();
+    ErrorInfo.ErrorRange = ErrorInfo.NoteRange = ElseBO->getSourceRange();
+    return false;
+  }
+
+  if (!checkIfTwoExprsAreSame(ContextRef, X, ElseBO->getRHS())) {
+    ErrorInfo.Error = ErrorTy::InvalidAssignment;
+    ErrorInfo.ErrorLoc = ErrorInfo.NoteLoc = ElseBO->getRHS()->getExprLoc();
+    ErrorInfo.ErrorRange = ErrorInfo.NoteRange =
+        ElseBO->getRHS()->getSourceRange();
+    return false;
+  }
+
+  V = ElseBO->getLHS();
+
+  return checkType(ErrorInfo);
+}
+
+bool OpenMPAtomicCompareCaptureChecker::checkForm45(Stmt *S,
+                                                    ErrorInfoTy &ErrorInfo) {
+  // We don't check here as they should be already done before call this
+  // function.
+  auto *CS = cast<CompoundStmt>(S);
+  assert(CS->size() == 2 && "CompoundStmt size is not expected");
+  auto *S1 = cast<BinaryOperator>(CS->body_front());
+  auto *S2 = cast<IfStmt>(CS->body_back());
+  assert(S1->getOpcode() == BO_Assign && "unexpected binary operator");
+
+  if (!checkIfTwoExprsAreSame(ContextRef, S1->getLHS(), S2->getCond())) {
+    ErrorInfo.Error = ErrorTy::InvalidComparison;
+    ErrorInfo.ErrorLoc = ErrorInfo.NoteLoc = S2->getCond()->getExprLoc();
+    ErrorInfo.ErrorRange = ErrorInfo.NoteRange = S1->getLHS()->getSourceRange();
+    return false;
+  }
+
+  R = S1->getLHS();
+
+  auto *Then = dyn_cast<BinaryOperator>(S2->getThen());
+  if (!Then) {
+    ErrorInfo.Error = ErrorTy::NotAnAssignment;
+    ErrorInfo.ErrorLoc = ErrorInfo.NoteLoc = S2->getBeginLoc();
+    ErrorInfo.ErrorRange = ErrorInfo.NoteRange = S2->getSourceRange();
+    return false;
+  }
+  if (Then->getOpcode() != BO_Assign) {
+    ErrorInfo.Error = ErrorTy::NotAnAssignment;
+    ErrorInfo.ErrorLoc = Then->getExprLoc();
+    ErrorInfo.NoteLoc = Then->getOperatorLoc();
+    ErrorInfo.ErrorRange = ErrorInfo.NoteRange = Then->getSourceRange();
+    return false;
+  }
+
+  X = Then->getLHS();
+  D = Then->getRHS();
+
+  auto *BO = dyn_cast<BinaryOperator>(S1->getRHS());
+  if (!BO) {
+    ErrorInfo.Error = ErrorTy::NotEQ;
+    ErrorInfo.ErrorLoc = ErrorInfo.NoteLoc = S1->getExprLoc();
+    ErrorInfo.ErrorRange = ErrorInfo.NoteRange = S1->getSourceRange();
+    return false;
+  }
+  if (BO->getOpcode() != BO_EQ) {
+    ErrorInfo.Error = ErrorTy::NotEQ;
+    ErrorInfo.ErrorLoc = BO->getExprLoc();
+    ErrorInfo.NoteLoc = BO->getOperatorLoc();
+    ErrorInfo.ErrorRange = ErrorInfo.NoteRange = BO->getSourceRange();
+    return false;
+  }
+
+  if (checkIfTwoExprsAreSame(ContextRef, X, BO->getLHS())) {
+    E = BO->getRHS();
+  } else if (checkIfTwoExprsAreSame(ContextRef, X, BO->getRHS())) {
+    E = BO->getLHS();
+  } else {
+    ErrorInfo.Error = ErrorTy::InvalidComparison;
+    ErrorInfo.ErrorLoc = ErrorInfo.NoteLoc = BO->getExprLoc();
+    ErrorInfo.ErrorRange = ErrorInfo.NoteRange = BO->getSourceRange();
+    return false;
+  }
+
+  if (S2->getElse()) {
+    IsFailOnly = true;
+
+    auto *Else = dyn_cast<BinaryOperator>(S2->getElse());
+    if (!Else) {
+      ErrorInfo.Error = ErrorTy::NotAnAssignment;
+      ErrorInfo.ErrorLoc = ErrorInfo.NoteLoc = S2->getElse()->getBeginLoc();
+      ErrorInfo.ErrorRange = ErrorInfo.NoteRange =
+          S2->getElse()->getSourceRange();
+      return false;
+    }
+    if (Else->getOpcode() != BO_Assign) {
+      ErrorInfo.Error = ErrorTy::NotAnAssignment;
+      ErrorInfo.ErrorLoc = Else->getExprLoc();
+      ErrorInfo.NoteLoc = Else->getOperatorLoc();
+      ErrorInfo.ErrorRange = ErrorInfo.NoteRange = Else->getSourceRange();
+      return false;
+    }
+    if (checkIfTwoExprsAreSame(ContextRef, X, Else->getRHS())) {
+      ErrorInfo.Error = ErrorTy::InvalidAssignment;
+      ErrorInfo.ErrorLoc = Else->getRHS()->getExprLoc();
+      ErrorInfo.NoteLoc = X->getExprLoc();
+      ErrorInfo.ErrorRange = Else->getRHS()->getSourceRange();
+      ErrorInfo.NoteRange = X->getSourceRange();
+    }
+
+    V = Else->getLHS();
+  }
+
+  return checkType(ErrorInfo);
+}
+
+bool OpenMPAtomicCompareCaptureChecker::checkStmt(Stmt *S,
+                                                  ErrorInfoTy &ErrorInfo) {
+  // if(x == e) { x = d; } else { v = x; }
+  if (auto *IS = dyn_cast<IfStmt>(S))
+    return checkForm3(IS, ErrorInfo);
+
+  auto *CS = dyn_cast<CompoundStmt>(S);
+  if (!CS) {
+    ErrorInfo.Error = ErrorTy::NotCompoundStmt;
+    ErrorInfo.ErrorLoc = ErrorInfo.NoteLoc = S->getBeginLoc();
+    ErrorInfo.ErrorRange = ErrorInfo.NoteRange = S->getSourceRange();
+    return false;
+  }
+  if (CS->body_empty()) {
+    ErrorInfo.Error = ErrorTy::NoStmt;
+    ErrorInfo.ErrorLoc = ErrorInfo.NoteLoc = CS->getBeginLoc();
+    ErrorInfo.ErrorRange = ErrorInfo.NoteRange = CS->getSourceRange();
+    return false;
+  }
+
+  // { if(x == e) { x = d; } else { v = x; } }
+  if (CS->size() == 1) {
+    auto *IS = dyn_cast<IfStmt>(CS->body_front());
+    if (!IS) {
+      ErrorInfo.Error = ErrorTy::NotIfStmt;
+      ErrorInfo.ErrorLoc = ErrorInfo.NoteLoc = CS->body_front()->getBeginLoc();
+      ErrorInfo.ErrorRange = ErrorInfo.NoteRange =
+          CS->body_front()->getSourceRange();
+      return false;
+    }
+
+    return checkForm3(IS, ErrorInfo);
+  } else if (CS->size() == 2) {
+    auto *S1 = CS->body_front();
+    auto *S2 = CS->body_back();
+
+    Stmt *UpdateStmt = nullptr;
+    Stmt *CondUpdateStmt = nullptr;
+
+    if (auto *BO = dyn_cast<BinaryOperator>(S1)) {
+      // { v = x; cond-update-stmt } or form 45.
+      UpdateStmt = S1;
+      CondUpdateStmt = S2;
+      // Check if form 45.
+      if (dyn_cast<BinaryOperator>(BO->getRHS()) && dyn_cast<IfStmt>(S2))
+        return checkForm45(CS, ErrorInfo);
+    } else {
+      // { cond-update-stmt v = x; }
+      UpdateStmt = S2;
+      CondUpdateStmt = S1;
+    }
+
+    auto CheckUpdateStmt = [this, &ErrorInfo](Stmt *US) {
+      auto *BO = dyn_cast<BinaryOperator>(US);
+      if (!BO) {
+        ErrorInfo.Error = ErrorTy::NotAnAssignment;
+        ErrorInfo.ErrorLoc = ErrorInfo.NoteLoc = US->getBeginLoc();
+        ErrorInfo.ErrorRange = ErrorInfo.NoteRange = US->getSourceRange();
+        return false;
+      }
+      if (BO->getOpcode() != BO_Assign) {
+        ErrorInfo.Error = ErrorTy::NotAnAssignment;
+        ErrorInfo.ErrorLoc = BO->getExprLoc();
+        ErrorInfo.NoteLoc = BO->getOperatorLoc();
+        ErrorInfo.ErrorRange = ErrorInfo.NoteRange = BO->getSourceRange();
+        return false;
+      }
+      if (this->X != BO->getRHS()) {
+        ErrorInfo.Error = ErrorTy::InvalidAssignment;
+        ErrorInfo.ErrorLoc = BO->getRHS()->getExprLoc();
+        ErrorInfo.NoteLoc = this->X->getExprLoc();
+        ErrorInfo.ErrorRange = BO->getRHS()->getSourceRange();
+        ErrorInfo.NoteRange = this->X->getSourceRange();
+        return false;
+      }
+
+      this->V = BO->getLHS();
+
+      return true;
+    };
+
+    auto CheckCondUpdateStmt = [this, &ErrorInfo](Stmt *CUS) {
+      auto *IS = dyn_cast<IfStmt>(CUS);
+      if (!IS) {
+        ErrorInfo.Error = ErrorTy::NotIfStmt;
+        ErrorInfo.ErrorLoc = ErrorInfo.NoteLoc = CUS->getBeginLoc();
+        ErrorInfo.ErrorRange = ErrorInfo.NoteRange = CUS->getSourceRange();
+        return false;
+      }
+
+      if (!checkCondUpdateStmt(IS, ErrorInfo))
+        return false;
+
+      return true;
+    };
+
+    if (!CheckUpdateStmt(UpdateStmt))
+      return false;
+    if (!CheckCondUpdateStmt(CondUpdateStmt))
+      return false;
+  } else {
+    ErrorInfo.Error = ErrorTy::MoreThanTwoStmts;
+    ErrorInfo.ErrorLoc = ErrorInfo.NoteLoc = CS->getBeginLoc();
+    ErrorInfo.ErrorRange = ErrorInfo.NoteRange = CS->getSourceRange();
+    return false;
+  }
+
+  return checkType(ErrorInfo);
+}
 } // namespace
 
 StmtResult Sema::ActOnOpenMPAtomicDirective(ArrayRef<OMPClause *> Clauses,
@@ -11785,6 +12165,15 @@
       UE = V = E = X = nullptr;
   } else if (AtomicKind == OMPC_compare) {
     if (IsCompareCapture) {
+      OpenMPAtomicCompareCaptureChecker::ErrorInfoTy ErrorInfo;
+      OpenMPAtomicCompareCaptureChecker Checker(*this);
+      if (!Checker.checkStmt(Body, ErrorInfo)) {
+        Diag(ErrorInfo.ErrorLoc, diag::err_omp_atomic_compare)
+            << ErrorInfo.ErrorRange;
+        Diag(ErrorInfo.NoteLoc, diag::note_omp_atomic_compare)
+            << ErrorInfo.Error << ErrorInfo.NoteRange;
+        return StmtError();
+      }
       // TODO: We don't set X, D, E, etc. here because in code gen we will emit
       // error directly.
     } else {