Index: clang/include/clang/AST/OpenMPClause.h
===================================================================
--- clang/include/clang/AST/OpenMPClause.h
+++ clang/include/clang/AST/OpenMPClause.h
@@ -2266,8 +2266,8 @@
   }
 };
 
-/// This represents 'seq_cst' clause in the '#pragma omp atomic'
-/// directive.
+/// This represents 'seq_cst' clause in the '#pragma omp atomic|flush'
+/// directives.
 ///
 /// \code
 /// #pragma omp atomic seq_cst
Index: clang/include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -10567,7 +10567,7 @@
 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<
-  "directive '#pragma omp %0' cannot contain more than one %select{'seq_cst', 'relaxed', |}1'acq_rel', 'acquire' or 'release' clause">;
+  "directive '#pragma omp %0' cannot contain more than one %select{'seq_cst', 'relaxed', |}1'seq_cst', 'acq_rel', 'acquire' or 'release' clause">;
 def err_omp_atomic_incompatible_mem_order_clause : Error<
   "directive '#pragma omp atomic%select{ %0|}1' cannot be used with '%2' clause">;
 def note_omp_previous_mem_order_clause : Note<
Index: clang/lib/Sema/SemaOpenMP.cpp
===================================================================
--- clang/lib/Sema/SemaOpenMP.cpp
+++ clang/lib/Sema/SemaOpenMP.cpp
@@ -10539,7 +10539,8 @@
   for (const OMPClause *C : Clauses) {
     if (C->getClauseKind() == OMPC_acq_rel ||
         C->getClauseKind() == OMPC_acquire ||
-        C->getClauseKind() == OMPC_release) {
+        C->getClauseKind() == OMPC_release ||
+        C->getClauseKind() == OMPC_seq_cst /*OpenMP 5.1*/) {
       if (MemOrderKind != OMPC_unknown) {
         Diag(C->getBeginLoc(), diag::err_omp_several_mem_order_clauses)
             << getOpenMPDirectiveName(OMPD_flush) << 1
Index: clang/test/OpenMP/flush_ast_print.cpp
===================================================================
--- clang/test/OpenMP/flush_ast_print.cpp
+++ clang/test/OpenMP/flush_ast_print.cpp
@@ -1,10 +1,10 @@
-// RUN: %clang_cc1 -verify -fopenmp -fopenmp-version=50 -ast-print %s | FileCheck %s
-// RUN: %clang_cc1 -fopenmp -fopenmp-version=50 -x c++ -std=c++11 -emit-pch -o %t %s
-// RUN: %clang_cc1 -fopenmp -fopenmp-version=50 -std=c++11 -include-pch %t -fsyntax-only -verify %s -ast-print | FileCheck %s
+// RUN: %clang_cc1 -verify -fopenmp -fopenmp-version=51 -ast-print %s | FileCheck %s
+// RUN: %clang_cc1 -fopenmp -fopenmp-version=51 -x c++ -std=c++11 -emit-pch -o %t %s
+// RUN: %clang_cc1 -fopenmp -fopenmp-version=51 -std=c++11 -include-pch %t -fsyntax-only -verify %s -ast-print | FileCheck %s
 
-// RUN: %clang_cc1 -verify -fopenmp-simd -fopenmp-version=50 -ast-print %s | FileCheck %s
-// RUN: %clang_cc1 -fopenmp-simd -fopenmp-version=50 -x c++ -std=c++11 -emit-pch -o %t %s
-// RUN: %clang_cc1 -fopenmp-simd -fopenmp-version=50 -std=c++11 -include-pch %t -fsyntax-only -verify %s -ast-print | FileCheck %s
+// RUN: %clang_cc1 -verify -fopenmp-simd -fopenmp-version=51 -ast-print %s | FileCheck %s
+// RUN: %clang_cc1 -fopenmp-simd -fopenmp-version=51 -x c++ -std=c++11 -emit-pch -o %t %s
+// RUN: %clang_cc1 -fopenmp-simd -fopenmp-version=51 -std=c++11 -include-pch %t -fsyntax-only -verify %s -ast-print | FileCheck %s
 // expected-no-diagnostics
 
 #ifndef HEADER
@@ -19,6 +19,7 @@
 #pragma omp flush acq_rel
 #pragma omp flush acquire
 #pragma omp flush release
+#pragma omp flush seq_cst
 #pragma omp flush(a)
   return a + argc;
 }
@@ -27,18 +28,21 @@
 // CHECK-NEXT: #pragma omp flush acq_rel{{$}}
 // CHECK-NEXT: #pragma omp flush acquire{{$}}
 // CHECK-NEXT: #pragma omp flush release{{$}}
+// CHECK-NEXT: #pragma omp flush seq_cst{{$}}
 // CHECK-NEXT: #pragma omp flush (a)
 // CHECK:      static int a;
 // CHECK-NEXT: #pragma omp flush
 // CHECK-NEXT: #pragma omp flush acq_rel{{$}}
 // CHECK-NEXT: #pragma omp flush acquire{{$}}
 // CHECK-NEXT: #pragma omp flush release{{$}}
+// CHECK-NEXT: #pragma omp flush seq_cst{{$}}
 // CHECK-NEXT: #pragma omp flush (a)
 // CHECK:      static char a;
 // CHECK-NEXT: #pragma omp flush
 // CHECK-NEXT: #pragma omp flush acq_rel{{$}}
 // CHECK-NEXT: #pragma omp flush acquire{{$}}
 // CHECK-NEXT: #pragma omp flush release{{$}}
+// CHECK-NEXT: #pragma omp flush seq_cst{{$}}
 // CHECK-NEXT: #pragma omp flush (a)
 
 int main(int argc, char **argv) {
@@ -48,11 +52,13 @@
 #pragma omp flush acq_rel
 #pragma omp flush acquire
 #pragma omp flush release
+#pragma omp flush seq_cst
 #pragma omp flush(a)
 // CHECK-NEXT: #pragma omp flush
 // CHECK-NEXT: #pragma omp flush acq_rel
 // CHECK-NEXT: #pragma omp flush acquire{{$}}
 // CHECK-NEXT: #pragma omp flush release
+// CHECK-NEXT: #pragma omp flush seq_cst
 // CHECK-NEXT: #pragma omp flush (a)
   return tmain(argc) + tmain(argv[0][0]) + a;
 }
Index: clang/test/OpenMP/flush_codegen.cpp
===================================================================
--- clang/test/OpenMP/flush_codegen.cpp
+++ clang/test/OpenMP/flush_codegen.cpp
@@ -1,13 +1,13 @@
-// RUN: %clang_cc1 -verify -fopenmp -x c++ -emit-llvm %s -fexceptions -fcxx-exceptions -o - | FileCheck %s
-// RUN: %clang_cc1 -fopenmp -x c++ -std=c++11 -triple x86_64-unknown-unknown -fexceptions -fcxx-exceptions -emit-pch -o %t %s
-// RUN: %clang_cc1 -fopenmp -x c++ -triple x86_64-unknown-unknown -fexceptions -fcxx-exceptions -debug-info-kind=limited -std=c++11 -include-pch %t -verify %s -emit-llvm -o - | FileCheck %s
-// RUN: %clang_cc1 -verify -fopenmp -fopenmp-enable-irbuilder -x c++ -emit-llvm %s -fexceptions -fcxx-exceptions -o - | FileCheck %s
-// RUN: %clang_cc1 -fopenmp -fopenmp-enable-irbuilder -x c++ -std=c++11 -triple x86_64-unknown-unknown -fexceptions -fcxx-exceptions -emit-pch -o %t %s
-// RUN: %clang_cc1 -fopenmp -fopenmp-enable-irbuilder -x c++ -triple x86_64-unknown-unknown -fexceptions -fcxx-exceptions -debug-info-kind=limited -std=c++11 -include-pch %t -verify %s -emit-llvm -o - | FileCheck %s
+// RUN: %clang_cc1 -verify -fopenmp -fopenmp-version=51 -x c++ -emit-llvm %s -fexceptions -fcxx-exceptions -o - | FileCheck %s
+// RUN: %clang_cc1 -fopenmp -fopenmp-version=51 -x c++ -std=c++11 -triple x86_64-unknown-unknown -fexceptions -fcxx-exceptions -emit-pch -o %t %s
+// RUN: %clang_cc1 -fopenmp -fopenmp-version=51 -x c++ -triple x86_64-unknown-unknown -fexceptions -fcxx-exceptions -debug-info-kind=limited -std=c++11 -include-pch %t -verify %s -emit-llvm -o - | FileCheck %s
+// RUN: %clang_cc1 -verify -fopenmp -fopenmp-version=51 -fopenmp-enable-irbuilder -x c++ -emit-llvm %s -fexceptions -fcxx-exceptions -o - | FileCheck %s
+// RUN: %clang_cc1 -fopenmp -fopenmp-version=51 -fopenmp-enable-irbuilder -x c++ -std=c++11 -triple x86_64-unknown-unknown -fexceptions -fcxx-exceptions -emit-pch -o %t %s
+// RUN: %clang_cc1 -fopenmp -fopenmp-version=51 -fopenmp-enable-irbuilder -x c++ -triple x86_64-unknown-unknown -fexceptions -fcxx-exceptions -debug-info-kind=limited -std=c++11 -include-pch %t -verify %s -emit-llvm -o - | FileCheck %s
 
-// RUN: %clang_cc1 -verify -fopenmp-simd -x c++ -emit-llvm %s -fexceptions -fcxx-exceptions -o - | FileCheck --check-prefix SIMD-ONLY0 %s
-// RUN: %clang_cc1 -fopenmp-simd -x c++ -std=c++11 -triple x86_64-unknown-unknown -fexceptions -fcxx-exceptions -emit-pch -o %t %s
-// RUN: %clang_cc1 -fopenmp-simd -x c++ -triple x86_64-unknown-unknown -fexceptions -fcxx-exceptions -debug-info-kind=limited -std=c++11 -include-pch %t -verify %s -emit-llvm -o - | FileCheck --check-prefix SIMD-ONLY0 %s
+// RUN: %clang_cc1 -verify -fopenmp-simd -fopenmp-version=51 -x c++ -emit-llvm %s -fexceptions -fcxx-exceptions -o - | FileCheck --check-prefix SIMD-ONLY0 %s
+// RUN: %clang_cc1 -fopenmp-simd -fopenmp-version=51 -x c++ -std=c++11 -triple x86_64-unknown-unknown -fexceptions -fcxx-exceptions -emit-pch -o %t %s
+// RUN: %clang_cc1 -fopenmp-simd -fopenmp-version=51 -x c++ -triple x86_64-unknown-unknown -fexceptions -fcxx-exceptions -debug-info-kind=limited -std=c++11 -include-pch %t -verify %s -emit-llvm -o - | FileCheck --check-prefix SIMD-ONLY0 %s
 // SIMD-ONLY0-NOT: {{__kmpc|__tgt}}
 // expected-no-diagnostics
 #ifndef HEADER
@@ -20,6 +20,7 @@
 #pragma omp flush acq_rel
 #pragma omp flush acquire
 #pragma omp flush release
+#pragma omp flush seq_cst
 #pragma omp flush(a)
   return a + argc;
 }
@@ -31,6 +32,7 @@
 #pragma omp flush acq_rel
 #pragma omp flush acquire
 #pragma omp flush release
+#pragma omp flush seq_cst
 #pragma omp flush(a)
   // CHECK: call {{.*}}void @__kmpc_flush(%{{.+}}* {{(@|%).+}})
   // CHECK: call {{.*}}void @__kmpc_flush(%{{.+}}* {{(@|%).+}})
Index: clang/test/OpenMP/flush_messages.cpp
===================================================================
--- clang/test/OpenMP/flush_messages.cpp
+++ clang/test/OpenMP/flush_messages.cpp
@@ -1,8 +1,8 @@
 // RUN: %clang_cc1 -verify=expected,omp45 -fopenmp -fopenmp-version=45 -ferror-limit 100 %s -Wuninitialized
-// RUN: %clang_cc1 -verify=expected,omp50 -fopenmp -ferror-limit 100 %s -Wuninitialized
+// RUN: %clang_cc1 -verify=expected,omp51 -fopenmp -ferror-limit 100 %s -Wuninitialized
 
 // RUN: %clang_cc1 -verify=expected,omp45 -fopenmp-simd -fopenmp-version=45 -ferror-limit 100 %s -Wuninitialized
-// RUN: %clang_cc1 -verify=expected,omp50 -fopenmp-simd -ferror-limit 100 %s -Wuninitialized
+// RUN: %clang_cc1 -verify=expected,omp51 -fopenmp-simd -ferror-limit 100 %s -Wuninitialized
 
 struct S1 { // expected-note 2 {{declared here}}
   int a;
@@ -140,9 +140,12 @@
 #pragma omp flush release // omp45-error {{unexpected OpenMP clause 'release' in directive '#pragma omp flush'}}
 #pragma omp flush relaxed // expected-error {{unexpected OpenMP clause 'relaxed' in directive '#pragma omp flush'}}
 #pragma omp flush seq_cst // expected-error {{unexpected OpenMP clause 'seq_cst' in directive '#pragma omp flush'}}
-#pragma omp flush acq_rel acquire // omp45-error {{unexpected OpenMP clause 'acq_rel' in directive '#pragma omp flush'}} omp45-error {{unexpected OpenMP clause 'acquire' in directive '#pragma omp flush'}} omp50-error {{directive '#pragma omp flush' cannot contain more than one 'acq_rel', 'acquire' or 'release' clause}} omp50-note {{'acq_rel' clause used here}}
-#pragma omp flush release acquire // omp45-error {{unexpected OpenMP clause 'release' in directive '#pragma omp flush'}} omp45-error {{unexpected OpenMP clause 'acquire' in directive '#pragma omp flush'}} omp50-error {{directive '#pragma omp flush' cannot contain more than one 'acq_rel', 'acquire' or 'release' clause}} omp50-note {{'release' clause used here}}
+#pragma omp flush acq_rel acquire // omp45-error {{unexpected OpenMP clause 'acq_rel' in directive '#pragma omp flush'}} omp45-error {{unexpected OpenMP clause 'acquire' in directive '#pragma omp flush'}} omp50-error {{directive '#pragma omp flush' cannot contain more than one 'seq_cst', 'acq_rel', 'acquire' or 'release' clause}} omp50-note {{'acq_rel' clause used here}} omp51-error {{directive '#pragma omp flush' cannot contain more than one 'seq_cst', 'acq_rel', 'acquire' or 'release' clause}} omp51-note {{'acq_rel' clause used here}}
+
+#pragma omp flush release acquire // omp45-error {{unexpected OpenMP clause 'release' in directive '#pragma omp flush'}} omp45-error {{unexpected OpenMP clause 'acquire' in directive '#pragma omp flush'}} omp50-error {{directive '#pragma omp flush' cannot contain more than one 'seq_cst', 'acq_rel', 'acquire' or 'release' clause}} omp50-note {{'release' clause used here}} omp51-error {{directive '#pragma omp flush' cannot contain more than one 'seq_cst', 'acq_rel', 'acquire' or 'release' clause}} omp51-note {{'release' clause used here}}
+
 #pragma omp flush acq_rel (argc) // omp45-error {{unexpected OpenMP clause 'acq_rel' in directive '#pragma omp flush'}} expected-warning {{extra tokens at the end of '#pragma omp flush' are ignored}}
-#pragma omp flush(argc) acq_rel // omp45-error {{unexpected OpenMP clause 'acq_rel' in directive '#pragma omp flush'}} omp50-error {{'flush' directive with memory order clause 'acq_rel' cannot have the list}} omp50-note {{memory order clause 'acq_rel' is specified here}}
+#pragma omp flush(argc) acq_rel // omp45-error {{unexpected OpenMP clause 'acq_rel' in directive '#pragma omp flush'}} omp50-error {{'flush' directive with memory order clause 'acq_rel' cannot have the list}} omp50-note {{memory order clause 'acq_rel' is specified here}} omp51-error {{'flush' directive with memory order clause 'acq_rel' cannot have the list}} omp51-note {{memory order clause 'acq_rel' is specified here}}
+
   return tmain(argc);
 }
Index: llvm/include/llvm/Frontend/OpenMP/OMP.td
===================================================================
--- llvm/include/llvm/Frontend/OpenMP/OMP.td
+++ llvm/include/llvm/Frontend/OpenMP/OMP.td
@@ -517,6 +517,7 @@
     VersionedClause<OMPC_AcqRel, 50>,
     VersionedClause<OMPC_Acquire, 50>,
     VersionedClause<OMPC_Release, 50>,
+    VersionedClause<OMPC_SeqCst, 51>,
     // TODO This should ne `none` instead. Comment carried over from
     // OMPKinds.def.
     VersionedClause<OMPC_Flush>