Index: clang/lib/CodeGen/CGAtomic.cpp
===================================================================
--- clang/lib/CodeGen/CGAtomic.cpp
+++ clang/lib/CodeGen/CGAtomic.cpp
@@ -427,6 +427,8 @@
     else
       switch ((llvm::AtomicOrderingCABI)FOS) {
       case llvm::AtomicOrderingCABI::relaxed:
+      // 31.7.2.18: "The failure argument shall not be memory_order_release
+      // nor memory_order_acq_rel". Fallback to monotonic.
       case llvm::AtomicOrderingCABI::release:
       case llvm::AtomicOrderingCABI::acq_rel:
         FailureOrder = llvm::AtomicOrdering::Monotonic;
@@ -439,12 +441,10 @@
         FailureOrder = llvm::AtomicOrdering::SequentiallyConsistent;
         break;
       }
-    if (isStrongerThan(FailureOrder, SuccessOrder)) {
-      // Don't assert on undefined behavior "failure argument shall be no
-      // stronger than the success argument".
-      FailureOrder =
-          llvm::AtomicCmpXchgInst::getStrongestFailureOrdering(SuccessOrder);
-    }
+    // Prior to c++17, "the failure argument shall be no stronger than the
+    // success argument". This condition has been lifted and the only
+    // precondition is 31.7.2.18. Effectively treat this as a DR and skip
+    // language version checks.
     emitAtomicCmpXchg(CGF, E, IsWeak, Dest, Ptr, Val1, Val2, Size, SuccessOrder,
                       FailureOrder, Scope);
     return;
@@ -454,8 +454,7 @@
   llvm::BasicBlock *MonotonicBB = nullptr, *AcquireBB = nullptr,
                    *SeqCstBB = nullptr;
   MonotonicBB = CGF.createBasicBlock("monotonic_fail", CGF.CurFn);
-  if (SuccessOrder != llvm::AtomicOrdering::Monotonic &&
-      SuccessOrder != llvm::AtomicOrdering::Release)
+  if (SuccessOrder != llvm::AtomicOrdering::Monotonic)
     AcquireBB = CGF.createBasicBlock("acquire_fail", CGF.CurFn);
   if (SuccessOrder == llvm::AtomicOrdering::SequentiallyConsistent)
     SeqCstBB = CGF.createBasicBlock("seqcst_fail", CGF.CurFn);
@@ -479,8 +478,9 @@
     emitAtomicCmpXchg(CGF, E, IsWeak, Dest, Ptr, Val1, Val2,
                       Size, SuccessOrder, llvm::AtomicOrdering::Acquire, Scope);
     CGF.Builder.CreateBr(ContBB);
-    SI->addCase(CGF.Builder.getInt32((int)llvm::AtomicOrderingCABI::consume),
-                AcquireBB);
+    if (SuccessOrder != llvm::AtomicOrdering::Release)
+      SI->addCase(CGF.Builder.getInt32((int)llvm::AtomicOrderingCABI::consume),
+                  AcquireBB);
     SI->addCase(CGF.Builder.getInt32((int)llvm::AtomicOrderingCABI::acquire),
                 AcquireBB);
   }
Index: clang/test/CodeGen/atomic-ops.c
===================================================================
--- clang/test/CodeGen/atomic-ops.c
+++ clang/test/CodeGen/atomic-ops.c
@@ -500,6 +500,7 @@
 
   // CHECK: [[RELEASE]]
   // CHECK: switch {{.*}}, label %[[RELEASE_MONOTONIC:[0-9a-zA-Z._]+]] [
+  // CHECK-NEXT: i32 2, label %[[RELEASE_ACQUIRE:[0-9a-zA-Z._]+]]
   // CHECK-NEXT: ]
 
   // CHECK: [[ACQREL]]
@@ -527,6 +528,14 @@
   // CHECK: cmpxchg {{.*}} acquire acquire, align
   // CHECK: br
 
+  // CHECK: [[RELEASE_MONOTONIC]]
+  // CHECK: cmpxchg {{.*}} release monotonic, align
+  // CHECK: br
+
+  // CHECK: [[RELEASE_ACQUIRE]]
+  // CHECK: cmpxchg {{.*}} release acquire, align
+  // CHECK: br
+
   // CHECK: [[ACQREL_MONOTONIC]]
   // CHECK: cmpxchg {{.*}} acq_rel monotonic, align
   // CHECK: br
@@ -562,6 +571,20 @@
   // CHECK-NOT: br
   // CHECK: cmpxchg weak {{.*}} seq_cst seq_cst, align
   // CHECK: br
+
+  __atomic_compare_exchange_n(ptr, ptr2, 42, weak, memory_order_release, memory_order_acquire);
+  // CHECK: switch i1 {{.*}}, label %[[WEAK:[0-9a-zA-Z._]+]] [
+  // CHECK-NEXT: i1 false, label %[[STRONG:[0-9a-zA-Z._]+]]
+
+  // CHECK: [[STRONG]]
+  // CHECK-NOT: br
+  // CHECK: cmpxchg {{.*}} release acquire
+  // CHECK: br
+
+  // CHECK: [[WEAK]]
+  // CHECK-NOT: br
+  // CHECK: cmpxchg weak {{.*}} release acquire
+  // CHECK: br
 }
 
 // Having checked the flow in the previous two cases, we'll trust clang to
@@ -576,7 +599,9 @@
   // CHECK: = cmpxchg weak {{.*}} acquire monotonic, align
   // CHECK: = cmpxchg weak {{.*}} acquire acquire, align
   // CHECK: = cmpxchg {{.*}} release monotonic, align
+  // CHECK: = cmpxchg {{.*}} release acquire, align
   // CHECK: = cmpxchg weak {{.*}} release monotonic, align
+  // CHECK: = cmpxchg weak {{.*}} release acquire, align
   // CHECK: = cmpxchg {{.*}} acq_rel monotonic, align
   // CHECK: = cmpxchg {{.*}} acq_rel acquire, align
   // CHECK: = cmpxchg weak {{.*}} acq_rel monotonic, align
Index: clang/test/CodeGenOpenCL/atomic-ops.cl
===================================================================
--- clang/test/CodeGenOpenCL/atomic-ops.cl
+++ clang/test/CodeGenOpenCL/atomic-ops.cl
@@ -227,6 +227,7 @@
 
   // CHECK: [[RELEASE]]
   // CHECK: switch {{.*}}, label %[[RELEASE_MONOTONIC:[0-9a-zA-Z._]+]] [
+  // CHECK-NEXT: i32 2, label %[[RELEASE_ACQUIRE:[0-9a-zA-Z._]+]]
   // CHECK-NEXT: ]
 
   // CHECK: [[ACQREL]]
@@ -254,6 +255,14 @@
   // CHECK: cmpxchg {{.*}} acquire acquire, align 4
   // CHECK: br
 
+  // CHECK: [[RELEASE_MONOTONIC]]
+  // CHECK: cmpxchg {{.*}} release monotonic, align 4
+  // CHECK: br
+
+  // CHECK: [[RELEASE_ACQUIRE]]
+  // CHECK: cmpxchg {{.*}} release acquire, align 4
+  // CHECK: br
+
   // CHECK: [[ACQREL_MONOTONIC]]
   // CHECK: cmpxchg {{.*}} acq_rel monotonic, align 4
   // CHECK: br
Index: llvm/docs/LangRef.rst
===================================================================
--- llvm/docs/LangRef.rst
+++ llvm/docs/LangRef.rst
@@ -9768,8 +9768,7 @@
 
 The success and failure :ref:`ordering <ordering>` arguments specify how this
 ``cmpxchg`` synchronizes with other atomic operations. Both ordering parameters
-must be at least ``monotonic``, the ordering constraint on failure must be no
-stronger than that on success, and the failure ordering cannot be either
+must be at least ``monotonic``, the failure ordering cannot be either
 ``release`` or ``acq_rel``.
 
 A ``cmpxchg`` instruction can also take an optional