Index: llvm/lib/Analysis/MemoryDependenceAnalysis.cpp
===================================================================
--- llvm/lib/Analysis/MemoryDependenceAnalysis.cpp
+++ llvm/lib/Analysis/MemoryDependenceAnalysis.cpp
@@ -424,6 +424,16 @@
     return false;
   };
 
+  // Return "true" if and only if the instruction I is either a non-unordered
+  // load or a non-unordered store.
+  auto isNonUnorderedLoadOrStore = [](Instruction *I) -> bool {
+    if (auto *LI = dyn_cast<LoadInst>(I))
+      return !LI->isUnordered();
+    if (auto *SI = dyn_cast<StoreInst>(I))
+      return !SI->isUnordered();
+    return false;
+  };
+
   // Return "true" if I is not a load and not a store, but it does access
   // memory.
   auto isOtherMemAccess = [](Instruction *I) -> bool {
@@ -549,11 +559,18 @@
       // A Monotonic store is OK if the query inst is itself not atomic.
       // FIXME: This is overly conservative.
       if (!SI->isUnordered() && SI->isAtomic()) {
-        if (!QueryInst || isNonSimpleLoadOrStore(QueryInst) ||
+        if (!QueryInst || isNonUnorderedLoadOrStore(QueryInst) ||
             isOtherMemAccess(QueryInst))
           return MemDepResult::getClobber(SI);
-        if (SI->getOrdering() != AtomicOrdering::Monotonic)
-          return MemDepResult::getClobber(SI);
+        // Ok, if we are here the guard above guarantee us that
+        // QueryInst is a non-atomic or unordered load/store.
+        // SI is atomic with monotonic or release semantic (seq_cst for store
+        // is actually a release semantic plus total order over other seq_cst
+        // instructions, as soon as QueryInst is not seq_cst we can consider it
+        // as simple release semantic).
+        // Monotonic and Release semantic allows re-ordering before store
+        // so we are safe to go further and check the aliasing. It will prohibit
+        // re-ordering in case locations are may or must alias.
       }
 
       // While volatile access cannot be eliminated, they do not have to clobber
Index: llvm/test/Analysis/MemoryDependenceAnalysis/reorder-over-store-atomic.ll
===================================================================
--- /dev/null
+++ llvm/test/Analysis/MemoryDependenceAnalysis/reorder-over-store-atomic.ll
@@ -0,0 +1,47 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -S -gvn -basic-aa < %s | FileCheck %s
+
+@u = global i32 5, align 4
+@w = global i32 10, align 4
+
+define i32 @test_load() {
+; CHECK-LABEL: @test_load(
+; CHECK-NEXT:    [[L1:%.*]] = load atomic i32, i32* @w unordered, align 4
+; CHECK-NEXT:    [[LV:%.*]] = load atomic i32, i32* @u seq_cst, align 4
+; CHECK-NEXT:    [[L2:%.*]] = load atomic i32, i32* @w unordered, align 4
+; CHECK-NEXT:    [[RES_1:%.*]] = sub i32 [[L1]], [[L2]]
+; CHECK-NEXT:    [[RES:%.*]] = add i32 [[RES_1]], [[LV]]
+; CHECK-NEXT:    ret i32 [[RES]]
+;
+  %l1 = load atomic i32, i32* @w unordered, align 4
+  %lv = load atomic i32, i32* @u seq_cst, align 4
+  %l2 = load atomic i32, i32* @w unordered, align 4
+  %res.1 = sub i32 %l1, %l2
+  %res = add i32 %res.1, %lv
+  ret i32 %res
+}
+
+define i32 @test_store(i32 %x) {
+; CHECK-LABEL: @test_store(
+; CHECK-NEXT:    store atomic i32 [[X:%.*]], i32* @u seq_cst, align 4
+; CHECK-NEXT:    ret i32 0
+;
+  %l1 = load atomic i32, i32* @w unordered, align 4
+  store atomic i32 %x, i32* @u seq_cst, align 4
+  %l2 = load atomic i32, i32* @w unordered, align 4
+  %res = sub i32 %l1, %l2
+  ret i32 %res
+}
+
+define i32 @test_store_store(i32 %x) {
+; CHECK-LABEL: @test_store_store(
+; CHECK-NEXT:    store atomic i32 [[X:%.*]], i32* @w unordered, align 4
+; CHECK-NEXT:    store atomic i32 [[X]], i32* @u release, align 4
+; CHECK-NEXT:    store atomic i32 0, i32* @w unordered, align 4
+; CHECK-NEXT:    ret i32 0
+;
+  store atomic i32 %x, i32* @w unordered, align 4
+  store atomic i32 %x, i32* @u release, align 4
+  store atomic i32 0, i32* @w unordered, align 4
+  ret i32 0
+}