Index: lib/Analysis/MemoryDependenceAnalysis.cpp
===================================================================
--- lib/Analysis/MemoryDependenceAnalysis.cpp
+++ lib/Analysis/MemoryDependenceAnalysis.cpp
@@ -407,10 +407,29 @@
 
     // Values depend on loads if the pointers are must aliased.  This means that
     // a load depends on another must aliased load from the same value.
+    // One exception is atomic loads: a value can depend on an atomic load that it
+    // does not alias with.
     if (LoadInst *LI = dyn_cast<LoadInst>(Inst)) {
       // Atomic loads have complications involved.
+      // A monotonic load is OK if the query inst is itself not atomic.
       // FIXME: This is overly conservative.
-      if (!LI->isUnordered())
+      if (!LI->isUnordered()) {
+        if (!QueryInst)
+          return MemDepResult::getClobber(LI);
+        if (LI->getOrdering() != Monotonic)
+          return MemDepResult::getClobber(LI);
+        if (auto *QueryLI = dyn_cast<LoadInst>(QueryInst))
+          if (!QueryLI->isSimple())
+            return MemDepResult::getClobber(LI);
+        if (auto *QuerySI = dyn_cast<StoreInst>(QueryInst))
+          if (!QuerySI->isSimple())
+            return MemDepResult::getClobber(LI);
+      }
+
+      // FIXME: this is overly conservative.
+      // While volatile access cannot be eliminated, they do not have to kill
+      // optimisation in that way.
+      if (LI->isVolatile())
         return MemDepResult::getClobber(LI);
 
       AliasAnalysis::Location LoadLoc = AA->getLocation(LI);
@@ -469,8 +488,25 @@
 
     if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) {
       // Atomic stores have complications involved.
+      // A monotonic store is OK if the query inst is itself not atomic.
       // FIXME: This is overly conservative.
-      if (!SI->isUnordered())
+      if (!SI->isUnordered()) {
+        if (!QueryInst)
+          return MemDepResult::getClobber(SI);
+        if (SI->getOrdering() != Monotonic)
+          return MemDepResult::getClobber(SI);
+        if (auto *QueryLI = dyn_cast<LoadInst>(QueryInst))
+          if (!QueryLI->isSimple())
+            return MemDepResult::getClobber(SI);
+        if (auto *QuerySI = dyn_cast<StoreInst>(QueryInst))
+          if (!QuerySI->isSimple())
+            return MemDepResult::getClobber(SI);
+      }
+
+      // FIXME: this is overly conservative.
+      // While volatile access cannot be eliminated, they do not have to kill
+      // optimisation in that way.
+      if (SI->isVolatile())
         return MemDepResult::getClobber(SI);
 
       // If alias analysis can tell that this store is guaranteed to not modify
Index: test/Transforms/DeadStoreElimination/atomic.ll
===================================================================
--- test/Transforms/DeadStoreElimination/atomic.ll
+++ test/Transforms/DeadStoreElimination/atomic.ll
@@ -105,3 +105,50 @@
   ret i32 %x
 }
 
+; DSE across monotonic load (allowed as long as the eliminated store isUnordered)
+define i32 @test9()  nounwind uwtable ssp {
+; CHECK: test9
+; CHECK-NOT: store i32 0
+; CHECK: store i32 1
+entry:
+  store i32 0, i32* @x
+  %x = load atomic i32* @y monotonic, align 4
+  store i32 1, i32* @x
+  ret i32 %x
+}
+
+; DSE across monotonic store (allowed as long as the eliminated store isUnordered)
+define void @test10()  nounwind uwtable ssp {
+; CHECK: test10
+; CHECK-NOT: store i32 0
+; CHECK: store i32 1
+entry:
+  store i32 0, i32* @x
+  store atomic i32 42, i32* @y monotonic, align 4
+  store i32 1, i32* @x
+  ret void
+}
+
+; DSE across monotonic load (forbidden since the eliminated store is atomic)
+define i32 @test11()  nounwind uwtable ssp {
+; CHECK: test11
+; CHECK: store atomic i32 0
+; CHECK: store atomic i32 1
+entry:
+  store atomic i32 0, i32* @x monotonic, align 4
+  %x = load atomic i32* @y monotonic, align 4
+  store atomic i32 1, i32* @x monotonic, align 4
+  ret i32 %x
+}
+
+; DSE across monotonic store (forbidden since the eliminated store is atomic)
+define void @test12()  nounwind uwtable ssp {
+; CHECK: test12
+; CHECK: store atomic i32 0
+; CHECK: store atomic i32 1
+entry:
+  store atomic i32 0, i32* @x monotonic, align 4
+  store atomic i32 42, i32* @y monotonic, align 4
+  store atomic i32 1, i32* @x monotonic, align 4
+  ret void
+}
Index: test/Transforms/GVN/atomic.ll
===================================================================
--- test/Transforms/GVN/atomic.ll
+++ test/Transforms/GVN/atomic.ll
@@ -78,3 +78,29 @@
   %x3 = add i32 %x, %x2
   ret i32 %x3
 }
+
+; GVN across monotonic store (allowed)
+define i32 @test7() nounwind uwtable ssp {
+; CHECK: test7
+; CHECK: add i32 %x, %x
+entry:
+  %x = load i32* @y
+  store atomic i32 %x, i32* @x monotonic, align 4
+  %y = load i32* @y
+  %z = add i32 %x, %y
+  ret i32 %z
+}
+
+; GVN of an unordered across monotonic load (not allowed)
+define i32 @test8() nounwind uwtable ssp {
+; CHECK: test8
+; CHECK: add i32 %x, %y
+entry:
+  %x = load atomic i32* @y unordered, align 4
+  %clobber = load atomic i32* @x monotonic, align 4
+  %y = load atomic i32* @y monotonic, align 4
+  %z = add i32 %x, %y
+  ret i32 %z
+}
+
+