Index: lib/Analysis/MemoryDependenceAnalysis.cpp
===================================================================
--- lib/Analysis/MemoryDependenceAnalysis.cpp
+++ lib/Analysis/MemoryDependenceAnalysis.cpp
@@ -370,6 +370,36 @@
   int64_t MemLocOffset = 0;
   unsigned Limit = BlockScanLimit;
   bool isInvariantLoad = false;
+
+  // We must be careful with atomic accesses, as they may allow another thread
+  //   to touch this location, cloberring it. We are conservative: if the
+  //   QueryInst is not a simple (non-atomic) memory access, we automatically
+  //   return getClobber.
+  // If it is simple, we know based on the results of
+  // "Compiler testing via a theory of sound optimisations in the C11/C++11
+  //   memory model" in PLDI 2013, that a non-atomic location can only be
+  //   clobbered between a pair of a release and an acquire action, with no
+  //   access to the location in between.
+  // Here is an example for giving the general intuition behind this rule.
+  // In the following code:
+  //   store x 0;
+  //   release action; [1]
+  //   acquire action; [4]
+  //   %val = load x;
+  // It is unsafe to replace %val by 0 because another thread may be running:
+  //   acquire action; [2]
+  //   store x 42;
+  //   release action; [3]
+  // with synchronization from 1 to 2 and from 3 to 4, resulting in %val
+  // being 42. A key property of this program however is that if either
+  // 1 or 4 were missing, there would be a race between the store of 42
+  // either the store of 0 or the load (making the whole progam racy).
+  // The paper mentionned above shows that the same property is respected
+  // by every program that can detect any optimisation of that kind: either
+  // it is racy (undefined) or there is a release followed by an acquire
+  // between the pair of accesses under consideration.
+  bool HasSeenAcquire = false;
+
   if (isLoad && QueryInst) {
     LoadInst *LI = dyn_cast<LoadInst>(QueryInst);
     if (LI && LI->getMetadata(LLVMContext::MD_invariant_load) != nullptr)
@@ -412,19 +442,21 @@
     // be accessing the location.
     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.
+      // A Monotonic (or higher) load is OK if the query inst is itself not atomic.
+      // An Acquire (or higher) load sets the HasSeenAcquire flag, so that any
+      //   release store will know to return getClobber.
       // FIXME: This is overly conservative.
       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);
+        if (isAtLeastAcquire(LI->getOrdering()))
+          HasSeenAcquire = true;
       }
 
       // FIXME: this is overly conservative.
@@ -490,19 +522,21 @@
 
     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.
+      // A Monotonic store is OK if the query inst is itself not atomic.
+      // A Release (or higher) store further requires that no acquire load
+      //   has been seen.
       // FIXME: This is overly conservative.
       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);
+        if (HasSeenAcquire && isAtLeastRelease(SI->getOrdering()))
+          return MemDepResult::getClobber(SI);
       }
 
       // FIXME: this is overly conservative.
Index: test/Transforms/DeadStoreElimination/atomic.ll
===================================================================
--- test/Transforms/DeadStoreElimination/atomic.ll
+++ test/Transforms/DeadStoreElimination/atomic.ll
@@ -5,7 +5,7 @@
 
 ; Sanity tests for atomic stores.
 ; Note that it turns out essentially every transformation DSE does is legal on
-; atomic ops, just some transformations are not allowed across them. 
+; atomic ops, just some transformations are not allowed across release-acquire pairs.
 
 @x = common global i32 0, align 4
 @y = common global i32 0, align 4
@@ -13,35 +13,32 @@
 declare void @randomop(i32*)
 
 ; DSE across unordered store (allowed)
-define void @test1()  nounwind uwtable ssp {
-; CHECK: test1
+define void @test1() {
+; CHECK-LABEL: test1
 ; CHECK-NOT: store i32 0
 ; CHECK: store i32 1
-entry:
   store i32 0, i32* @x
   store atomic i32 0, i32* @y unordered, align 4
   store i32 1, i32* @x
   ret void
 }
 
-; DSE across seq_cst load (allowed in theory; not implemented ATM)
-define i32 @test2()  nounwind uwtable ssp {
-; CHECK: test2
-; CHECK: store i32 0
+; DSE across seq_cst load (allowed)
+define i32 @test2() {
+; CHECK-LABEL: test2
+; CHECK-NOT: store i32 0
 ; CHECK: store i32 1
-entry:
   store i32 0, i32* @x
   %x = load atomic i32* @y seq_cst, align 4
   store i32 1, i32* @x
   ret i32 %x
 }
 
-; DSE across seq_cst store (store before atomic store must not be removed)
-define void @test3()  nounwind uwtable ssp {
-; CHECK: test3
-; CHECK: store i32
+; DSE across seq_cst store (allowed)
+define void @test3() {
+; CHECK-LABEL: test3
+; CHECK-NOT: store i32 0
 ; CHECK: store atomic i32 2
-entry:
   store i32 0, i32* @x
   store atomic i32 2, i32* @y seq_cst, align 4
   store i32 1, i32* @x
@@ -49,32 +46,29 @@
 }
 
 ; DSE remove unordered store (allowed)
-define void @test4()  nounwind uwtable ssp {
-; CHECK: test4
+define void @test4() {
+; CHECK-LABEL: test4
 ; CHECK-NOT: store atomic
 ; CHECK: store i32 1
-entry:
   store atomic i32 0, i32* @x unordered, align 4
   store i32 1, i32* @x
   ret void
 }
 
 ; DSE unordered store overwriting non-atomic store (allowed)
-define void @test5()  nounwind uwtable ssp {
-; CHECK: test5
+define void @test5() {
+; CHECK-LABEL: test5
 ; CHECK: store atomic i32 1
-entry:
   store i32 0, i32* @x
   store atomic i32 1, i32* @x unordered, align 4
   ret void
 }
 
 ; DSE no-op unordered atomic store (allowed)
-define void @test6()  nounwind uwtable ssp {
-; CHECK: test6
+define void @test6() {
+; CHECK-LABEL: test6
 ; CHECK-NOT: store
 ; CHECK: ret void
-entry:
   %x = load atomic i32* @x unordered, align 4
   store atomic i32 %x, i32* @x unordered, align 4
   ret void
@@ -82,10 +76,9 @@
 
 ; DSE seq_cst store (be conservative; DSE doesn't have infrastructure
 ; to reason about atomic operations).
-define void @test7()  nounwind uwtable ssp {
-; CHECK: test7
-; CHECK: store atomic 
-entry:
+define void @test7() {
+; CHECK-LABEL: test7
+; CHECK: store atomic
   %a = alloca i32
   store atomic i32 0, i32* %a seq_cst, align 4
   ret void
@@ -93,11 +86,10 @@
 
 ; DSE and seq_cst load (be conservative; DSE doesn't have infrastructure
 ; to reason about atomic operations).
-define i32 @test8()  nounwind uwtable ssp {
-; CHECK: test8
+define i32 @test8() {
+; CHECK-LABEL: test8
 ; CHECK: store
-; CHECK: load atomic 
-entry:
+; CHECK: load atomic
   %a = alloca i32
   call void @randomop(i32* %a)
   store i32 0, i32* %a, align 4
@@ -106,11 +98,10 @@
 }
 
 ; DSE across monotonic load (allowed as long as the eliminated store isUnordered)
-define i32 @test9()  nounwind uwtable ssp {
-; CHECK: test9
+define i32 @test9() {
+; CHECK-LABEL: 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
@@ -118,11 +109,10 @@
 }
 
 ; DSE across monotonic store (allowed as long as the eliminated store isUnordered)
-define void @test10()  nounwind uwtable ssp {
-; CHECK: test10
+define void @test10() {
+; CHECK-LABEL: 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
@@ -130,11 +120,10 @@
 }
 
 ; DSE across monotonic load (forbidden since the eliminated store is atomic)
-define i32 @test11()  nounwind uwtable ssp {
-; CHECK: test11
+define i32 @test11() {
+; CHECK-LABEL: 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
@@ -142,13 +131,48 @@
 }
 
 ; DSE across monotonic store (forbidden since the eliminated store is atomic)
-define void @test12()  nounwind uwtable ssp {
-; CHECK: test12
+define void @test12() {
+; CHECK-LABEL: 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
 }
+
+; DSE is allowed across a pair of an atomic read and then write.
+define i32 @test13() {
+; CHECK-LABEL: test13
+; CHECK-NOT: store i32 0
+; CHECK: store i32 1
+  store i32 0, i32* @x
+  %x = load atomic i32* @y seq_cst, align 4
+  store atomic i32 %x, i32* @y seq_cst, align 4
+  store i32 1, i32* @x
+  ret i32 %x
+}
+
+; Same if it is acquire-release instead of seq_cst/seq_cst
+define i32 @test14() {
+; CHECK-LABEL: test14
+; CHECK-NOT: store i32 0
+; CHECK: store i32 1
+  store i32 0, i32* @x
+  %x = load atomic i32* @y acquire, align 4
+  store atomic i32 %x, i32* @y release, align 4
+  store i32 1, i32* @x
+  ret i32 %x
+}
+
+; But DSE is not allowed across a release-acquire pair.
+define i32 @test15() {
+; CHECK-LABEL: test15
+; CHECK: store i32 0
+; CHECK: store i32 1
+  store i32 0, i32* @x
+  store atomic i32 0, i32* @y release, align 4
+  %x = load atomic i32* @y acquire, align 4
+  store i32 1, i32* @x
+  ret i32 %x
+}
Index: test/Transforms/GVN/atomic.ll
===================================================================
--- test/Transforms/GVN/atomic.ll
+++ test/Transforms/GVN/atomic.ll
@@ -8,7 +8,7 @@
 
 ; GVN across unordered store (allowed)
 define i32 @test1() nounwind uwtable ssp {
-; CHECK: test1
+; CHECK-LABEL: test1
 ; CHECK: add i32 %x, %x
 entry:
   %x = load i32* @y
@@ -18,10 +18,10 @@
   ret i32 %z
 }
 
-; GVN across seq_cst store (allowed in theory; not implemented ATM)
+; GVN across seq_cst store (allowed)
 define i32 @test2() nounwind uwtable ssp {
-; CHECK: test2
-; CHECK: add i32 %x, %y
+; CHECK-LABEL: test2
+; CHECK: add i32 %x, %x
 entry:
   %x = load i32* @y
   store atomic i32 %x, i32* @x seq_cst, align 4
@@ -32,7 +32,7 @@
 
 ; GVN across unordered load (allowed)
 define i32 @test3() nounwind uwtable ssp {
-; CHECK: test3
+; CHECK-LABEL: test3
 ; CHECK: add i32 %x, %x
 entry:
   %x = load i32* @y
@@ -43,11 +43,11 @@
   ret i32 %b
 }
 
-; GVN across acquire load (load after atomic load must not be removed)
+; GVN across acquire load (allowed as the original load was not atomic)
 define i32 @test4() nounwind uwtable ssp {
-; CHECK: test4
+; CHECK-LABEL: test4
 ; CHECK: load atomic i32* @x
-; CHECK: load i32* @y
+; CHECK-NOT: load i32* @y
 entry:
   %x = load i32* @y
   %y = load atomic i32* @x seq_cst, align 4
@@ -59,7 +59,7 @@
 
 ; GVN load to unordered load (allowed)
 define i32 @test5() nounwind uwtable ssp {
-; CHECK: test5
+; CHECK-LABEL: test5
 ; CHECK: add i32 %x, %x
 entry:
   %x = load atomic i32* @x unordered, align 4
@@ -70,7 +70,7 @@
 
 ; GVN unordered load to load (unordered load must not be removed)
 define i32 @test6() nounwind uwtable ssp {
-; CHECK: test6
+; CHECK-LABEL: test6
 ; CHECK: load atomic i32* @x unordered
 entry:
   %x = load i32* @x
@@ -79,9 +79,35 @@
   ret i32 %x3
 }
 
-; GVN across monotonic store (allowed)
+; GVN across release-acquire pair (forbidden)
 define i32 @test7() nounwind uwtable ssp {
-; CHECK: test7
+; CHECK-LABEL: test7
+; CHECK: add i32 %x, %y
+entry:
+  %x = load i32* @y
+  store atomic i32 %x, i32* @x release, align 4
+  %w = load atomic i32* @x acquire, align 4
+  %y = load i32* @y
+  %z = add i32 %x, %y
+  ret i32 %z
+}
+
+; GVN across acquire-release pair (allowed)
+define i32 @test8() nounwind uwtable ssp {
+; CHECK-LABEL: test8
+; CHECK: add i32 %x, %x
+entry:
+  %x = load i32* @y
+  %w = load atomic i32* @x acquire, align 4
+  store atomic i32 %x, i32* @x release, align 4
+  %y = load i32* @y
+  %z = add i32 %x, %y
+  ret i32 %z
+}
+
+; GVN across monotonic store (allowed)
+define i32 @test9() nounwind uwtable ssp {
+; CHECK-LABEL: test9
 ; CHECK: add i32 %x, %x
 entry:
   %x = load i32* @y
@@ -92,8 +118,8 @@
 }
 
 ; GVN of an unordered across monotonic load (not allowed)
-define i32 @test8() nounwind uwtable ssp {
-; CHECK: test8
+define i32 @test10() nounwind uwtable ssp {
+; CHECK-LABEL: test10
 ; CHECK: add i32 %x, %y
 entry:
   %x = load atomic i32* @y unordered, align 4
@@ -103,4 +129,3 @@
   ret i32 %z
 }
 
-