Index: lib/Transforms/Scalar/EarlyCSE.cpp
===================================================================
--- lib/Transforms/Scalar/EarlyCSE.cpp
+++ lib/Transforms/Scalar/EarlyCSE.cpp
@@ -271,8 +271,11 @@
       return false;
 
     CallInst *CI = dyn_cast<CallInst>(Inst);
-    if (!CI || !CI->onlyReadsMemory())
+
+    // Handle readnone, readonly and writeonly.
+    if (!CI || !(CI->doesNotReadMemory() || CI->onlyReadsMemory()))
       return false;
+
     return true;
   }
 };
@@ -996,7 +999,7 @@
         !(MemInst.isValid() && !MemInst.mayReadFromMemory()))
       LastStore = nullptr;
 
-    // If this is a read-only call, process it.
+    // If this is a read-only or writeonly call, process it.
     if (CallValue::canHandle(Inst)) {
       // If we have an available version of this call, and if it is the right
       // generation, replace this instruction.
@@ -1010,6 +1013,13 @@
           LLVM_DEBUG(dbgs() << "Skipping due to debug counter\n");
           continue;
         }
+        // When handling writeonly function calls we need to update lastStore
+        // and currentGen.
+        auto *CI = dyn_cast<CallInst>(Inst);
+        if (CI && CI->doesNotReadMemory() && !CI->doesNotAccessMemory()) {
+          ++CurrentGeneration;
+          LastStore = nullptr;
+        }
         if (!Inst->use_empty())
           Inst->replaceAllUsesWith(InVal.first);
         removeMSSA(Inst);
@@ -1018,10 +1028,18 @@
         ++NumCSECall;
         continue;
       }
-
       // Otherwise, remember that we have this instruction.
       AvailableCalls.insert(
           Inst, std::pair<Instruction *, unsigned>(Inst, CurrentGeneration));
+
+      // When handling writeonly function calls we need to update lastStore
+      // and currentGen.
+      auto *CI = dyn_cast<CallInst>(Inst);
+      if (CI && CI->doesNotReadMemory() && !CI->doesNotAccessMemory()) {
+        ++CurrentGeneration;
+        LastStore = nullptr;
+      }
+
       continue;
     }
 
Index: test/Transforms/EarlyCSE/memoryssa.ll
===================================================================
--- test/Transforms/EarlyCSE/memoryssa.ll
+++ test/Transforms/EarlyCSE/memoryssa.ll
@@ -106,3 +106,34 @@
   store i32 %v2, i32* @G3
   ret void
 }
+
+;; Check that writeonly call gets CSE'd when using MSSA
+declare i32 @write_only() writeonly
+
+define void @test_write_only(i1 %c, i32* %p) {
+; CHECK-LABEL: entry:
+; CHECK-NOMEMSSA-LABEL: entry:
+entry:
+; CHECK: call i32 @write_only()
+; CHECK-NOMEMSSA: call i32 @write_only()
+  %call = call i32 @write_only()
+; CHECK: %v1 = load i32, i32* @G1
+; CHECK-NOMEMSSA: %v1 = load i32, i32* @G1
+  %v1 = load i32, i32* @G1
+; CHECK-NOT: call i32 @write_only()
+; CHECK-NOMEMSSA: call i32 @write_only()
+  %call2 =  call i32 @write_only()
+; CHECK: store
+; CHECK-NOMEMSSA: store
+  store i32 %v1, i32* @G2
+; CHECK: call i32 @write_only()
+; CHECK-NOMEMSSA: call i32 @write_only()
+  %call3 = call i32 @write_only()
+; CHECK: store
+; CHECK-NOMEMSSA: store
+  store i32 %v1, i32* @G2
+; CHECK: call i32 @write_only()
+; CHECK-NOMEMSSA: call i32 @write_only()
+  %call4 = call i32 @write_only()
+  ret void
+}