Index: llvm/trunk/lib/Transforms/InstCombine/InstCombineAtomicRMW.cpp
===================================================================
--- llvm/trunk/lib/Transforms/InstCombine/InstCombineAtomicRMW.cpp
+++ llvm/trunk/lib/Transforms/InstCombine/InstCombineAtomicRMW.cpp
@@ -57,15 +57,23 @@
 }
 
 /// Return true if the given instruction always produces a value in memory
-/// equivelent to its value operand.
+/// equivalent to its value operand.
 bool isSaturating(AtomicRMWInst& RMWI) {
+  if (auto CF = dyn_cast<ConstantFP>(RMWI.getValOperand()))
+    switch(RMWI.getOperation()) {
+    case AtomicRMWInst::FAdd:
+    case AtomicRMWInst::FSub:
+      return CF->isNaN();
+    default:
+      return false;
+    };
+
   auto C = dyn_cast<ConstantInt>(RMWI.getValOperand());
   if(!C)
     return false;
 
   switch(RMWI.getOperation()) {
   default:
-    // TODO: fadd, fsub w/Nan
     return false;
   case AtomicRMWInst::Xchg:
     return true;
Index: llvm/trunk/test/Transforms/InstCombine/atomicrmw.ll
===================================================================
--- llvm/trunk/test/Transforms/InstCombine/atomicrmw.ll
+++ llvm/trunk/test/Transforms/InstCombine/atomicrmw.ll
@@ -214,7 +214,7 @@
 }
 
 ; CHECK-LABEL: sat_fadd_nan
-; CHECK-NEXT: %res = atomicrmw fadd double* %addr, double 0x7FF00000FFFFFFFF release
+; CHECK-NEXT: %res = atomicrmw xchg double* %addr, double 0x7FF00000FFFFFFFF release
 ; CHECK-NEXT: ret double %res
 define double @sat_fadd_nan(double* %addr) {
   %res = atomicrmw fadd double* %addr, double 0x7FF00000FFFFFFFF release
@@ -222,7 +222,7 @@
 }
 
 ; CHECK-LABEL: sat_fsub_nan
-; CHECK-NEXT: %res = atomicrmw fsub double* %addr, double 0x7FF00000FFFFFFFF release
+; CHECK-NEXT: %res = atomicrmw xchg double* %addr, double 0x7FF00000FFFFFFFF release
 ; CHECK-NEXT: ret double %res
 define double @sat_fsub_nan(double* %addr) {
   %res = atomicrmw fsub double* %addr, double 0x7FF00000FFFFFFFF release
@@ -230,7 +230,7 @@
 }
 
 ; CHECK-LABEL: sat_fsub_nan_unused
-; CHECK-NEXT: atomicrmw fsub double* %addr, double 0x7FF00000FFFFFFFF monotonic
+; CHECK-NEXT: store atomic double 0x7FF00000FFFFFFFF, double* %addr monotonic, align 8
 ; CHECK-NEXT: ret void
 define void @sat_fsub_nan_unused(double* %addr) {
   atomicrmw fsub double* %addr, double 0x7FF00000FFFFFFFF monotonic