[X86] LowerATOMIC_STORE - on 32-bit targets see if i64 values were originally legal f64 values that we can store directly. (#171602)

Based off feedback from #171478

Author: RKSimon

llvm/lib/Target/X86/X86ISelLowering.cpp

@@ -33171,7 +33171,14 @@ static SDValue LowerATOMIC_STORE(SDValue Op, SelectionDAG &DAG,
     // For illegal i64 atomic_stores, we can try to use MOVQ or MOVLPS if SSE
     // is enabled.
     if (VT == MVT::i64) {
-      if (Subtarget.hasSSE1()) {
+      SDValue BCValue = peekThroughBitcasts(Node->getVal());
+      if (BCValue.getValueType() == MVT::f64 &&
+          (Subtarget.hasX87() || Subtarget.hasSSE2())) {
+        // If the i64 was bitcast from a f64 then we can do the f64 atomic store
+        // directly with FSTPL/MOVSD.
+        Chain = DAG.getStore(Node->getChain(), dl, BCValue, Node->getBasePtr(),
+                             Node->getMemOperand());
+      } else if (Subtarget.hasSSE1()) {
         SDValue SclToVec =
             DAG.getNode(ISD::SCALAR_TO_VECTOR, dl, MVT::v2i64, Node->getVal());
         MVT StVT = Subtarget.hasSSE2() ? MVT::v2i64 : MVT::v4f32;