[llvm] DAG: Simplify demanded bits for truncating atomic_store (PR #90113)

[Jay Foad via llvm-commits]

================
@@ -21096,6 +21098,24 @@ SDValue DAGCombiner::replaceStoreOfInsertLoad(StoreSDNode *ST) {
                       ST->getMemOperand()->getFlags());
 }
 
+SDValue DAGCombiner::visitATOMIC_STORE(SDNode *N) {
+  AtomicSDNode *ST = cast<AtomicSDNode>(N);
+  SDValue Val = ST->getVal();
+  EVT VT = Val.getValueType();
+  EVT MemVT = ST->getMemoryVT();
+
+  if (MemVT.bitsLT(VT)) { // Is truncating store
+    APInt TruncDemandedBits = APInt::getLowBitsSet(VT.getScalarSizeInBits(),
+                                                   MemVT.getScalarSizeInBits());
+    // See if we can simplify the operation with SimplifyDemandedBits, which
+    // only works if the value has a single use.
+    if (SimplifyDemandedBits(Val, TruncDemandedBits))
----------------
jayfoad wrote:

Do you need any of the worklist fiddling that the corresponding code in visitSTORE does?

https://github.com/llvm/llvm-project/pull/90113