[llvm] [AArch64][SVE] Fold integer lane extract and store to FPR store (PR #129756)

[Sander de Smalen via llvm-commits]

================
@@ -23946,17 +23960,62 @@ static SDValue performSTORECombine(SDNode *N,
   if (SDValue Store = combineBoolVectorAndTruncateStore(DAG, ST))
     return Store;
 
-  if (ST->isTruncatingStore()) {
-    EVT StoreVT = ST->getMemoryVT();
-    if (!isHalvingTruncateOfLegalScalableType(ValueVT, StoreVT))
-      return SDValue();
+  if (ST->isTruncatingStore() &&
+      isHalvingTruncateOfLegalScalableType(ValueVT, MemVT)) {
     if (SDValue Rshrnb =
             trySimplifySrlAddToRshrnb(ST->getOperand(1), DAG, Subtarget)) {
       return DAG.getTruncStore(ST->getChain(), ST, Rshrnb, ST->getBasePtr(),
-                               StoreVT, ST->getMemOperand());
+                               MemVT, ST->getMemOperand());
     }
   }
 
+  // This is an integer vector_extract_elt followed by a (possibly truncating)
+  // store. We may be able to replace this with a store of an FP subregister.
+  if (DCI.isAfterLegalizeDAG() && ST->isUnindexed() &&
+      Value.getOpcode() == ISD::EXTRACT_VECTOR_ELT) {
+
+    SDValue Vector = Value.getOperand(0);
+    SDValue ExtIdx = Value.getOperand(1);
+    EVT VectorVT = Vector.getValueType();
+    EVT ElemVT = VectorVT.getVectorElementType();
+    if (!ValueVT.isInteger() || ElemVT == MVT::i8 || MemVT == MVT::i8)
+      return SDValue();
+    if (ValueVT != MemVT && !ST->isTruncatingStore())
+      return SDValue();
+
+    // Heuristic: If there are other users of integer scalars extracted from
+    // this vector that won't fold into the store -- abandon folding. This may
+    // extend the vector lifetime and disrupt paired stores.
+    for (auto Use = Vector->use_begin(), End = Vector->use_end(); Use != End;
+         ++Use) {
+      if (Use->getResNo() != Vector.getResNo())
+        continue;
+      SDNode *User = Use->getUser();
----------------
sdesmalen-arm wrote:

nit:
```suggestion
    for (const auto &Use : Vector->uses()) {
      if (Use.getResNo() != Vector.getResNo())
        continue;
      const SDNode *User = Use.getUser();
```

https://github.com/llvm/llvm-project/pull/129756