[libcxxabi] [clang] [openmp] [compiler-rt] [llvm] [libc] [libcxx] [clang-tools-extra] [flang] [mlir] [AArch64] Combine store (trunc X to <3 x i8>) to sequence of ST1.b. (PR #78637)

[Eli Friedman via llvm-commits]

================
@@ -21471,6 +21471,53 @@ bool isHalvingTruncateOfLegalScalableType(EVT SrcVT, EVT DstVT) {
          (SrcVT == MVT::nxv2i64 && DstVT == MVT::nxv2i32);
 }
 
+// Combine store (trunc X to <3 x i8>) to sequence of ST1.b.
+static SDValue combineI8TruncStore(StoreSDNode *ST, SelectionDAG &DAG,
+                                   const AArch64Subtarget *Subtarget) {
+  SDValue Value = ST->getValue();
+  EVT ValueVT = Value.getValueType();
+
+  if (ST->isVolatile() || !Subtarget->isLittleEndian() ||
+      Value.getOpcode() != ISD::TRUNCATE ||
+      ValueVT != EVT::getVectorVT(*DAG.getContext(), MVT::i8, 3))
+    return SDValue();
+
+  assert(ST->getOffset().isUndef() && "undef offset expected");
+  SDLoc DL(ST);
+  auto WideVT = EVT::getVectorVT(
+      *DAG.getContext(),
+      Value->getOperand(0).getValueType().getVectorElementType(), 4);
+  SDValue UndefVector = DAG.getUNDEF(WideVT);
+  SDValue WideTrunc = DAG.getNode(
+      ISD::INSERT_SUBVECTOR, DL, WideVT,
+      {UndefVector, Value->getOperand(0), DAG.getVectorIdxConstant(0, DL)});
+  SDValue Cast = DAG.getNode(
+      ISD::BITCAST, DL, WideVT.getSizeInBits() == 64 ? MVT::v8i8 : MVT::v16i8,
+      WideTrunc);
+
+  MachineFunction &MF = DAG.getMachineFunction();
+  SDValue Chain = ST->getChain();
+  MachineMemOperand *MMO = ST->getMemOperand();
+  unsigned IdxScale = WideVT.getScalarSizeInBits() / 8;
+  SDValue E2 = DAG.getNode(ISD::EXTRACT_VECTOR_ELT, DL, MVT::i8, Cast,
+                           DAG.getConstant(2 * IdxScale, DL, MVT::i64));
+  TypeSize Offset2 = TypeSize::getFixed(2);
+  SDValue Ptr2 = DAG.getMemBasePlusOffset(ST->getBasePtr(), Offset2, DL);
+  Chain = DAG.getStore(Chain, DL, E2, Ptr2, MF.getMachineMemOperand(MMO, 2, 1));
+
+  SDValue E1 = DAG.getNode(ISD::EXTRACT_VECTOR_ELT, DL, MVT::i8, Cast,
+                           DAG.getConstant(1 * IdxScale, DL, MVT::i64));
+  TypeSize Offset1 = TypeSize::getFixed(1);
+  SDValue Ptr1 = DAG.getMemBasePlusOffset(ST->getBasePtr(), Offset1, DL);
+  Chain = DAG.getStore(Chain, DL, E1, Ptr1, MF.getMachineMemOperand(MMO, 1, 1));
+
+  SDValue E0 = DAG.getNode(ISD::EXTRACT_VECTOR_ELT, DL, MVT::i8, Cast,
+                           DAG.getConstant(0, DL, MVT::i64));
+  Chain = DAG.getStore(Chain, DL, E0, ST->getBasePtr(), ST->getMemOperand());
----------------
efriedma-quic wrote:

Missing getMachineMemOperand call here?  (The MachineMemOperand for the original store has the wrong size.)

https://github.com/llvm/llvm-project/pull/78637