[llvm] Add an all-in-one histogram intrinsic, along with lowering for AArch64 (PR #88106)

[Graham Hunter via llvm-commits]

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@@ -6673,6 +6677,56 @@ SDValue AArch64TargetLowering::LowerOperation(SDValue Op,
     return LowerFunnelShift(Op, DAG);
   case ISD::FLDEXP:
     return LowerFLDEXP(Op, DAG);
+  case ISD::EXPERIMENTAL_HISTOGRAM: {
+    // FIXME: Move to another function.
+    // FIXME: Maybe share some code with LowerMGather/Scatter?
+    MaskedHistogramSDNode *HG = cast<MaskedHistogramSDNode>(Op);
+    SDLoc DL(HG);
+    SDValue Chain = HG->getOperand(0);
+    SDValue Inc = HG->getOperand(1);
+    SDValue Mask = HG->getOperand(2);
+    SDValue Ptr = HG->getOperand(3);
+    SDValue Index = HG->getOperand(4);
+    SDValue Scale = HG->getOperand(5);
+
+    EVT IncVT = Inc.getValueType();
+    EVT IndexVT = Index.getValueType();
+    EVT MemVT = EVT::getVectorVT(*DAG.getContext(), IncVT,
+                                 IndexVT.getVectorElementCount());
+    SDValue Zero = DAG.getConstant(0, DL, MVT::i64);
+    SDValue PassThru = DAG.getSplatVector(MemVT, DL, Zero);
+    SDValue IncSplat = DAG.getSplatVector(MemVT, DL, Inc);
+    SDValue Ops[] = {Chain, PassThru, Mask, Ptr, Index, Scale};
+
+    // Set the MMO to load only, rather than load|store.
+    MachineMemOperand *GMMO = HG->getMemOperand();
+    GMMO->setFlags(MachineMemOperand::MOLoad);
+    ISD::MemIndexType IndexType = HG->getIndexType();
+    SDValue Gather =
+                DAG.getMaskedGather(DAG.getVTList(MemVT, MVT::Other), MemVT, DL,
+                                    Ops, HG->getMemOperand(),
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huntergr-arm wrote:

MemSDNodes have a member variable of a pointer to a MachineMemOperand, so to correctly have separate MOLoad semantics for the gather and MOStore semantics for the scatter we need two different MMO instances.

(I'm not sure if it matters if we just let both be `MOLoad | MOStore` at this point, I'm just being a bit cautious)

https://github.com/llvm/llvm-project/pull/88106