[llvm] [Intrinsics][AArch64] Add intrinsic to mask off aliasing vector lanes (PR #117007)

[Nicholas Guy via llvm-commits]

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@@ -8284,6 +8284,50 @@ void SelectionDAGBuilder::visitIntrinsicCall(const CallInst &I,
     visitVectorExtractLastActive(I, Intrinsic);
     return;
   }
+  case Intrinsic::experimental_get_alias_lane_mask: {
+    SDValue SourceValue = getValue(I.getOperand(0));
+    SDValue SinkValue = getValue(I.getOperand(1));
+    SDValue EltSize = getValue(I.getOperand(2));
+    bool IsWriteAfterRead = cast<ConstantSDNode>(getValue(I.getOperand(3)))->getZExtValue() != 0;
+    auto IntrinsicVT = EVT::getEVT(I.getType());
+    auto PtrVT = SourceValue->getValueType(0);
+
+    if (!TLI.shouldExpandGetAliasLaneMask(IntrinsicVT, PtrVT, cast<ConstantSDNode>(EltSize)->getSExtValue())) {
+      visitTargetIntrinsic(I, Intrinsic);
+      return;
+    }
+
+    SDValue Diff = DAG.getNode(ISD::SUB, sdl,
+                              PtrVT, SinkValue, SourceValue);
+    if (!IsWriteAfterRead)
+      Diff = DAG.getNode(ISD::ABS, sdl, PtrVT, Diff);
+
+    Diff = DAG.getNode(ISD::SDIV, sdl, PtrVT, Diff, EltSize);
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NickGuy-Arm wrote:

>From my understanding: `!IsWriteAfterRead` would imply that `Diff` is negative, so this inserts the `ISD::ABS` immediately before then using the new `Diff` as an operand to the `SDIV` node, ensuring that it is positive.

Arguably the `if` isn't needed there, as `ABS`-ing a positive number just returns the same number, but then we're adding nodes that we know don't do anything.

https://github.com/llvm/llvm-project/pull/117007