[llvm] [LLVM] Add `llvm.masked.compress` intrinsic (PR #92289)

[Craig Topper via llvm-commits]

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@@ -7197,6 +7200,47 @@ SDValue SelectionDAG::getNode(unsigned Opcode, const SDLoc &DL, EVT VT,
       return N1.getOperand(1);
     break;
   }
+  case ISD::MCOMPRESS: {
+    EVT VecVT = N1.getValueType();
+    [[maybe_unused]] EVT MaskVT = N2.getValueType();
+    assert(VT == VecVT && "Vector and result type don't match.");
+    assert(VecVT.isVector() && MaskVT.isVector() &&
+           "Both inputs must be vectors.");
+    assert(VecVT.isScalableVector() == MaskVT.isScalableVector() &&
+           "Inputs must both be either fixed or scalable vectors.");
+    assert(VecVT.getVectorElementCount() == MaskVT.getVectorElementCount() &&
+           "Vector and mask must have same number of elements.");
+
+    APInt SplatVal;
+    if (ISD::isConstantSplatVector(N2.getNode(), SplatVal))
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topperc wrote:

I'm not sure it makes sense to do a lot of folding in getNode. It would only trigger if the mcompress is created with a constant vector. It's more likely the mask becomes a constant as part of DAG combine which this code won't catch. So I think its better to do this in DAGCominer.

https://github.com/llvm/llvm-project/pull/92289