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

[Simon Pilgrim via llvm-commits]

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@@ -12078,6 +12080,57 @@ SDValue DAGCombiner::visitVP_STRIDED_STORE(SDNode *N) {
   return SDValue();
 }
 
+SDValue DAGCombiner::visitMASKED_COMPRESS(SDNode *N) {
+  SDLoc DL(N);
+  SDValue Vec = N->getOperand(0);
+  SDValue Mask = N->getOperand(1);
+  SDValue Passthru = N->getOperand(2);
+  EVT VecVT = Vec.getValueType();
+
+  bool HasPassthru = !Passthru.isUndef();
+
+  APInt SplatVal;
+  if (ISD::isConstantSplatVector(Mask.getNode(), SplatVal)) {
+    return TLI.isConstTrueVal(Mask)
+               ? Vec
+               : (HasPassthru ? Passthru : DAG.getUNDEF(VecVT));
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RKSimon wrote:

`return TLI.isConstTrueVal(Mask) ? Vec : Passthru;` - I'm not sure if you gain anything from recreating the UNDEF now

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