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

[Simon Pilgrim via llvm-commits]

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@@ -7510,6 +7512,94 @@ LegalizerHelper::lowerShuffleVector(MachineInstr &MI) {
   return Legalized;
 }
 
+LegalizerHelper::LegalizeResult
+LegalizerHelper::lowerMASKED_COMPRESS(llvm::MachineInstr &MI) {
+  auto [Dst, DstTy, Vec, VecTy, Mask, MaskTy, Passthru, PassthruTy] =
+      MI.getFirst4RegLLTs();
+
+  if (VecTy.isScalableVector())
+    report_fatal_error("Cannot expand masked_compress for scalable vectors.");
+
+  Align VecAlign = getStackTemporaryAlignment(VecTy);
+  MachinePointerInfo PtrInfo;
+  Register StackPtr =
+      createStackTemporary(TypeSize::getFixed(VecTy.getSizeInBytes()), VecAlign,
+                           PtrInfo)
+          .getReg(0);
+  MachinePointerInfo ValPtrInfo =
+      MachinePointerInfo::getUnknownStack(*MI.getMF());
+
+  LLT IdxTy = LLT::scalar(32);
+  LLT ValTy = VecTy.getElementType();
+  Align ValAlign = getStackTemporaryAlignment(ValTy);
+
+  auto OutPos = MIRBuilder.buildConstant(IdxTy, 0);
+
+  bool HasPassthru =
+      MRI.getVRegDef(Passthru)->getOpcode() != TargetOpcode::G_IMPLICIT_DEF;
+
+  if (HasPassthru)
+    MIRBuilder.buildStore(Passthru, StackPtr, PtrInfo, VecAlign);
+
+  Register LastWriteVal;
+  std::optional<APInt> PassthruSplatVal =
+      isConstantOrConstantSplatVector(*MRI.getVRegDef(Passthru), MRI);
+  ;
----------------
RKSimon wrote:

superfluous

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