[llvm] [RISCV][TTI] Implement cost of llvm.experimental.vector.extract.last.active (PR #184067)

[Luke Lau via llvm-commits]

================
@@ -1702,6 +1702,35 @@ RISCVTTIImpl::getIntrinsicInstrCost(const IntrinsicCostAttributes &ICA,
                                CmpInst::FCMP_UNO, CostKind);
     return Cost;
   }
+  case Intrinsic::experimental_vector_extract_last_active: {
+    Type *ValTy = ICA.getArgTypes()[0];
+    Type *MaskTy = ICA.getArgTypes()[1];
+
+    auto ValLT = getTypeLegalizationCost(ValTy);
+    auto MaskLT = getTypeLegalizationCost(MaskTy);
+
+    if (!ValLT.first.isValid() || !MaskLT.first.isValid())
+      return InstructionCost::getInvalid();
+
+    // TODO: Return cheaper cost when the entire lane is inactive.
+    // The expected asm sequence is:
+    // vcpop.m a0, v0
+    // beqz a0, exit # Return passthru when the entire lane is inactive.
+    // vid v10, v0.t
+    // vredmaxu.vs v10, v10, v10
+    // vmv.x.s a0, v10
+    // zext.b a0, a0
+    // vslidedown v8, v8, a0
----------------
lukel97 wrote:

Nit
```suggestion
    // vslidedown.vx v8, v8, a0
```

https://github.com/llvm/llvm-project/pull/184067