[llvm] [VPlan] Handle FirstActiveLane when unrolling. (PR #145394)

[via llvm-commits]

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@@ -856,9 +856,32 @@ Value *VPInstruction::generate(VPTransformState &State) {
     return Builder.CreateOrReduce(Res);
   }
   case VPInstruction::FirstActiveLane: {
-    Value *Mask = State.get(getOperand(0));
-    return Builder.CreateCountTrailingZeroElems(Builder.getInt64Ty(), Mask,
-                                                true, Name);
+    if (getNumOperands() == 1) {
+      Value *Mask = State.get(getOperand(0));
+      return Builder.CreateCountTrailingZeroElems(Builder.getInt64Ty(), Mask,
+                                                  true, Name);
+    }
+    // If there are multiple operands, create a chain of selects to pick the
+    // first operand with an active lane and add the number of lanes of the
+    // preceding operands.
+    Value *RuntimeVF =
+        getRuntimeVF(State.Builder, State.Builder.getInt64Ty(), State.VF);
+    unsigned LastOpIdx = getNumOperands() - 1;
+    Value *Res = nullptr;
+    for (int Idx = LastOpIdx; Idx >= 0; --Idx) {
+      Value *TrailingZeros = Builder.CreateCountTrailingZeroElems(
+          Builder.getInt64Ty(), State.get(getOperand(Idx)), true, Name);
+      Value *Current = Builder.CreateAdd(
+          Builder.CreateMul(RuntimeVF, Builder.getInt64(Idx)), TrailingZeros);
----------------
ayalz wrote:

```suggestion
      Value *NumPreceedingLanes = Builder.CreateMul(RuntimeVF, Builder.getInt64(Idx));
      Value *LastActiveLane = Builder.CreateAdd(NumPreceedingLanes, LastActiveLaneInMask);
```
splitting is easier to read, as in cmp-select below?

https://github.com/llvm/llvm-project/pull/145394