[llvm] [LV] Support EVL for partial reduction and VPExpressionRecipe. (PR #205741)

[Elvis Wang via llvm-commits]

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@@ -3356,7 +3356,27 @@ void VPReductionEVLRecipe::execute(VPTransformState &State) {
     Mask = Builder.CreateVectorSplat(State.VF, Builder.getTrue());
 
   Value *NewRed;
-  if (isOrdered()) {
+  if (isPartialReduction()) {
+    // For partial reduction, we need to generate the predicated select
+    // (vp.merge) since `@llvm.vector.partial.reduce()` doesn't have vector
+    // predicated version.
+    VectorType *VecTy = cast<VectorType>(VecOp->getType());
+    Value *Identity = getRecurrenceIdentity(Kind, VecTy->getElementType(),
+                                            getFastMathFlagsOrNone());
+    if (State.VF.isVector())
+      Identity =
+          State.Builder.CreateVectorSplat(VecTy->getElementCount(), Identity);
+
+    Value *NewVecOp = State.Builder.CreateIntrinsic(
+        VecTy, Intrinsic::vp_merge, {Mask, VecOp, Identity, EVL});
+    assert((Kind == RecurKind::Add || Kind == RecurKind::FAdd) &&
+           "Unexpected partial reduction kind");
+    NewRed = State.Builder.CreateIntrinsic(
+        Prev->getType(),
+        Kind == RecurKind::Add ? Intrinsic::vector_partial_reduce_add
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ElvisWang123 wrote:

We can, but we need to handle the identity construction here and only partial EVL reduction needs predicated select. 

Generate the predicated `vp.merge` match the behavior with non-EVL reduction recipe.
I think a possible approach is that we can generate the predicated select for all reduction when the reduction construction(https://github.com/llvm/llvm-project/pull/201740). 

https://github.com/llvm/llvm-project/pull/205741