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

[Luke Lau via llvm-commits]

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@@ -3149,6 +3157,28 @@ static VPRecipeBase *optimizeMaskToEVL(VPValue *HeaderMask,
                                            Mask ? Mask : Plan->getTrue(), &EVL},
                                           IntrR->getScalarType(), {}, {}, DL);
 
+  // Transforms the VPReductionRecipe inside the VPExpressionRecipe.
+  if (auto *Expr = dyn_cast<VPExpressionRecipe>(&CurRecipe))
+    if (match(Expr->getOperand(Expr->getNumOperands() - 1),
+              m_RemoveMask(HeaderMask, Mask))) {
+      // Decompose first and construct with VPReductionEVLRecipe later.
+      SmallVector<VPSingleDefRecipe *> ExpressionRecipes = Expr->decompose();
+      VPReductionRecipe *Red =
+          cast<VPReductionRecipe>(ExpressionRecipes.pop_back_val());
+
+      // Convert to VPReductionEVLRecipe.
+      auto *NewRed = new VPReductionEVLRecipe(
+          *Red, EVL, Mask ? Mask : Plan->getTrue(), Red->getDebugLoc());
+      NewRed->insertBefore(Expr);
+      ExpressionRecipes.push_back(NewRed);
+      auto *NewExpr =
+          new VPExpressionRecipe(Expr->getExpressionType(), ExpressionRecipes);
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lukel97 wrote:

I prefer calling decompose, since I think it's important to keep the EVL specific code in one place instead of spreading it across the loop vectorizer. It's also the easiest way to keep `ExpressionRecipes` internal, and I don't see any particular hazards to using decompose. 

Would like to hear what others think

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