[llvm] [LLVM][VPlan] Keep all VPBlend masks until VPlan transformation. (PR #104015)

[Paul Walker via llvm-commits]

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@@ -989,15 +989,47 @@ void VPlanTransforms::clearReductionWrapFlags(VPlan &Plan) {
 /// Try to simplify recipe \p R.
 static void simplifyRecipe(VPRecipeBase &R, VPTypeAnalysis &TypeInfo) {
   using namespace llvm::VPlanPatternMatch;
-  // Try to remove redundant blend recipes.
+
   if (auto *Blend = dyn_cast<VPBlendRecipe>(&R)) {
-    VPValue *Inc0 = Blend->getIncomingValue(0);
+    // Try to remove redundant blend recipes.
+    SmallSet<VPValue *, 4> UniqueValues;
+    if (Blend->isNormalized() || !match(Blend->getMask(0), m_False()))
+      UniqueValues.insert(Blend->getIncomingValue(0));
     for (unsigned I = 1; I != Blend->getNumIncomingValues(); ++I)
-      if (Inc0 != Blend->getIncomingValue(I) &&
-          !match(Blend->getMask(I), m_False()))
-        return;
-    Blend->replaceAllUsesWith(Inc0);
+      if (!match(Blend->getMask(I), m_False()))
+        UniqueValues.insert(Blend->getIncomingValue(I));
+
+    if (UniqueValues.size() == 1) {
+      Blend->replaceAllUsesWith(*UniqueValues.begin());
+      Blend->eraseFromParent();
+      return;
+    }
+
+    if (Blend->isNormalized())
+      return;
+
+    // Normalize the blend so its first incomming value is used as the initial
+    // value with the others blended into it.
+
+    unsigned StartIndex = 0;
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paulwalker-arm wrote:

There's no relationship between `StartIndex` and the code above.  The code above is walking through all operands hence starting at 0, whereas the intent of StartIndex is to capture the new start value for the normalised blends.

There's an associated PR (https://github.com/llvm/llvm-project/pull/104019) that adds code to update `StartIndex` which is why it's declared where it is and not a const.  I can add the const if you prefer but it will be short lived.

https://github.com/llvm/llvm-project/pull/104015