[llvm] [VPlan] Compute blend masks from minimum set of edge masks (PR #201783)

[Luke Lau via llvm-commits]

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@@ -233,6 +245,115 @@ void VPPredicator::createSwitchEdgeMasks(const VPInstruction *SI) {
   setEdgeMask(Src, DefaultDst, DefaultMask);
 }
 
+// Compute the "furthest up" set of edges for each incoming value of a phi.
+//
+// Start by keeping track of what edges lead to which value. Then see if any
+// node has the same value for all outgoing edges. If so then propagate that
+// value up to every node it postdominates.
+MapVector<VPPredicator::EdgeTy, VPValue *>
+VPPredicator::computeBlendEdges(VPPhi *Phi) {
+  MapVector<EdgeTy, VPValue *> Edges;
+
+  // Mark the given edge as providing the value \p V.
+  auto AddEdge = [&Edges](const VPBlockBase *From, const VPBlockBase *To,
+                          VPValue *V) {
+    EdgeTy Edge = {cast<VPBasicBlock>(From), cast<VPBasicBlock>(To)};
+    assert((!Edges.contains(Edge) || Edges.lookup(Edge) == V) &&
+           "Clobbering an edge?");
+    Edges[Edge] = V;
+  };
+
+  for (auto [InVal, InVPBB] : Phi->incoming_values_and_blocks())
+    AddEdge(InVPBB, Phi->getParent(), InVal);
+
+  // The root phi must postdominate every incoming block. Also don't touch
+  // phis in a reduction chain since they need to be in a specific structure
+  // for handle*Reductions.
+  for (auto [InVal, InVPBB] : Phi->incoming_values_and_blocks())
+    if (!VPPDT.dominates(Phi->getParent(), InVPBB) ||
+        isa<VPReductionPHIRecipe>(InVal))
+      return Edges;
+
+  // Given a list of edges, check if they all have the same value and return it.
+  auto GetAllEqual = [&Edges](ArrayRef<EdgeTy> OutEdges) -> VPValue * {
+    VPValue *Common = nullptr;
+    for (EdgeTy E : OutEdges) {
+      VPValue *V = Edges.lookup(E);
+      if (!V)
+        return nullptr;
+      if (match(V, m_Poison()))
+        continue;
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lukel97 wrote:

I've removed the poison coalescing logic for now so we can focus on it in another PR 0b553a5201ce649021ad88e9b0eb05aa3f12f3fc

https://github.com/llvm/llvm-project/pull/201783