[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 don't think phis block poison propagation if that's what you mean. So if a phi has poison on a particular edge, and there's multiple users of that phi, each user of that phi is free to treat the value from that edge differently. 

Does this alive2 example illustrate what you're talking about? https://alive2.llvm.org/ce/z/nwmt4K

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