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

[Luke Lau via llvm-commits]

================
@@ -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
----------------
lukel97 wrote:

Clarified in 5d82ee1510ee4e456ba17a14d559dfdd1d85785a

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