[llvm] [Uniformity] Avoid marking all PHIs as divergent in join blocks (PR #157808)

[David Stuttard via llvm-commits]

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@@ -930,6 +933,44 @@ void GenericUniformityAnalysisImpl<ContextT>::taintAndPushPhiNodes(
     // https://reviews.llvm.org/D19013
     if (ContextT::isConstantOrUndefValuePhi(Phi))
       continue;
+
+    // Attempt to maintain uniformity for PHIs by considering control
+    // dependencies.
+    SmallVector<ConstValueRefT> Values;
+    SmallVector<const BlockT *> Blocks;
+    Context.getPhiInputs(Phi, Values, Blocks);
+    assert(Blocks.size() == Values.size());
+
+    // Allow an empty Blocks/Values list to signify getPhiInputs is not
+    // implemented; in which case no uniformity is possible.
+    bool Uniform = !Values.empty();
+
+    std::optional<ConstValueRefT> CommonValue;
+    for (unsigned I = 0; I < Blocks.size() && Uniform; ++I) {
+      if (DivDesc.CycleDivBlocks.contains(Blocks[I])) {
+        // If PHI is reachable via divergent exit it is divergent.
+        Uniform = false;
+      } else if (DT.dominates(&DivTermBlock, Blocks[I]) ||
+                 DivDesc.BlockLabels.lookup_or(Blocks[I], nullptr)) {
+        // If all edges from the marked path share a common value then
+        // uniformity is preserved when the value is itself uniform.
+        if (!CommonValue)
+          CommonValue = Values[I];
+        else
+          Uniform = Values[I] == *CommonValue;
+      }
+      // Ignore undefined values when checking definitions.
+      if (!Values[I])
----------------
dstutt wrote:

If Values[I] is undefined, and it isn't the first one - won't Uniform already be set to false?

https://github.com/llvm/llvm-project/pull/157808