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

[Carl Ritson via llvm-commits]

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@@ -917,19 +919,61 @@ void GenericUniformityAnalysisImpl<ContextT>::taintAndPushAllDefs(
 /// Mark divergent phi nodes in a join block
 template <typename ContextT>
 void GenericUniformityAnalysisImpl<ContextT>::taintAndPushPhiNodes(
-    const BlockT &JoinBlock) {
+    const BlockT &JoinBlock, const BlockT &DivTermBlock,
+    const DivergenceDescriptorT &DivDesc) {
   LLVM_DEBUG(dbgs() << "taintAndPushPhiNodes in " << Context.print(&JoinBlock)
                     << "\n");
   for (const auto &Phi : JoinBlock.phis()) {
-    // FIXME: The non-undef value is not constant per se; it just happens to be
-    // uniform and may not dominate this PHI. So assuming that the same value
-    // reaches along all incoming edges may itself be undefined behaviour. This
-    // particular interpretation of the undef value was added to
-    // DivergenceAnalysis in the following review:
-    //
-    // https://reviews.llvm.org/D19013
-    if (ContextT::isConstantOrUndefValuePhi(Phi))
+    // Attempt to maintain uniformity for PHIs by considering control
+    // dependencies before marking them.
+    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();
----------------
perlfu wrote:

Loop is already predicated on `Uniform` so it won't go through blocks in this case.

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