[llvm] [AMDGPU] Rewrite undef PHIs with a unique constant value (PR #214319)

[via llvm-commits]

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@@ -140,23 +140,25 @@ bool rewritePHIs(Function &F, UniformityInfo &UA, DominatorTree *DT) {
       }
       // We only need to replace the undef for the PHI which is merging
       // defined/undefined values from divergent threads.
-      // TODO: We should still be able to replace undef value if the unique
-      // value is a Constant.
-      if (!UniqueDefinedIncoming || Undefs.empty() ||
-          UA.isUniformTerminator(DominateBB->getTerminator()))
+      if (!UniqueDefinedIncoming || Undefs.empty())
         continue;
 
       // We only replace the undef when DominateBB truly dominates all the
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robertvirany wrote:

Thanks. The first commit was intended as an intentionally failing regression test. After the second commit the test passes with the desired output. 

I also updated the comment to describe the constant case.

https://github.com/llvm/llvm-project/pull/214319