[llvm] [VPlan] Don't rely on region check in isUniformAfterVectorization. (PR #137883)

[Florian Hahn via llvm-commits]

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@@ -39,19 +39,43 @@ const SCEV *getSCEVExprForVPValue(VPValue *V, ScalarEvolution &SE);
 
 /// Returns true if \p VPV is uniform after vectorization.
 inline bool isUniformAfterVectorization(const VPValue *VPV) {
-  // A value defined outside the vector region must be uniform after
-  // vectorization inside a vector region.
-  if (VPV->isDefinedOutsideLoopRegions())
+  auto PreservesUniformity = [](unsigned Opcode) -> bool {
+    if (Instruction::isBinaryOp(Opcode) || Instruction::isCast(Opcode))
+      return true;
+    switch (Opcode) {
+    case Instruction::GetElementPtr:
+    case Instruction::ICmp:
+    case Instruction::FCmp:
+    case VPInstruction::Broadcast:
+    case VPInstruction::PtrAdd:
+      return true;
+    default:
+      return false;
+    }
+  };
+
+  // A live-in must be uniform across the scope of VPlan.
+  if (VPV->isLiveIn())
     return true;
-  if (auto *Rep = dyn_cast<VPReplicateRecipe>(VPV))
-    return Rep->isUniform();
+
+  if (auto *Rep = dyn_cast<VPReplicateRecipe>(VPV)) {
+    const VPRegionBlock *RegionOfR = Rep->getParent()->getParent();
+    if (RegionOfR && RegionOfR->isReplicator())
+      return false;
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fhahn wrote:

Yep, we could consider them uniform, the main issue is that this is also used when executing replicate recipes to see if we can simply retrieve the first lane; but that's not possible for replicate regions, as the uniform value created for lane 0 won't dominate the user when executing the region for lane 1 and so on. added a comment

https://github.com/llvm/llvm-project/pull/137883