[llvm] [LV] Fix miscompile with conditional scalar assignment + tail folding (PR #182492)

[Sander de Smalen via llvm-commits]

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@@ -1381,11 +1391,23 @@ bool VPlanTransforms::handleFindLastReductions(VPlan &Plan) {
       return NumIncomingDataValues == 1;
     };
 
+    VPSingleDefRecipe *SelectR =
+        cast<VPSingleDefRecipe>(CondSelect->getDefiningRecipe());
     if (!match(SelectR,
                m_Select(m_VPValue(Cond), m_VPValue(Op1), m_VPValue(Op2))) &&
         !MatchBlend(SelectR))
       return false;
 
+    assert(Cond != HeaderMask && "Cond must not be HeaderMask");
+
+    // Find final reduction computation and replace it with an
+    // extract.last.active intrinsic.
+    auto *RdxResult =
+        vputils::findUserOf<VPInstruction::ComputeReductionResult>(
+            BackedgeSelect);
+    if (!RdxResult)
+      return false;
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sdesmalen-arm wrote:

Should this actually be an assert that `RdxResult` is non-zero?

https://github.com/llvm/llvm-project/pull/182492