[llvm] [SLP][NFC] Fix uninitialized ReductionRoot in getTreeCost (PR #189994)

[Alexey Bataev via llvm-commits]

https://github.com/alexey-bataev created https://github.com/llvm/llvm-project/pull/189994

ReductionRoot was initialized to nullptr instead of the RdxRoot
parameter. This caused two ScaleCost calls (for MinBWs cast cost and
ReductionBitWidth resize cost) to pass nullptr as the user instruction,
and suppressed the "Reduction Cost" line in debug output. In practice
the scale factor is the same because the tree root's main op and the
reduction root share the same basic block, so this is NFC.


>From 63782f65232c502618580071fd10a6985d532094 Mon Sep 17 00:00:00 2001
From: Alexey Bataev <a.bataev at outlook.com>
Date: Wed, 1 Apr 2026 09:20:58 -0700
Subject: [PATCH] =?UTF-8?q?[=F0=9D=98=80=F0=9D=97=BD=F0=9D=97=BF]=20initia?=
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Created using spr 1.3.7
---
 llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp b/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
index e66801659e454..39ab9932c006f 100644
--- a/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
+++ b/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
@@ -18082,11 +18082,11 @@ InstructionCost BoUpSLP::getTreeCost(InstructionCost TreeCost,
     LLVM_DEBUG(dbgs() << "Scale " << Scale << " For entry " << TE.Idx << "\n");
     return C * Scale;
   };
-  Instruction *ReductionRoot = nullptr;
+  Instruction *ReductionRoot = RdxRoot;
   if (UserIgnoreList) {
     // Scale reduction cost to the factor of the loop nest trip count.
     ReductionCost = ScaleCost(ReductionCost, *VectorizableTree.front().get(),
-                              /*Scalar=*/nullptr, RdxRoot);
+                              /*Scalar=*/nullptr, ReductionRoot);
   }
 
   // Add the cost for reduction.