[llvm] [GVN] Use SmallDenseSet for visited node tracking (PR #215258)

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<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-transforms

Author: Sven van Haastregt (svenvh)

<details>
<summary>Changes</summary>

Commit 0a321f38147f ("[GVN] Track visited nodes in equality propagation to avoid OOM (#<!-- -->212265)", 2026-08-05) caused some compilation time regressions.  Use a SmallDenseSet to mitigate the impact of that change.

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Full diff: https://github.com/llvm/llvm-project/pull/215258.diff


1 Files Affected:

- (modified) llvm/lib/Transforms/Scalar/GVN.cpp (+1-1) 


``````````diff
diff --git a/llvm/lib/Transforms/Scalar/GVN.cpp b/llvm/lib/Transforms/Scalar/GVN.cpp
index a698b839b7f3a..64fd14fcba1c5 100644
--- a/llvm/lib/Transforms/Scalar/GVN.cpp
+++ b/llvm/lib/Transforms/Scalar/GVN.cpp
@@ -3092,7 +3092,7 @@ bool GVNPass::propagateEquality(
     Value *LHS, Value *RHS,
     const std::variant<BasicBlockEdge, Instruction *> &Root) {
   SmallVector<std::pair<Value*, Value*>, 4> Worklist;
-  DenseSet<std::pair<Value *, Value *>> Visited;
+  SmallDenseSet<std::pair<Value *, Value *>, 4> Visited;
   Worklist.push_back(std::make_pair(LHS, RHS));
   bool Changed = false;
   SmallVector<const BasicBlock *> DominatedBlocks;

``````````

</details>


https://github.com/llvm/llvm-project/pull/215258