[llvm] [InstCombine] Use MapVector for SourceAggregates. (PR #132564)

[Florian Hahn via llvm-commits]

https://github.com/fhahn created https://github.com/llvm/llvm-project/pull/132564

foldAggregateConstructionIntoAggregateReuse iterates over the entries of SourceAggregates and the order of inserted instructions depends on the order of the iterator. Using a regular DenseMap can lead to non-deterministic value naming/numbering.

I don't think it can actually impact the generated binary, but it makes diffing IR more difficult. If desired, I could provide a test case showing the difference in naming.

>From fd8c1779b338e0f20eee7686b58fb539cbce6c4d Mon Sep 17 00:00:00 2001
From: Florian Hahn <flo at fhahn.com>
Date: Sat, 22 Mar 2025 20:44:56 +0000
Subject: [PATCH] [InstCombine] Use MapVector for SourceAggregates.

foldAggregateConstructionIntoAggregateReuse iterates over the entries of
SourceAggregates and the order of inserted instructions depends on the
order of the iterator. Using a regular DenseMap can lead to
non-deterministic value naming/numbering.

I don't think it can actually impact the generated binary, but it makes
diffing IR more difficult. If desired, I could provide a test case showing
the difference in naming.
---
 llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp b/llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp
index 6860a7cd07b78..f897cc7855d2d 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp
@@ -1111,7 +1111,7 @@ Instruction *InstCombinerImpl::foldAggregateConstructionIntoAggregateReuse(
   // For each predecessor, what is the source aggregate,
   // from which all the elements were originally extracted from?
   // Note that we want for the map to have stable iteration order!
-  SmallDenseMap<BasicBlock *, Value *, 4> SourceAggregates;
+  SmallMapVector<BasicBlock *, Value *, 4> SourceAggregates;
   bool FoundSrcAgg = false;
   for (BasicBlock *Pred : Preds) {
     std::pair<decltype(SourceAggregates)::iterator, bool> IV =