[llvm] Use a reference to DataLayout instead of copying the underlying string data (PR #128269)

[via llvm-commits]

https://github.com/cooperp created https://github.com/llvm/llvm-project/pull/128269

I noticed this when looking at all allocations by clang.  For a medium sized file this was around 6000 calls to operator new, although i suspect there were more allocations in total as the SmallVectors in DataLayout may have their own allocations in some cases.

In a follow-up i'm tempted to make the DataLayout copy constructor private, to avoid this in future. There are a few tests which copy the DataLayout, and perhaps need to (I didn't check yet), but we could provide a clone() method for them if needed.  Its only accidental copying I think we should consider avoiding, not people who really do need to copy it for reasons.

>From 450ce7492fd51de1161052a6bcced3dd1868df5c Mon Sep 17 00:00:00 2001
From: Peter Cooper <peter_cooper at apple.com>
Date: Fri, 21 Feb 2025 17:52:00 -0800
Subject: [PATCH] Use a reference to DataLayout instead of copying the
 underlying string data

---
 llvm/lib/Transforms/Scalar/Reassociate.cpp | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/llvm/lib/Transforms/Scalar/Reassociate.cpp b/llvm/lib/Transforms/Scalar/Reassociate.cpp
index 7cb9bace47bf4..d3476ab992269 100644
--- a/llvm/lib/Transforms/Scalar/Reassociate.cpp
+++ b/llvm/lib/Transforms/Scalar/Reassociate.cpp
@@ -420,7 +420,7 @@ static bool LinearizeExprTree(Instruction *I,
   using LeafMap = DenseMap<Value *, uint64_t>;
   LeafMap Leaves; // Leaf -> Total weight so far.
   SmallVector<Value *, 8> LeafOrder; // Ensure deterministic leaf output order.
-  const DataLayout DL = I->getDataLayout();
+  const DataLayout& DL = I->getDataLayout();
 
 #ifndef NDEBUG
   SmallPtrSet<Value *, 8> Visited; // For checking the iteration scheme.