[llvm] SelectionDAG store merging default implementations are unreasonable #90714 (PR #131424)

[Devesh Yadav via llvm-commits]

https://github.com/Devesh-Yadav10 created https://github.com/llvm/llvm-project/pull/131424

### Summary
The default implementation of `mergeStoresAfterLegalization` in `TargetLowering` unconditionally returned `true`, which could cause infinite loops when a backend decomposes stores during legalization. 

This patch updates the default behavior to return `isOperationLegal(ISD::STORE, MemVT)`, ensuring that merging only happens when storing `MemVT` is explicitly legal. This provides a more conservative and safer default.

### Impact
- Prevents infinite loops in custom store legalization.  
- Some targets that previously relied on the unconditional `true` may need to override this function explicitly.  
- A manageable number of test cases break, which suggests that some backends need updates to handle this more correctly.

### Next Steps
If necessary, the API may need to be extended to consider both register and memory value types for more precise store merging decisions.


>From 452a6e136c601196ba55bca5a512ab85f2ddcf07 Mon Sep 17 00:00:00 2001
From: Devesh Yadav <160987201+Devesh-Yadav10 at users.noreply.github.com>
Date: Sat, 15 Mar 2025 08:44:11 +0530
Subject: [PATCH] Updated unconditional true in mergeStoresAfterLegalization to
 checking isOperationLegal(ISD::STORE, MemVT)

---
 llvm/include/llvm/CodeGen/TargetLowering.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/llvm/include/llvm/CodeGen/TargetLowering.h b/llvm/include/llvm/CodeGen/TargetLowering.h
index a3fb4e9a8513b..a52dcf68dd243 100644
--- a/llvm/include/llvm/CodeGen/TargetLowering.h
+++ b/llvm/include/llvm/CodeGen/TargetLowering.h
@@ -695,7 +695,7 @@ class TargetLoweringBase {
   /// to before legalization. This may transform stores that do not exist
   /// earlier (for example, stores created from intrinsics).
   virtual bool mergeStoresAfterLegalization(EVT MemVT) const {
-    return true;
+    return isOperationLegal(ISD::STORE, MemVT);
   }
 
   /// Returns if it's reasonable to merge stores to MemVT size.