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

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-support

Author: Devesh Yadav (Devesh-Yadav10)

<details>
<summary>Changes</summary>

### 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.


---
Full diff: https://github.com/llvm/llvm-project/pull/131424.diff


2 Files Affected:

- (modified) llvm/include/llvm/CodeGen/TargetLowering.h (+1-1) 
- (modified) llvm/include/llvm/Support/TargetOpcodes.def (+2) 


``````````diff
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.
diff --git a/llvm/include/llvm/Support/TargetOpcodes.def b/llvm/include/llvm/Support/TargetOpcodes.def
index 5ef3707b81fe9..b9f7c57c91c6b 100644
--- a/llvm/include/llvm/Support/TargetOpcodes.def
+++ b/llvm/include/llvm/Support/TargetOpcodes.def
@@ -297,6 +297,8 @@ HANDLE_TARGET_OPCODE(G_ABDU)
 
 
 HANDLE_TARGET_OPCODE(G_IMPLICIT_DEF)
+HANDLE_TARGET_OPCODE(G_POISON)
+
 
 /// Generic PHI instruction with types.
 HANDLE_TARGET_OPCODE(G_PHI)

``````````

</details>


https://github.com/llvm/llvm-project/pull/131424