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

[Devesh Yadav via llvm-commits]

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

>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 1/2] 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.

>From 24c8a57695adecbcb2036773f98827dea050c08f Mon Sep 17 00:00:00 2001
From: Devesh Yadav <160987201+Devesh-Yadav10 at users.noreply.github.com>
Date: Sat, 15 Mar 2025 19:27:02 +0530
Subject: [PATCH 2/2] [GlobalISel] Introduce G_POISON to distinguish poison
 from undef

This patch adds G_POISON as a new opcode in GlobalISel to represent poison values separately from G_IMPLICIT_DEF. This distinction is necessary to mirror LLVM IR semantics more accurately and enable better optimizations, particularly when legalization introduces padding vector elements.
---
 llvm/include/llvm/Support/TargetOpcodes.def | 2 ++
 1 file changed, 2 insertions(+)

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)