[llvm] [AArch64] canCreateUndefOrPoisonForTargetNode - AArch64ISD::MOVI opcodes can't create undef/poison (PR #149323)

[Simon Pilgrim via llvm-commits]

https://github.com/RKSimon created https://github.com/llvm/llvm-project/pull/149323

Possible fix for failed fold in #148191

>From 8c0a8829c9889b6f1e2a2fa86092c74f5d881b85 Mon Sep 17 00:00:00 2001
From: Simon Pilgrim <llvm-dev at redking.me.uk>
Date: Thu, 17 Jul 2025 15:39:06 +0100
Subject: [PATCH] [AArch64] canCreateUndefOrPoisonForTargetNode -
 AArch64ISD::MOVI opcodes can't create undef/poison

Possible fix for failed fold in #148191
---
 llvm/lib/Target/AArch64/AArch64ISelLowering.cpp | 17 +++++++++++++++++
 llvm/lib/Target/AArch64/AArch64ISelLowering.h   |  6 ++++++
 2 files changed, 23 insertions(+)

diff --git a/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp b/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp
index ff23f76fadccd..ea8a8e1c0219e 100644
--- a/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp
+++ b/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp
@@ -30398,6 +30398,23 @@ bool AArch64TargetLowering::SimplifyDemandedBitsForTargetNode(
       Op, OriginalDemandedBits, OriginalDemandedElts, Known, TLO, Depth);
 }
 
+bool AArch64TargetLowering::canCreateUndefOrPoisonForTargetNode(
+    SDValue Op, const APInt &DemandedElts, const SelectionDAG &DAG,
+    bool PoisonOnly, bool ConsiderFlags, unsigned Depth) const {
+
+  // TODO: Add more target nodes.
+  switch (Op.getOpcode()) {
+  case AArch64ISD::MOVI:
+  case AArch64ISD::MOVIedit:
+  case AArch64ISD::MOVImsl:
+  case AArch64ISD::MOVIshift:
+  case AArch64ISD::MVNIshift:
+    return false;
+  }
+  return TargetLowering::canCreateUndefOrPoisonForTargetNode(
+      Op, DemandedElts, DAG, PoisonOnly, ConsiderFlags, Depth);
+}
+
 bool AArch64TargetLowering::isTargetCanonicalConstantNode(SDValue Op) const {
   return Op.getOpcode() == AArch64ISD::DUP ||
          Op.getOpcode() == AArch64ISD::MOVI ||
diff --git a/llvm/lib/Target/AArch64/AArch64ISelLowering.h b/llvm/lib/Target/AArch64/AArch64ISelLowering.h
index 7b1de3d3254f2..12f07682868e5 100644
--- a/llvm/lib/Target/AArch64/AArch64ISelLowering.h
+++ b/llvm/lib/Target/AArch64/AArch64ISelLowering.h
@@ -851,6 +851,12 @@ class AArch64TargetLowering : public TargetLowering {
                                          TargetLoweringOpt &TLO,
                                          unsigned Depth) const override;
 
+  bool canCreateUndefOrPoisonForTargetNode(SDValue Op,
+                                           const APInt &DemandedElts,
+                                           const SelectionDAG &DAG,
+                                           bool PoisonOnly, bool ConsiderFlags,
+                                           unsigned Depth) const override;
+
   bool isTargetCanonicalConstantNode(SDValue Op) const override;
 
   // With the exception of data-predicate transitions, no instructions are