[llvm] [ARM] Return the correct chain when expanding READ_REGISTER (PR #145237)

[David Green via llvm-commits]

https://github.com/davemgreen created https://github.com/llvm/llvm-project/pull/145237

This prevents it CSEing multiple nodes together from "volatile" registers as they would end up with the same chain. The new chain out should be the chain from the new READ_REGISTER node.

Fixes #144845

>From 9cd435c2e4818f7347c71fe7db81a0559d9aee54 Mon Sep 17 00:00:00 2001
From: David Green <david.green at arm.com>
Date: Sun, 22 Jun 2025 15:28:18 +0100
Subject: [PATCH] [ARM] Return the correct chain when expanding READ_REGISTER

This prevents it CSEing multiple nodes together from "volatile" registers as
they would end up with the same chain. The new chain out should be the chain
from the new READ_REGISTER node.

Fixes #144845
---
 llvm/lib/Target/ARM/ARMISelLowering.cpp |  2 +-
 llvm/test/CodeGen/ARM/special-reg.ll    | 12 ++++++++----
 2 files changed, 9 insertions(+), 5 deletions(-)

diff --git a/llvm/lib/Target/ARM/ARMISelLowering.cpp b/llvm/lib/Target/ARM/ARMISelLowering.cpp
index 4567081fe78dc..9cb2235d2c196 100644
--- a/llvm/lib/Target/ARM/ARMISelLowering.cpp
+++ b/llvm/lib/Target/ARM/ARMISelLowering.cpp
@@ -6192,7 +6192,7 @@ static void ExpandREAD_REGISTER(SDNode *N, SmallVectorImpl<SDValue> &Results,
 
   Results.push_back(DAG.getNode(ISD::BUILD_PAIR, DL, MVT::i64, Read.getValue(0),
                     Read.getValue(1)));
-  Results.push_back(Read.getOperand(0));
+  Results.push_back(Read.getValue(2)); // Chain
 }
 
 /// \p BC is a bitcast that is about to be turned into a VMOVDRR.
diff --git a/llvm/test/CodeGen/ARM/special-reg.ll b/llvm/test/CodeGen/ARM/special-reg.ll
index e966550e673d4..cc95f79d2c73b 100644
--- a/llvm/test/CodeGen/ARM/special-reg.ll
+++ b/llvm/test/CodeGen/ARM/special-reg.ll
@@ -25,14 +25,18 @@ entry:
 define i64 @read_volatile_i64_twice() {
 ; ACORE-LABEL: read_volatile_i64_twice:
 ; ACORE:       @ %bb.0: @ %entry
-; ACORE-NEXT:    mov r0, #0
-; ACORE-NEXT:    mov r1, #0
+; ACORE-NEXT:    mrrc p15, #1, r0, r1, c14
+; ACORE-NEXT:    mrrc p15, #1, r2, r3, c14
+; ACORE-NEXT:    eor r0, r2, r0
+; ACORE-NEXT:    eor r1, r3, r1
 ; ACORE-NEXT:    bx lr
 ;
 ; MCORE-LABEL: read_volatile_i64_twice:
 ; MCORE:       @ %bb.0: @ %entry
-; MCORE-NEXT:    movs r0, #0
-; MCORE-NEXT:    movs r1, #0
+; MCORE-NEXT:    mrrc p15, #1, r0, r1, c14
+; MCORE-NEXT:    mrrc p15, #1, r2, r3, c14
+; MCORE-NEXT:    eors r0, r2
+; MCORE-NEXT:    eors r1, r3
 ; MCORE-NEXT:    bx lr
 entry:
   %0 = tail call i64 @llvm.read_volatile_register.i64(metadata !5)