[llvm] CodeGen: Record tied virtual register operands in finalizeBundle (PR #166209)

[Nicolai Hähnle via llvm-commits]

https://github.com/nhaehnle created https://github.com/llvm/llvm-project/pull/166209

This is in preparation of a future AMDGPU change where we are going to
create bundles before register allocation and want to rely on the
TwoAddressInstructionPass handling those bundles correctly.

>From 4c9997f6f11a5b9b5baff5929fc69101d9b42c33 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Nicolai=20H=C3=A4hnle?= <nicolai.haehnle at amd.com>
Date: Thu, 18 Sep 2025 16:38:50 -0700
Subject: [PATCH] CodeGen: Record tied virtual register operands in
 finalizeBundle

This is in preparation of a future AMDGPU change where we are going to
create bundles before register allocation and want to rely on the
TwoAddressInstructionPass handling those bundles correctly.

commit-id:64872792

Pull Request: https://github.com/nhaehnle/llvm-project/pull/8
---
 llvm/lib/CodeGen/MachineInstrBundle.cpp       | 23 ++++++-
 llvm/unittests/Target/AMDGPU/CMakeLists.txt   |  1 +
 .../Target/AMDGPU/MachineInstrBundleTest.cpp  | 61 +++++++++++++++++++
 3 files changed, 84 insertions(+), 1 deletion(-)
 create mode 100644 llvm/unittests/Target/AMDGPU/MachineInstrBundleTest.cpp

diff --git a/llvm/lib/CodeGen/MachineInstrBundle.cpp b/llvm/lib/CodeGen/MachineInstrBundle.cpp
index da29ffc9d2fed..1f47dc50351d2 100644
--- a/llvm/lib/CodeGen/MachineInstrBundle.cpp
+++ b/llvm/lib/CodeGen/MachineInstrBundle.cpp
@@ -136,6 +136,7 @@ void llvm::finalizeBundle(MachineBasicBlock &MBB,
   SmallSetVector<Register, 8> ExternUses;
   SmallSet<Register, 8> KilledUseSet;
   SmallSet<Register, 8> UndefUseSet;
+  SmallVector<std::pair<Register, Register>> TiedOperands;
   for (auto MII = FirstMI; MII != LastMI; ++MII) {
     // Debug instructions have no effects to track.
     if (MII->isDebugInstr())
@@ -161,6 +162,16 @@ void llvm::finalizeBundle(MachineBasicBlock &MBB,
           // External def is now killed.
           KilledUseSet.insert(Reg);
         }
+        if (MO.isTied()) {
+          // Record tied operand constraints that involve virtual registers so
+          // that bundles that are formed pre-register allocation reflect the
+          // relevant constraints.
+          unsigned TiedIdx = MII->findTiedOperandIdx(MO.getOperandNo());
+          MachineOperand &TiedMO = MII->getOperand(TiedIdx);
+          Register DefReg = TiedMO.getReg();
+          if (Reg.isVirtual() || DefReg.isVirtual())
+            TiedOperands.emplace_back(DefReg, Reg);
+        }
       }
     }
 
@@ -203,7 +214,17 @@ void llvm::finalizeBundle(MachineBasicBlock &MBB,
     bool isKill = KilledUseSet.contains(Reg);
     bool isUndef = UndefUseSet.contains(Reg);
     MIB.addReg(Reg, getKillRegState(isKill) | getUndefRegState(isUndef) |
-               getImplRegState(true));
+                        getImplRegState(true));
+  }
+
+  for (auto [DefReg, UseReg] : TiedOperands) {
+    unsigned DefIdx =
+        std::distance(LocalDefs.begin(), llvm::find(LocalDefs, DefReg));
+    unsigned UseIdx =
+        std::distance(ExternUses.begin(), llvm::find(ExternUses, UseReg));
+    assert(DefIdx < LocalDefs.size());
+    assert(UseIdx < ExternUses.size());
+    MIB->tieOperands(DefIdx, LocalDefs.size() + UseIdx);
   }
 }
 
diff --git a/llvm/unittests/Target/AMDGPU/CMakeLists.txt b/llvm/unittests/Target/AMDGPU/CMakeLists.txt
index d6cbaf3f3fb5d..e3748bcea9422 100644
--- a/llvm/unittests/Target/AMDGPU/CMakeLists.txt
+++ b/llvm/unittests/Target/AMDGPU/CMakeLists.txt
@@ -24,5 +24,6 @@ add_llvm_target_unittest(AMDGPUTests
   DwarfRegMappings.cpp
   ExecMayBeModifiedBeforeAnyUse.cpp
   LiveRegUnits.cpp
+  MachineInstrBundleTest.cpp
   PALMetadata.cpp
   )
diff --git a/llvm/unittests/Target/AMDGPU/MachineInstrBundleTest.cpp b/llvm/unittests/Target/AMDGPU/MachineInstrBundleTest.cpp
new file mode 100644
index 0000000000000..3ec4875958e73
--- /dev/null
+++ b/llvm/unittests/Target/AMDGPU/MachineInstrBundleTest.cpp
@@ -0,0 +1,61 @@
+//===- MachineInstrBundleTest.cpp -----------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "AMDGPUTargetMachine.h"
+#include "AMDGPUUnitTests.h"
+#include "llvm/CodeGen/MachineFunction.h"
+#include "llvm/CodeGen/MachineInstrBuilder.h"
+#include "llvm/CodeGen/MachineModuleInfo.h"
+#include "gtest/gtest.h"
+
+using namespace llvm;
+
+namespace {
+
+TEST(AMDGPU, TiedOperandsBundle) {
+  auto TM = createAMDGPUTargetMachine("amdgcn-amd-", "gfx1100", "");
+
+  GCNSubtarget ST(TM->getTargetTriple(), std::string(TM->getTargetCPU()),
+                  std::string(TM->getTargetFeatureString()), *TM);
+
+  LLVMContext Ctx;
+  Module Mod("Module", Ctx);
+  Mod.setDataLayout(TM->createDataLayout());
+
+  auto *Type = FunctionType::get(Type::getVoidTy(Ctx), false);
+  auto *F = Function::Create(Type, GlobalValue::ExternalLinkage, "Test", &Mod);
+
+  MachineModuleInfo MMI(TM.get());
+  auto MF =
+      std::make_unique<MachineFunction>(*F, *TM, ST, MMI.getContext(), 42);
+  auto *BB = MF->CreateMachineBasicBlock();
+  MF->push_back(BB);
+
+  const auto &TII = *ST.getInstrInfo();
+  auto &MRI = MF->getRegInfo();
+
+  Register RX = MRI.createVirtualRegister(&AMDGPU::SReg_32_XEXEC_HIRegClass);
+  Register RY = MRI.createVirtualRegister(&AMDGPU::SReg_32_XEXEC_HIRegClass);
+
+  MachineInstr *MI =
+      BuildMI(*BB, BB->begin(), {}, TII.get(AMDGPU::COPY), RX).addReg(RY);
+  MI->tieOperands(0, 1);
+
+  finalizeBundle(*BB, MI->getIterator(), std::next(MI->getIterator()));
+
+  MachineInstr *Bundle = &*BB->begin();
+  EXPECT_TRUE(Bundle->getOpcode() == AMDGPU::BUNDLE);
+  EXPECT_EQ(Bundle->getNumOperands(), 2u);
+  EXPECT_EQ(Bundle->getOperand(0).getReg(), RX);
+  EXPECT_EQ(Bundle->getOperand(1).getReg(), RY);
+  EXPECT_TRUE(Bundle->getOperand(0).isTied());
+  EXPECT_TRUE(Bundle->getOperand(1).isTied());
+  EXPECT_EQ(Bundle->findTiedOperandIdx(0), 1u);
+}
+
+} // end namespace