[AArch64][GlobalISel] Add a post-legalizer combiner + remove undef stores

[paquette]

Summary

Add a post-legalizer combiner with a very simple combine. This supersedes D77250, which did equivalent work in the selector.

This can be done pre-legalization or post-legalization. Post-legalization is more likely to hit, since G_IMPLICIT_DEFs tend to appear during legalization. There's no reason to not do it pre-legalization though-- if it can be caught earlier, great.

(I also think that it might be worth reimplementing D78769 using a target-specific post-legalization combine too after thinking about it for a while.)

[aemerson]

llvm/lib/Target/AArch64/AArch64PostLegalizerCombiner.cpp

94

@dsanders is this SelectionDAGFallbackAnalysis necessary for a combiner?

[dsanders]

llvm/lib/Target/AArch64/AArch64PostLegalizerCombiner.cpp

94

Unfortunately yes but it should be removable when fallbacks are disabled. The only thing it's doing is marking StackProtector as preserved.

When I was adding -debugify-and-strip-all I found that it's not safe to let some analysis passes run more than once and IIRC StackProtector was one of them.

[aemerson]

At the moment, I don't think this is really necessary for -O0 (unless you have some data to suggest otherwise). (I also think at some point we should audit the combines for necessity at -O0, since the PreLegalizerCombiner is matching all of them.)

[aemerson]

I committed this on behalf of Jessica in 49a4f3f7d88f61a81279de3d4e1c734ab0363228 with the change of only enabling with optimizations enabled.

llvm/include/llvm/CodeGen/GlobalISel/CombinerHelper.h

public:
 
   /// Return true if all register explicit use operands on \p MI are defined by
   /// a G_IMPLICIT_DEF.
   bool matchAllExplicitUsesAreUndef(MachineInstr &MI);
 
   /// Return true if a G_SHUFFLE_VECTOR instruction \p MI has an undef mask.
   bool matchUndefShuffleVectorMask(MachineInstr &MI);
 
+  /// Return true if a G_STORE instruction \p MI is storing an undef value.
+  bool matchUndefStore(MachineInstr &MI);
+
   /// Replace an instruction with a G_FCONSTANT with value \p C.
   bool replaceInstWithFConstant(MachineInstr &MI, double C);
 
   /// Replace an instruction with a G_CONSTANT with value \p C.
   bool replaceInstWithConstant(MachineInstr &MI, int64_t C);
 
   /// Replace an instruction with a G_IMPLICIT_DEF.
   bool replaceInstWithUndef(MachineInstr &MI);
public:
   bool matchSelectSameVal(MachineInstr &MI);
 
   /// Optimize (x op x) -> x
   bool matchBinOpSameVal(MachineInstr &MI);
 
   /// Check if operand \p OpIdx is zero.
   bool matchOperandIsZero(MachineInstr &MI, unsigned OpIdx);
 
+  /// Erase \p MI
+  bool eraseInst(MachineInstr &MI);
+
   /// Try to transform \p MI by using all of the above
   /// combine functions. Returns true if changed.
   bool tryCombine(MachineInstr &MI);
 
 private:
   // Memcpy family optimization helpers.
   bool optimizeMemcpy(MachineInstr &MI, Register Dst, Register Src,
                       unsigned KnownLen, Align DstAlign, Align SrcAlign,

llvm/include/llvm/Target/GlobalISel/Combine.td

 // Fold (x op 0) - > 0
 def binop_right_to_zero: GICombineRule<
   (defs root:$root),
   (match (wip_match_opcode G_MUL):$root,
     [{ return Helper.matchOperandIsZero(*${root}, 2); }]),
   (apply [{ return Helper.replaceSingleDefInstWithOperand(*${root}, 2); }])
 >;
 
+// Erase stores of undef values.
+def erase_undef_store : GICombineRule<
+  (defs root:$root),
+  (match (wip_match_opcode G_STORE):$root,
+    [{ return Helper.matchUndefStore(*${root}); }]),
+  (apply [{ return Helper.eraseInst(*${root}); }])
+>;
+
 // FIXME: These should use the custom predicate feature once it lands.
 def undef_combines : GICombineGroup<[undef_to_fp_zero, undef_to_int_zero,
                                      undef_to_negative_one,
                                      propagate_undef_any_op,
                                      propagate_undef_all_ops,
-                                     propagate_undef_shuffle_mask]>;
+                                     propagate_undef_shuffle_mask,
+                                     erase_undef_store]>;
 
 def identity_combines : GICombineGroup<[select_same_val, right_identity_zero,
                                         binop_same_val, binop_left_to_zero,
                                         binop_right_to_zero]>;
 
 def trivial_combines : GICombineGroup<[copy_prop, mul_to_shl]>;
 def all_combines : GICombineGroup<[trivial_combines, ptr_add_immed_chain,
     combines_for_extload, combine_indexed_load_store, undef_combines,
     identity_combines]>;

llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp

 }
 
 bool CombinerHelper::matchUndefShuffleVectorMask(MachineInstr &MI) {
   assert(MI.getOpcode() == TargetOpcode::G_SHUFFLE_VECTOR);
   ArrayRef<int> Mask = MI.getOperand(3).getShuffleMask();
   return all_of(Mask, [](int Elt) { return Elt < 0; });
 }
 
+bool CombinerHelper::matchUndefStore(MachineInstr &MI) {
+  assert(MI.getOpcode() == TargetOpcode::G_STORE);
+  return getOpcodeDef(TargetOpcode::G_IMPLICIT_DEF, MI.getOperand(0).getReg(),
+                      MRI);
+}
+
+bool CombinerHelper::eraseInst(MachineInstr &MI) {
+  MI.eraseFromParent();
+  return true;
+}
+
 bool CombinerHelper::matchEqualDefs(const MachineOperand &MOP1,
                                     const MachineOperand &MOP2) {
   if (!MOP1.isReg() || !MOP2.isReg())
     return false;
   MachineInstr *I1 = getDefIgnoringCopies(MOP1.getReg(), MRI);
   if (!I1)
     return false;
   MachineInstr *I2 = getDefIgnoringCopies(MOP2.getReg(), MRI);

llvm/lib/Target/AArch64/AArch64.h

 FunctionPass *createAArch64CleanupLocalDynamicTLSPass();
 
 FunctionPass *createAArch64CollectLOHPass();
 ModulePass *createSVEIntrinsicOptsPass();
 InstructionSelector *
 createAArch64InstructionSelector(const AArch64TargetMachine &,
                                  AArch64Subtarget &, AArch64RegisterBankInfo &);
 FunctionPass *createAArch64PreLegalizeCombiner(bool IsOptNone);
+FunctionPass *createAArch64PostLegalizeCombiner(bool IsOptNone);
 FunctionPass *createAArch64StackTaggingPass(bool MergeInit);
 FunctionPass *createAArch64StackTaggingPreRAPass();
 
 void initializeAArch64A53Fix835769Pass(PassRegistry&);
 void initializeAArch64A57FPLoadBalancingPass(PassRegistry&);
 void initializeAArch64AdvSIMDScalarPass(PassRegistry&);
 void initializeAArch64BranchTargetsPass(PassRegistry&);
 void initializeAArch64CollectLOHPass(PassRegistry&);
 void initializeAArch64CondBrTuningPass(PassRegistry &);
 void initializeAArch64CompressJumpTablesPass(PassRegistry&);
 void initializeAArch64ConditionalComparesPass(PassRegistry&);
 void initializeAArch64ConditionOptimizerPass(PassRegistry&);
 void initializeAArch64DeadRegisterDefinitionsPass(PassRegistry&);
 void initializeAArch64ExpandPseudoPass(PassRegistry&);
 void initializeAArch64SpeculationHardeningPass(PassRegistry&);
 void initializeAArch64LoadStoreOptPass(PassRegistry&);
 void initializeAArch64SIMDInstrOptPass(PassRegistry&);
 void initializeAArch64PreLegalizerCombinerPass(PassRegistry&);
+void initializeAArch64PostLegalizerCombinerPass(PassRegistry &);
 void initializeAArch64PromoteConstantPass(PassRegistry&);
 void initializeAArch64RedundantCopyEliminationPass(PassRegistry&);
 void initializeAArch64StorePairSuppressPass(PassRegistry&);
 void initializeFalkorHWPFFixPass(PassRegistry&);
 void initializeFalkorMarkStridedAccessesLegacyPass(PassRegistry&);
 void initializeLDTLSCleanupPass(PassRegistry&);
 void initializeSVEIntrinsicOptsPass(PassRegistry&);
 void initializeAArch64StackTaggingPass(PassRegistry&);
 void initializeAArch64StackTaggingPreRAPass(PassRegistry&);
 } // end namespace llvm
 
 #endif

llvm/lib/Target/AArch64/AArch64Combine.td

def fconstant_to_constant : GICombineRule<
   (apply [{ applyFConstantToConstant(*${root}); }])>;
 
 def AArch64PreLegalizerCombinerHelper: GICombinerHelper<
   "AArch64GenPreLegalizerCombinerHelper", [all_combines,
                                            elide_br_by_inverting_cond,
                                            fconstant_to_constant]> {
   let DisableRuleOption = "aarch64prelegalizercombiner-disable-rule";
 }
+
+def AArch64PostLegalizerCombinerHelper: GICombinerHelper<
+  "AArch64GenPostLegalizerCombinerHelper", [erase_undef_store]> {
+  let DisableRuleOption = "aarch64postlegalizercombiner-disable-rule";
+}

llvm/lib/Target/AArch64/AArch64PostLegalizerCombiner.cpp

+//=== lib/CodeGen/GlobalISel/AArch64PostLegalizerCombiner.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
+//
+//===----------------------------------------------------------------------===//
+//
+// This performs post-legalization combines on generic MachineInstrs.
+//
+// Any combine that this pass performs must preserve instruction legality.
+// Combines unconcerned with legality should be handled by the
+// PreLegalizerCombiner instead.
+//
+//===----------------------------------------------------------------------===//
+
+#include "AArch64TargetMachine.h"
+#include "llvm/CodeGen/GlobalISel/Combiner.h"
+#include "llvm/CodeGen/GlobalISel/CombinerHelper.h"
+#include "llvm/CodeGen/GlobalISel/CombinerInfo.h"
+#include "llvm/CodeGen/GlobalISel/GISelKnownBits.h"
+#include "llvm/CodeGen/MachineDominators.h"
+#include "llvm/CodeGen/MachineFunctionPass.h"
+#include "llvm/CodeGen/TargetPassConfig.h"
+#include "llvm/Support/Debug.h"
+
+#define DEBUG_TYPE "aarch64-postlegalizer-combiner"
+
+using namespace llvm;
+
+#define AARCH64POSTLEGALIZERCOMBINERHELPER_GENCOMBINERHELPER_DEPS
+#include "AArch64GenPostLegalizeGICombiner.inc"
+#undef AARCH64POSTLEGALIZERCOMBINERHELPER_GENCOMBINERHELPER_DEPS
+
+namespace {
+#define AARCH64POSTLEGALIZERCOMBINERHELPER_GENCOMBINERHELPER_H
+#include "AArch64GenPostLegalizeGICombiner.inc"
+#undef AARCH64POSTLEGALIZERCOMBINERHELPER_GENCOMBINERHELPER_H
+
+class AArch64PostLegalizerCombinerInfo : public CombinerInfo {
+  GISelKnownBits *KB;
+  MachineDominatorTree *MDT;
+
+public:
+  AArch64GenPostLegalizerCombinerHelper Generated;
+
+  AArch64PostLegalizerCombinerInfo(bool EnableOpt, bool OptSize, bool MinSize,
+                                   GISelKnownBits *KB,
+                                   MachineDominatorTree *MDT)
+      : CombinerInfo(/*AllowIllegalOps*/ true, /*ShouldLegalizeIllegal*/ false,
+                     /*LegalizerInfo*/ nullptr, EnableOpt, OptSize, MinSize),
+        KB(KB), MDT(MDT) {
+    if (!Generated.parseCommandLineOption())
+      report_fatal_error("Invalid rule identifier");
+  }
+
+  virtual bool combine(GISelChangeObserver &Observer, MachineInstr &MI,
+                       MachineIRBuilder &B) const override;
+};
+
+bool AArch64PostLegalizerCombinerInfo::combine(GISelChangeObserver &Observer,
+                                               MachineInstr &MI,
+                                               MachineIRBuilder &B) const {
+  CombinerHelper Helper(Observer, B, KB, MDT);
+  return Generated.tryCombineAll(Observer, MI, B, Helper);
+}
+
+#define AARCH64POSTLEGALIZERCOMBINERHELPER_GENCOMBINERHELPER_CPP
+#include "AArch64GenPostLegalizeGICombiner.inc"
+#undef AARCH64POSTLEGALIZERCOMBINERHELPER_GENCOMBINERHELPER_CPP
+
+class AArch64PostLegalizerCombiner : public MachineFunctionPass {
+public:
+  static char ID;
+
+  AArch64PostLegalizerCombiner(bool IsOptNone = false);
+
+  StringRef getPassName() const override {
+    return "AArch64PostLegalizerCombiner";
+  }
+
+  bool runOnMachineFunction(MachineFunction &MF) override;
+  void getAnalysisUsage(AnalysisUsage &AU) const override;
+
+private:
+  bool IsOptNone;
+};
+} // end anonymous namespace
+
+void AArch64PostLegalizerCombiner::getAnalysisUsage(AnalysisUsage &AU) const {
+  AU.addRequired<TargetPassConfig>();
+  AU.setPreservesCFG();
+  getSelectionDAGFallbackAnalysisUsage(AU);
+  AU.addRequired<GISelKnownBitsAnalysis>();
+  AU.addPreserved<GISelKnownBitsAnalysis>();
+  if (!IsOptNone) {
+    AU.addRequired<MachineDominatorTree>();
+    AU.addPreserved<MachineDominatorTree>();
+  }
+  MachineFunctionPass::getAnalysisUsage(AU);
+}
+
+AArch64PostLegalizerCombiner::AArch64PostLegalizerCombiner(bool IsOptNone)
+    : MachineFunctionPass(ID), IsOptNone(IsOptNone) {
+  initializeAArch64PostLegalizerCombinerPass(*PassRegistry::getPassRegistry());
+}
+
+bool AArch64PostLegalizerCombiner::runOnMachineFunction(MachineFunction &MF) {
+  if (MF.getProperties().hasProperty(
+          MachineFunctionProperties::Property::FailedISel))
+    return false;
+  assert(MF.getProperties().hasProperty(
+             MachineFunctionProperties::Property::Legalized) &&
+         "Expected a legalized function?");
+  auto *TPC = &getAnalysis<TargetPassConfig>();
+  const Function &F = MF.getFunction();
+  bool EnableOpt =
+      MF.getTarget().getOptLevel() != CodeGenOpt::None && !skipFunction(F);
+  GISelKnownBits *KB = &getAnalysis<GISelKnownBitsAnalysis>().get(MF);
+  MachineDominatorTree *MDT =
+      IsOptNone ? nullptr : &getAnalysis<MachineDominatorTree>();
+  AArch64PostLegalizerCombinerInfo PCInfo(EnableOpt, F.hasOptSize(),
+                                          F.hasMinSize(), KB, MDT);
+  Combiner C(PCInfo, TPC);
+  return C.combineMachineInstrs(MF, /*CSEInfo*/ nullptr);
+}
+
+char AArch64PostLegalizerCombiner::ID = 0;
+INITIALIZE_PASS_BEGIN(AArch64PostLegalizerCombiner, DEBUG_TYPE,
+                      "Combine AArch64 MachineInstrs after legalization", false,
+                      false)
+INITIALIZE_PASS_DEPENDENCY(TargetPassConfig)
+INITIALIZE_PASS_DEPENDENCY(GISelKnownBitsAnalysis)
+INITIALIZE_PASS_END(AArch64PostLegalizerCombiner, DEBUG_TYPE,
+                    "Combine AArch64 MachineInstrs after legalization", false,
+                    false)
+
+namespace llvm {
+FunctionPass *createAArch64PostLegalizeCombiner(bool IsOptNone) {
+  return new AArch64PostLegalizerCombiner(IsOptNone);
+}
+} // end namespace llvm

llvm/lib/Target/AArch64/AArch64PreLegalizerCombiner.cpp

static void applyFConstantToConstant(MachineInstr &MI) {
   assert(MI.getOpcode() == TargetOpcode::G_FCONSTANT);
   MachineIRBuilder MIB(MI);
   const APFloat &ImmValAPF = MI.getOperand(1).getFPImm()->getValueAPF();
   MIB.buildConstant(MI.getOperand(0).getReg(), ImmValAPF.bitcastToAPInt());
   MI.eraseFromParent();
 }
 
 #define AARCH64PRELEGALIZERCOMBINERHELPER_GENCOMBINERHELPER_DEPS
-#include "AArch64GenGICombiner.inc"
+#include "AArch64GenPreLegalizeGICombiner.inc"
 #undef AARCH64PRELEGALIZERCOMBINERHELPER_GENCOMBINERHELPER_DEPS
 
 namespace {
 #define AARCH64PRELEGALIZERCOMBINERHELPER_GENCOMBINERHELPER_H
-#include "AArch64GenGICombiner.inc"
+#include "AArch64GenPreLegalizeGICombiner.inc"
 #undef AARCH64PRELEGALIZERCOMBINERHELPER_GENCOMBINERHELPER_H
 
 class AArch64PreLegalizerCombinerInfo : public CombinerInfo {
   GISelKnownBits *KB;
   MachineDominatorTree *MDT;
 
 public:
   AArch64GenPreLegalizerCombinerHelper Generated;
bool AArch64PreLegalizerCombinerInfo::combine(GISelChangeObserver &Observer,
   case TargetOpcode::G_SHUFFLE_VECTOR:
     return Helper.tryCombineShuffleVector(MI);
   }
 
   return false;
 }
 
 #define AARCH64PRELEGALIZERCOMBINERHELPER_GENCOMBINERHELPER_CPP
-#include "AArch64GenGICombiner.inc"
+#include "AArch64GenPreLegalizeGICombiner.inc"
 #undef AARCH64PRELEGALIZERCOMBINERHELPER_GENCOMBINERHELPER_CPP
 
 // Pass boilerplate
 // ================
 
 class AArch64PreLegalizerCombiner : public MachineFunctionPass {
 public:
   static char ID;

llvm/lib/Target/AArch64/AArch64TargetMachine.cpp

extern "C" LLVM_EXTERNAL_VISIBILITY void LLVMInitializeAArch64Target() {
   initializeAArch64CompressJumpTablesPass(*PR);
   initializeAArch64ConditionalComparesPass(*PR);
   initializeAArch64ConditionOptimizerPass(*PR);
   initializeAArch64DeadRegisterDefinitionsPass(*PR);
   initializeAArch64ExpandPseudoPass(*PR);
   initializeAArch64LoadStoreOptPass(*PR);
   initializeAArch64SIMDInstrOptPass(*PR);
   initializeAArch64PreLegalizerCombinerPass(*PR);
+  initializeAArch64PostLegalizerCombinerPass(*PR);
   initializeAArch64PromoteConstantPass(*PR);
   initializeAArch64RedundantCopyEliminationPass(*PR);
   initializeAArch64StorePairSuppressPass(*PR);
   initializeFalkorHWPFFixPass(*PR);
   initializeFalkorMarkStridedAccessesLegacyPass(*PR);
   initializeLDTLSCleanupPass(*PR);
   initializeSVEIntrinsicOptsPass(*PR);
   initializeAArch64SpeculationHardeningPass(*PR);
public:
   }
 
   void addIRPasses()  override;
   bool addPreISel() override;
   bool addInstSelector() override;
   bool addIRTranslator() override;
   void addPreLegalizeMachineIR() override;
   bool addLegalizeMachineIR() override;
+  void addPreRegBankSelect() override;
   bool addRegBankSelect() override;
   void addPreGlobalInstructionSelect() override;
   bool addGlobalInstructionSelect() override;
   bool addILPOpts() override;
   void addPreRegAlloc() override;
   void addPostRegAlloc() override;
   void addPreSched2() override;
   void addPreEmitPass() override;
void AArch64PassConfig::addPreLegalizeMachineIR() {
   addPass(createAArch64PreLegalizeCombiner(IsOptNone));
 }
 
 bool AArch64PassConfig::addLegalizeMachineIR() {
   addPass(new Legalizer());
   return false;
 }
 
+void AArch64PassConfig::addPreRegBankSelect() {
+  // For now we don't add this to the pipeline for -O0. We could do in future
+  // if we split the combines into separate O0/opt groupings.
+  bool IsOptNone = getOptLevel() == CodeGenOpt::None;
+  if (!IsOptNone)
+    addPass(createAArch64PostLegalizeCombiner(IsOptNone));
+}
+
 bool AArch64PassConfig::addRegBankSelect() {
   addPass(new RegBankSelect());
   return false;
 }
 
 void AArch64PassConfig::addPreGlobalInstructionSelect() {
   addPass(new Localizer());
 }

llvm/lib/Target/AArch64/CMakeLists.txt

 set(LLVM_TARGET_DEFINITIONS AArch64.td)
 
 tablegen(LLVM AArch64GenAsmMatcher.inc -gen-asm-matcher)
 tablegen(LLVM AArch64GenAsmWriter.inc -gen-asm-writer)
 tablegen(LLVM AArch64GenAsmWriter1.inc -gen-asm-writer -asmwriternum=1)
 tablegen(LLVM AArch64GenCallingConv.inc -gen-callingconv)
 tablegen(LLVM AArch64GenDAGISel.inc -gen-dag-isel)
 tablegen(LLVM AArch64GenDisassemblerTables.inc -gen-disassembler)
 tablegen(LLVM AArch64GenFastISel.inc -gen-fast-isel)
 tablegen(LLVM AArch64GenGlobalISel.inc -gen-global-isel)
-tablegen(LLVM AArch64GenGICombiner.inc -gen-global-isel-combiner
+tablegen(LLVM AArch64GenPreLegalizeGICombiner.inc -gen-global-isel-combiner
               -combiners="AArch64PreLegalizerCombinerHelper")
+tablegen(LLVM AArch64GenPostLegalizeGICombiner.inc -gen-global-isel-combiner
+              -combiners="AArch64PostLegalizerCombinerHelper")
 tablegen(LLVM AArch64GenInstrInfo.inc -gen-instr-info)
 tablegen(LLVM AArch64GenMCCodeEmitter.inc -gen-emitter)
 tablegen(LLVM AArch64GenMCPseudoLowering.inc -gen-pseudo-lowering)
 tablegen(LLVM AArch64GenRegisterBank.inc -gen-register-bank)
 tablegen(LLVM AArch64GenRegisterInfo.inc -gen-register-info)
 tablegen(LLVM AArch64GenSubtargetInfo.inc -gen-subtarget)
 tablegen(LLVM AArch64GenSystemOperands.inc -gen-searchable-tables)
 tablegen(LLVM AArch64GenExegesis.inc -gen-exegesis)
add_llvm_target(AArch64CodeGen
   AArch64InstrInfo.cpp
   AArch64InstructionSelector.cpp
   AArch64LegalizerInfo.cpp
   AArch64LoadStoreOptimizer.cpp
   AArch64MachineFunctionInfo.cpp
   AArch64MacroFusion.cpp
   AArch64MCInstLower.cpp
   AArch64PreLegalizerCombiner.cpp
+  AArch64PostLegalizerCombiner.cpp
   AArch64PromoteConstant.cpp
   AArch64PBQPRegAlloc.cpp
   AArch64RegisterBankInfo.cpp
   AArch64RegisterInfo.cpp
   AArch64SelectionDAGInfo.cpp
   AArch64SpeculationHardening.cpp
   AArch64StackTagging.cpp
   AArch64StackTaggingPreRA.cpp

llvm/test/CodeGen/AArch64/GlobalISel/gisel-commandline-option.ll

 ; VERIFY-NEXT:   Verify generated machine code
 ; ENABLED-NEXT:  Analysis for ComputingKnownBits
 ; ENABLED-O1-NEXT:  MachineDominator Tree Construction
 ; ENABLED-NEXT:  PreLegalizerCombiner
 ; VERIFY-NEXT:   Verify generated machine code
 ; ENABLED-NEXT:  Analysis containing CSE Info
 ; ENABLED-NEXT:  Legalizer
 ; VERIFY-NEXT:   Verify generated machine code
-; ENABLED-NEXT:  RegBankSelect
+; ENABLED:  RegBankSelect
 ; VERIFY-NEXT:   Verify generated machine code
 ; ENABLED-NEXT:  Localizer
 ; VERIFY-O0-NEXT:   Verify generated machine code
 ; ENABLED-NEXT: Analysis for ComputingKnownBits
 ; ENABLED-NEXT:  InstructionSelect
 ; VERIFY-NEXT:   Verify generated machine code
 ; ENABLED-NEXT:  ResetMachineFunction
 

llvm/test/CodeGen/AArch64/GlobalISel/postlegalizer-combiner-store-undef.mir

+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py
+# RUN: llc -mtriple aarch64 -run-pass=aarch64-postlegalizer-combiner -verify-machineinstrs %s -o - | FileCheck %s
+
+...
+---
+name:            delete_store_undef
+legalized:       true
+tracksRegLiveness: true
+body:             |
+  bb.1.entry:
+    liveins: $x0
+
+    ; A store of an implicit def can safely be deleted before or after
+    ; legalization.
+
+    ; CHECK-LABEL: name: delete_store_undef
+    ; CHECK: liveins: $x0
+    ; CHECK-NOT: G_STORE
+    ; CHECK: RET_ReallyLR
+    %0:_(p0) = COPY $x0
+    %1:_(s32) = G_IMPLICIT_DEF
+    G_STORE %1(s32), %0(p0) :: (store 4)
+    RET_ReallyLR
+
+...