[ARM] MVE VPT Block Pass

[SjoerdMeijer]

Summary

Initial commit of a new pass to create vector predication blocks, called VPT
blocks, that are supported by the Armv8.1-M MVE architecture.

This is a first naive implementation. I.e., for 2 consecutive predicated
instructions I1 and I2, for example, it will generate 2 VPT blocks:

VPST
I1
VPST
I2

A more optimal implementation would obviously put instructions in the same VPT
block when they are predicated on the same condition and when it is allowed to
do this:

VPTT
I1
I2

We will address this optimisation with follow up patches when the groundwork is
in. Creating VPT Blocks is very similar to IT Blocks, which is the reason I
added this to Thumb2ITBlocks.cpp. This allows reuse of the def use analysis
that we need for the more optimal implementation.

VPT blocks cannot be nested in IT blocks, and vice versa, and so these 2 passes
cannot interact with each other. Instructions allowed in VPT blocks must
be MVE instructions that are marked as VPT compatible.

This depends on D62669 because of the VPST tablegen instruction description.

[SjoerdMeijer]

Obviously a test is missing here. I will have a look what is easier: extract a few tablegen instructions descriptions from later patches so that we can create/add a test here, or we can wait for the later patches to be in.
Also, the pass will run, but won't trigger because there are no instructions yet that have getVPTInstrPredicate.

[ostannard]

D62669 includes a few VPT predicable MVE instructions to test the assembly side of this, is that enough to add some MIR tests for this?

llvm/lib/Target/ARM/Thumb2ITBlockPass.cpp

395

Can this one be a range-base for loop?

[samparker]

llvm/lib/Target/ARM/Thumb2ITBlockPass.cpp

369

What's 8? Can we use an enum from somewhere?

[SjoerdMeijer]

D62669 includes a few VPT predicable MVE instructions to test the assembly side of this, is that enough to add some MIR tests for this?

Yep, nice one, will do.

llvm/lib/Target/ARM/Thumb2ITBlockPass.cpp

369

That is the Mask Value (0b1000) for the VPST instruction as defined in the ARM ARM, see section B5.6. Predication/conditional execution. I'm not sure there's an enum for this, but will have a look. I will at least rename and comment on the Mask value here.

395

easy piecy lemon squeezy. :-)

[SjoerdMeijer]

• registered the pass,
• so that I could create and add a MIR test,
• and wrote a range based for-loop.

[samparker]

llvm/lib/Target/ARM/Thumb2ITBlockPass.cpp

369

Ah yes, cheers. Probably worth creating an enum for the values in Table Rvwgt and have it live where ever ARMVCC is defined.

[simon_tatham]

llvm/lib/Target/ARM/Thumb2ITBlockPass.cpp

365

I suggest adding an assertion checking that the predicate is not ARMVCC::Else.

(The idea of that enum is that None, Then and Else are for use when handling assembly language: they correspond to the three possible suffixes "", "t" and "e" on the mnemonic. So when instructions are read from assembly source or disassembled from object code, you expect to see a mixture whenever there's a long VPT block. But in code generation, I hope we'll never generate an Else as input to this pass – though once this pass becomes able to generate nontrivial blocks, there will surely have to be Else-tagged instructions in its output, so that the clang -S output has the right suffixes on the mnemonics.

369

I don't know of an enum for it, but as of the recent D63219, there's a comment explaining the format in ARMAsmParser::ParseInstruction.

[SjoerdMeijer]

Thanks for your suggestion and explanation Simon. I have shamelessly almost literally copied and added it as a comment as I think that's very useful.

I have also added an enum declaration, locally for now, as I think I indeed need it later.

[samparker]

LGTM.

llvm/lib/Target/ARM/ARM.h

FunctionPass *createARMISelDag(ARMBaseTargetMachine &TM,
                                CodeGenOpt::Level OptLevel);
 FunctionPass *createA15SDOptimizerPass();
 FunctionPass *createARMLoadStoreOptimizationPass(bool PreAlloc = false);
 FunctionPass *createARMExpandPseudoPass();
 FunctionPass *createARMCodeGenPreparePass();
 FunctionPass *createARMConstantIslandPass();
 FunctionPass *createMLxExpansionPass();
 FunctionPass *createThumb2ITBlockPass();
+FunctionPass *createMVEVPTBlockPass();
 FunctionPass *createARMOptimizeBarriersPass();
 FunctionPass *createThumb2SizeReductionPass(
     std::function<bool(const Function &)> Ftor = nullptr);
 InstructionSelector *
 createARMInstructionSelector(const ARMBaseTargetMachine &TM, const ARMSubtarget &STI,
                              const ARMRegisterBankInfo &RBI);
 
 void LowerARMMachineInstrToMCInst(const MachineInstr *MI, MCInst &OutMI,

llvm/lib/Target/ARM/ARMTargetMachine.cpp

if (getOptLevel() != CodeGenOpt::None) {
     addPass(createThumb2SizeReductionPass([this](const Function &F) {
       return this->TM->getSubtarget<ARMSubtarget>(F).restrictIT();
     }));
 
     addPass(createIfConverter([](const MachineFunction &MF) {
       return !MF.getSubtarget<ARMSubtarget>().isThumb1Only();
     }));
   }
+  addPass(createMVEVPTBlockPass());
   addPass(createThumb2ITBlockPass());
 }
 
 void ARMPassConfig::addPreEmitPass() {
   addPass(createThumb2SizeReductionPass());
 
   // Constant island pass work on unbundled instructions.
   addPass(createUnpackMachineBundles([](const MachineFunction &MF) {

llvm/lib/Target/ARM/Thumb2ITBlockPass.cpp

bool Thumb2ITBlockPass::runOnMachineFunction(MachineFunction &Fn) {
   return Modified;
 }
 
 /// createThumb2ITBlockPass - Returns an instance of the Thumb2 IT blocks
 /// insertion pass.
 FunctionPass *llvm::createThumb2ITBlockPass() {
   return new Thumb2ITBlockPass();
 }
+
+#undef DEBUG_TYPE
+#define DEBUG_TYPE "mve-vpt"
+
+namespace {
+  class MVEVPTBlockPass : public MachineFunctionPass {
+  public:
+    static char ID;
+    const Thumb2InstrInfo *TII;
+    const TargetRegisterInfo *TRI;
+
+    MVEVPTBlockPass() : MachineFunctionPass(ID) {}
+
+    bool runOnMachineFunction(MachineFunction &Fn) override;
+
+    MachineFunctionProperties getRequiredProperties() const override {
+      return MachineFunctionProperties().set(
+          MachineFunctionProperties::Property::NoVRegs);
+    }
+
+    StringRef getPassName() const override {
+      return "MVE VPT blocks insertion pass";
+    }
+
+  private:
+    bool InsertVPTBlocks(MachineBasicBlock &MBB);
+  };
+
+  char MVEVPTBlockPass::ID = 0;
+
+} // end anonymous namespace
+
+bool MVEVPTBlockPass::InsertVPTBlocks(MachineBasicBlock &Block) {
+  bool Modified = false;
+  MachineBasicBlock::iterator MBIter = Block.begin();
+  MachineBasicBlock::iterator EndIter = Block.end();
+
+  while (MBIter != EndIter) {
+    MachineInstr *MI = &*MBIter;
+    unsigned PredReg = 0;
+    DebugLoc dl = MI->getDebugLoc();
+
+    if (getVPTInstrPredicate(*MI, PredReg) != ARMVCC::None) {
+      MachineInstrBuilder MIBuilder =
+          BuildMI(Block, MBIter, dl, TII->get(ARM::t2VPST));
+      MachineInstr *LastITMI = MI;
+      MachineBasicBlock::iterator InsertPos = MIBuilder.getInstr();
+
+      const unsigned Mask = 8U;
+      MIBuilder.addImm(Mask);
+
+      finalizeBundle(Block, InsertPos.getInstrIterator(),
+                     ++LastITMI->getIterator());
+      Modified = true;
+    }
+    ++MBIter;
+  }
+  return Modified;
+}
+
+bool MVEVPTBlockPass::runOnMachineFunction(MachineFunction &Fn) {
+  const ARMSubtarget &STI =
+      static_cast<const ARMSubtarget &>(Fn.getSubtarget());
+
+  if (!STI.isThumb2() || !STI.hasMVEIntegerOps())
+    return false;
+
+  TII = static_cast<const Thumb2InstrInfo *>(STI.getInstrInfo());
+  TRI = STI.getRegisterInfo();
+
+  LLVM_DEBUG(dbgs() << "********** ARM MVE VPT BLOCKS **********\n"
+                    << "********** Function: " << Fn.getName() << '\n');
+
+  bool Modified = false;
+  for (MachineFunction::iterator MFI = Fn.begin(), E = Fn.end(); MFI != E;) {
+    MachineBasicBlock &MBB = *MFI;
+    ++MFI;
+    Modified |= InsertVPTBlocks(MBB);
+  }
+  LLVM_DEBUG(dbgs() << "**************************************\n");
+  return Modified;
+}
+
+/// createMVEVPTBlockPass - Returns an instance of the MVE VPT block
+/// insertion pass.
+FunctionPass *llvm::createMVEVPTBlockPass() { return new MVEVPTBlockPass(); }

llvm/lib/Target/ARM/Thumb2InstrInfo.h

 private:
   void expandLoadStackGuard(MachineBasicBlock::iterator MI) const override;
 };
 
 /// getITInstrPredicate - Valid only in Thumb2 mode. This function is identical
 /// to llvm::getInstrPredicate except it returns AL for conditional branch
 /// instructions which are "predicated", but are not in IT blocks.
 ARMCC::CondCodes getITInstrPredicate(const MachineInstr &MI, unsigned &PredReg);
+
+// getVPTInstrPredicate: VPT analogue of that, plus a helper function
+// corresponding to MachineInstr::findFirstPredOperandIdx.
+int findFirstVPTPredOperandIdx(const MachineInstr &MI);
+ARMVCC::VPTCodes getVPTInstrPredicate(const MachineInstr &MI,
+                                      unsigned &PredReg);
 }
 
 #endif

llvm/lib/Target/ARM/Thumb2InstrInfo.cpp

 
 ARMCC::CondCodes llvm::getITInstrPredicate(const MachineInstr &MI,
                                            unsigned &PredReg) {
   unsigned Opc = MI.getOpcode();
   if (Opc == ARM::tBcc || Opc == ARM::t2Bcc)
     return ARMCC::AL;
   return getInstrPredicate(MI, PredReg);
 }
+
+int llvm::findFirstVPTPredOperandIdx(const MachineInstr &MI) {
+  const MCInstrDesc &MCID = MI.getDesc();
+
+  if (!MCID.OpInfo)
+    return -1;
+
+  for (unsigned i = 0, e = MCID.getNumOperands(); i != e; ++i)
+    if (ARM::isVpred(MCID.OpInfo[i].OperandType))
+      return i;
+
+  return -1;
+}
+
+ARMVCC::VPTCodes llvm::getVPTInstrPredicate(const MachineInstr &MI,
+                                            unsigned &PredReg) {
+  int PIdx = findFirstVPTPredOperandIdx(MI);
+  if (PIdx == -1) {
+    PredReg = 0;
+    return ARMVCC::None;
+  }
+
+  PredReg = MI.getOperand(PIdx+1).getReg();
+  return (ARMVCC::VPTCodes)MI.getOperand(PIdx).getImm();
+}

llvm/test/CodeGen/ARM/O3-pipeline.ll

 ; CHECK-NEXT:      ARM Execution Domain Fix
 ; CHECK-NEXT:      BreakFalseDeps
 ; CHECK-NEXT:      ARM pseudo instruction expansion pass
 ; CHECK-NEXT:      Thumb2 instruction size reduce pass
 ; CHECK-NEXT:      MachineDominator Tree Construction
 ; CHECK-NEXT:      Machine Natural Loop Construction
 ; CHECK-NEXT:      Machine Block Frequency Analysis
 ; CHECK-NEXT:      If Converter
+; CHECK-NEXT:      MVE VPT block insertion pass
 ; CHECK-NEXT:      Thumb IT blocks insertion pass
 ; CHECK-NEXT:      MachineDominator Tree Construction
 ; CHECK-NEXT:      Machine Natural Loop Construction
 ; CHECK-NEXT:      Post RA top-down list latency scheduler
 ; CHECK-NEXT:      Analyze Machine Code For Garbage Collection
 ; CHECK-NEXT:      Machine Block Frequency Analysis
 ; CHECK-NEXT:      MachinePostDominator Tree Construction
 ; CHECK-NEXT:      Branch Probability Basic Block Placement