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//===-- MVEVPTBlockPass.cpp - Insert MVE VPT blocks -----------------------===//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
#include "ARM.h"
#include "ARMMachineFunctionInfo.h"
#include "ARMSubtarget.h"
#include "MCTargetDesc/ARMBaseInfo.h"
#include "Thumb2InstrInfo.h"
#include "llvm/ADT/SmallSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/StringRef.h"
#include "llvm/CodeGen/MachineBasicBlock.h"
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstr.h"
#include "llvm/CodeGen/MachineInstrBuilder.h"
#include "llvm/CodeGen/MachineInstrBundle.h"
#include "llvm/CodeGen/MachineOperand.h"
#include "llvm/IR/DebugLoc.h"
#include "llvm/MC/MCInstrDesc.h"
#include "llvm/MC/MCRegisterInfo.h"
#include "llvm/Support/Debug.h"
#include <cassert>
#include <new>
using namespace llvm;
#define DEBUG_TYPE "arm-mve-vpt"
namespace {
class MVEVPTBlock : public MachineFunctionPass {
static char ID;
const Thumb2InstrInfo *TII;
const TargetRegisterInfo *TRI;
MVEVPTBlock() : MachineFunctionPass(ID) {}
bool runOnMachineFunction(MachineFunction &Fn) override;
MachineFunctionProperties getRequiredProperties() const override {
return MachineFunctionProperties().set(
StringRef getPassName() const override {
return "MVE VPT block insertion pass";
bool InsertVPTBlocks(MachineBasicBlock &MBB);
char MVEVPTBlock::ID = 0;
} // end anonymous namespace
INITIALIZE_PASS(MVEVPTBlock, DEBUG_TYPE, "ARM MVE VPT block pass", false, false)
static MachineInstr *findVCMPToFoldIntoVPST(MachineBasicBlock::iterator MI,
const TargetRegisterInfo *TRI,
unsigned &NewOpcode) {
// Search backwards to the instruction that defines VPR. This may or not
// be a VCMP, we check that after this loop. If we find another instruction
// that reads cpsr, we return nullptr.
MachineBasicBlock::iterator CmpMI = MI;
while (CmpMI != MI->getParent()->begin()) {
if (CmpMI->modifiesRegister(ARM::VPR, TRI))
if (CmpMI->readsRegister(ARM::VPR, TRI))
if (CmpMI == MI)
return nullptr;
NewOpcode = VCMPOpcodeToVPT(CmpMI->getOpcode());
if (NewOpcode == 0)
return nullptr;
// Search forward from CmpMI to MI, checking if either register was def'd
if (registerDefinedBetween(CmpMI->getOperand(1).getReg(), std::next(CmpMI),
return nullptr;
if (registerDefinedBetween(CmpMI->getOperand(2).getReg(), std::next(CmpMI),
return nullptr;
return &*CmpMI;
// Advances Iter past a block of predicated instructions.
// Returns true if it successfully skipped the whole block of predicated
// instructions. Returns false when it stopped early (due to MaxSteps), or if
// Iter didn't point to a predicated instruction.
static bool StepOverPredicatedInstrs(MachineBasicBlock::instr_iterator &Iter,
MachineBasicBlock::instr_iterator EndIter,
unsigned MaxSteps,
unsigned &NumInstrsSteppedOver) {
ARMVCC::VPTCodes NextPred = ARMVCC::None;
Register PredReg;
NumInstrsSteppedOver = 0;
while (Iter != EndIter) {
if (Iter->isDebugInstr()) {
// Skip debug instructions
NextPred = getVPTInstrPredicate(*Iter, PredReg);
assert(NextPred != ARMVCC::Else &&
"VPT block pass does not expect Else preds");
if (NextPred == ARMVCC::None || MaxSteps == 0)
return NumInstrsSteppedOver != 0 &&
(NextPred == ARMVCC::None || Iter == EndIter);
// Returns true if at least one instruction in the range [Iter, End) defines
// or kills VPR.
static bool IsVPRDefinedOrKilledByBlock(MachineBasicBlock::iterator Iter,
MachineBasicBlock::iterator End) {
for (; Iter != End; ++Iter)
if (Iter->definesRegister(ARM::VPR) || Iter->killsRegister(ARM::VPR))
return true;
return false;
// Creates a T, TT, TTT or TTTT BlockMask depending on BlockSize.
static ARM::PredBlockMask GetInitialBlockMask(unsigned BlockSize) {
switch (BlockSize) {
case 1:
return ARM::PredBlockMask::T;
case 2:
return ARM::PredBlockMask::TT;
case 3:
return ARM::PredBlockMask::TTT;
case 4:
return ARM::PredBlockMask::TTTT;
llvm_unreachable("Invalid BlockSize!");
// Given an iterator (Iter) that points at an instruction with a "Then"
// predicate, tries to create the largest block of continuous predicated
// instructions possible, and returns the VPT Block Mask of that block.
// This will try to perform some minor optimization in order to maximize the
// size of the block.
static ARM::PredBlockMask
CreateVPTBlock(MachineBasicBlock::instr_iterator &Iter,
MachineBasicBlock::instr_iterator EndIter,
SmallVectorImpl<MachineInstr *> &DeadInstructions) {
MachineBasicBlock::instr_iterator BlockBeg = Iter;
assert(getVPTInstrPredicate(*Iter) == ARMVCC::Then &&
"Expected a Predicated Instruction");
LLVM_DEBUG(dbgs() << "VPT block created for: "; Iter->dump());
unsigned BlockSize;
StepOverPredicatedInstrs(Iter, EndIter, 4, BlockSize);
LLVM_DEBUG(for (MachineBasicBlock::instr_iterator AddedInstIter =
AddedInstIter != Iter; ++AddedInstIter) {
if (AddedInstIter->isDebugInstr())
dbgs() << " adding: ";
// Generate the initial BlockMask
ARM::PredBlockMask BlockMask = GetInitialBlockMask(BlockSize);
// Remove VPNOTs while there's still room in the block, so we can make the
// largest block possible.
ARMVCC::VPTCodes CurrentPredicate = ARMVCC::Else;
while (BlockSize < 4 && Iter != EndIter &&
Iter->getOpcode() == ARM::MVE_VPNOT) {
// Try to skip all of the predicated instructions after the VPNOT, stopping
// after (4 - BlockSize). If we can't skip them all, stop.
unsigned ElseInstCnt = 0;
MachineBasicBlock::instr_iterator VPNOTBlockEndIter = std::next(Iter);
if (!StepOverPredicatedInstrs(VPNOTBlockEndIter, EndIter, (4 - BlockSize),
// Check if this VPNOT can be removed or not: It can only be removed if at
// least one of the predicated instruction that follows it kills or sets
// VPR.
if (!IsVPRDefinedOrKilledByBlock(Iter, VPNOTBlockEndIter))
LLVM_DEBUG(dbgs() << " removing VPNOT: "; Iter->dump());
// Record the new size of the block
BlockSize += ElseInstCnt;
assert(BlockSize <= 4 && "Block is too large!");
// Record the VPNot to remove it later.
// Replace the predicates of the instructions we're adding.
// Note that we are using "Iter" to iterate over the block so we can update
// it at the same time.
for (; Iter != VPNOTBlockEndIter; ++Iter) {
if (Iter->isDebugInstr())
// Find the register in which the predicate is
int OpIdx = findFirstVPTPredOperandIdx(*Iter);
assert(OpIdx != -1);
// Change the predicate and update the mask
BlockMask = expandPredBlockMask(BlockMask, CurrentPredicate);
LLVM_DEBUG(dbgs() << " adding : "; Iter->dump());
CurrentPredicate =
(CurrentPredicate == ARMVCC::Then ? ARMVCC::Else : ARMVCC::Then);
return BlockMask;
bool MVEVPTBlock::InsertVPTBlocks(MachineBasicBlock &Block) {
bool Modified = false;
MachineBasicBlock::instr_iterator MBIter = Block.instr_begin();
MachineBasicBlock::instr_iterator EndIter = Block.instr_end();
SmallVector<MachineInstr *, 4> DeadInstructions;
while (MBIter != EndIter) {
MachineInstr *MI = &*MBIter;
Register PredReg;
DebugLoc DL = MI->getDebugLoc();
ARMVCC::VPTCodes Pred = getVPTInstrPredicate(*MI, PredReg);
// The idea of the predicate 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, we hope we'll never generate an Else as input to this pass.
assert(Pred != ARMVCC::Else && "VPT block pass does not expect Else preds");
if (Pred == ARMVCC::None) {
ARM::PredBlockMask BlockMask =
CreateVPTBlock(MBIter, EndIter, DeadInstructions);
// Search back for a VCMP that can be folded to create a VPT, or else
// create a VPST directly
MachineInstrBuilder MIBuilder;
unsigned NewOpcode;
LLVM_DEBUG(dbgs() << " final block mask: " << (unsigned)BlockMask << "\n");
if (MachineInstr *VCMP = findVCMPToFoldIntoVPST(MI, TRI, NewOpcode)) {
LLVM_DEBUG(dbgs() << " folding VCMP into VPST: "; VCMP->dump());
MIBuilder = BuildMI(Block, MI, DL, TII->get(NewOpcode));
// We need to remove any kill flags between the original VCMP and the new
// insertion point.
for (MachineInstr &MII :
make_range(VCMP->getIterator(), MI->getIterator())) {
MII.clearRegisterKills(VCMP->getOperand(1).getReg(), TRI);
MII.clearRegisterKills(VCMP->getOperand(2).getReg(), TRI);
} else {
MIBuilder = BuildMI(Block, MI, DL, TII->get(ARM::MVE_VPST));
// Erase all dead instructions (VPNOT's). Do that now so that they do not
// mess with the bundle creation.
for (MachineInstr *DeadMI : DeadInstructions)
Block, MachineBasicBlock::instr_iterator(MIBuilder.getInstr()), MBIter);
Modified = true;
return Modified;
bool MVEVPTBlock::runOnMachineFunction(MachineFunction &Fn) {
const ARMSubtarget &STI = Fn.getSubtarget<ARMSubtarget>();
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 (MachineBasicBlock &MBB : Fn)
Modified |= InsertVPTBlocks(MBB);
LLVM_DEBUG(dbgs() << "**************************************\n");
return Modified;
/// createMVEVPTBlock - Returns an instance of the MVE VPT block
/// insertion pass.
FunctionPass *llvm::createMVEVPTBlockPass() { return new MVEVPTBlock(); }