llvm/llvm/lib/CodeGen/InitUndef.cpp

//===- InitUndef.cpp - Initialize undef value to pseudo ----===//
//
// 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 file implements a function pass that initializes undef value to
// temporary pseudo instruction to prevent register allocation resulting in a
// constraint violated result for the particular instruction. It also rewrites
// the NoReg tied operand back to an IMPLICIT_DEF.
//
// Certain instructions have register overlapping constraints, and
// will cause illegal instruction trap if violated, we use early clobber to
// model this constraint, but it can't prevent register allocator allocating
// same or overlapped if the input register is undef value, so convert
// IMPLICIT_DEF to temporary pseudo instruction and remove it later could
// prevent that happen, it's not best way to resolve this, and it might
// change the order of program or increase the register pressure, so ideally we
// should model the constraint right, but before we model the constraint right,
// it's the only way to prevent that happen.
//
// When we enable the subregister liveness option, it will also trigger the same
// issue due to the partial of register is undef. If we pseudoinit the whole
// register, then it will generate redundant COPY instruction. Currently, it
// will generate INSERT_SUBREG to make sure the whole register is occupied
// when program encounter operation that has early-clobber constraint.
//
//
// See also: https://github.com/llvm/llvm-project/issues/50157
//
// Additionally, this pass rewrites tied operands of instructions
// from NoReg to IMPLICIT_DEF.  (Not that this is a non-overlapping set of
// operands to the above.)  We use NoReg to side step a MachineCSE
// optimization quality problem but need to convert back before
// TwoAddressInstruction.  See pr64282 for context.
//
//===----------------------------------------------------------------------===//

#include "llvm/ADT/SmallSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/CodeGen/DetectDeadLanes.h"
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/CodeGen/TargetInstrInfo.h"
#include "llvm/CodeGen/TargetRegisterInfo.h"
#include "llvm/CodeGen/TargetSubtargetInfo.h"
#include "llvm/InitializePasses.h"
#include "llvm/MC/MCRegister.h"
#include "llvm/Pass.h"
#include "llvm/Support/Debug.h"

usingnamespacellvm;

#define DEBUG_TYPE …
#define INIT_UNDEF_NAME …

namespace {

class InitUndef : public MachineFunctionPass { … };

} // end anonymous namespace

char InitUndef::ID = …;
INITIALIZE_PASS(…)
char &llvm::InitUndefID = …;

static bool isEarlyClobberMI(MachineInstr &MI) { … }

static bool findImplictDefMIFromReg(Register Reg, MachineRegisterInfo *MRI) { … }

bool InitUndef::handleReg(MachineInstr *MI) { … }

bool InitUndef::handleSubReg(MachineFunction &MF, MachineInstr &MI,
                             const DeadLaneDetector &DLD) { … }

bool InitUndef::fixupIllOperand(MachineInstr *MI, MachineOperand &MO) { … }

bool InitUndef::processBasicBlock(MachineFunction &MF, MachineBasicBlock &MBB,
                                  const DeadLaneDetector *DLD) { … }

bool InitUndef::runOnMachineFunction(MachineFunction &MF) { … }