llvm/llvm/lib/CodeGen/RemoveLoadsIntoFakeUses.cpp

//===---- RemoveLoadsIntoFakeUses.cpp - Remove loads with no real uses ----===//
//
// 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
//
//===----------------------------------------------------------------------===//
///
/// \file
/// The FAKE_USE instruction is used to preserve certain values through
/// optimizations for the sake of debugging. This may result in spilled values
/// being loaded into registers that are only used by FAKE_USEs; this is not
/// necessary for debugging purposes, because at that point the value must be on
/// the stack and hence available for debugging. Therefore, this pass removes
/// loads that are only used by FAKE_USEs.
///
/// This pass should run very late, to ensure that we don't inadvertently
/// shorten stack lifetimes by removing these loads, since the FAKE_USEs will
/// also no longer be in effect. Running immediately before LiveDebugValues
/// ensures that LDV will have accurate information of the machine location of
/// debug values.
///
//===----------------------------------------------------------------------===//

#include "llvm/ADT/PostOrderIterator.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/CodeGen/LiveRegUnits.h"
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/CodeGen/TargetSubtargetInfo.h"
#include "llvm/IR/Function.h"
#include "llvm/InitializePasses.h"
#include "llvm/Support/Debug.h"

usingnamespacellvm;

#define DEBUG_TYPE …

STATISTIC(NumLoadsDeleted, "Number of dead load instructions deleted");
STATISTIC(NumFakeUsesDeleted, "Number of FAKE_USE instructions deleted");

class RemoveLoadsIntoFakeUses : public MachineFunctionPass { … };

char RemoveLoadsIntoFakeUses::ID = …;
char &llvm::RemoveLoadsIntoFakeUsesID = …;

INITIALIZE_PASS_BEGIN(RemoveLoadsIntoFakeUses, DEBUG_TYPE,
                      "Remove Loads Into Fake Uses", false, false)
INITIALIZE_PASS_END(RemoveLoadsIntoFakeUses, DEBUG_TYPE,
                    "Remove Loads Into Fake Uses", false, false)

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