DELPHI用const来提高应用程序在多核多线程下的性能

来自：http://bbs.csdn.net/topics/330048800

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我们经常在DELPHI中用const来定义常量，用const来保护函数参数，其实在用const保护函数参数还有另一个更为重要的作用，提高应用程序的执行效率，尤其是在多线程多核下效果更明显。原因是：普通的函数参数如Add(AValue: string)，编译器在传入参数的时候先把变量复制一份，然后当成AValue传入Add，函数结束的时候进行销毁，你在参数上加了const，编译器在传入参数的时候不会进行复制，而是直接传地址，并在编译期间检查不能修改AValue值，我们知道DELPHI的内存管理在申请内存的时候是会加锁的，因此如果调用函数频繁，而且没有加const，这样会造成线程排队等候，性能会不如单线程，const只是对string、结构体等非基本类型有提高效率的作用，对Integer等基本类型（栈变量）不起作用。

1、const的类型检查，以下代码可以修改const参数的值

procedure TFmMain.EditConstParameter(const ARecordTest: TRecordTest);
var
  pPoint: PRecordTest;
begin
  pPoint := @ARecordTest;
  pPoint.A := 1;
  ShowMessage(IntToStr(ARecordTest.A));
end;

procedure TFmMain.btnEditConstClick(Sender: TObject);
var
  ARecordTest: TRecordTest;
begin
  ARecordTest.A := 0;
  EditConstParameter(ARecordTest);
  Inc(ARecordTest.A);
  ShowMessage(IntToStr(ARecordTest.A));
end;

2、const提高代码性能，使用const提高代码性能，大家可以把以下例子在自己电脑上测试。

unit Main;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls, DateUtils;

const
  WM_Complete = WM_USER + 1001;
type
  TRecordTest = record
    A: Integer;
    B: Integer;
    C: Integer;
    D: Integer;
    E: Integer;
    F: Integer;
    AStr: string;
    BStr: string;
    CStr: string;
    DStr: string;
    EStr: string;
    FStr: string;
    FCommit: array[0..15*1024*4] of Char;
  end;
  PRecordTest = ^TRecordTest;

  TTestThread = class;

  TFmMain = class(TForm)
    grpConst: TGroupBox;
    cbbConstThreadNum: TComboBox;
    lblThreadConst: TLabel;
    btnConstStart: TButton;
    btnConstStop: TButton;
    grp1: TGroupBox;
    lbl1: TLabel;
    cbbUnConstThreadNum: TComboBox;
    btnUnConstStart: TButton;
    btnUnConstStop: TButton;
    mmoText: TMemo;
    btnEditConst: TButton;
    procedure btnConstStartClick(Sender: TObject);
    procedure btnConstStopClick(Sender: TObject);
    procedure btnUnConstStartClick(Sender: TObject);
    procedure btnUnConstStopClick(Sender: TObject);
    procedure btnEditConstClick(Sender: TObject);
  private
    { Private declarations }
    FStartTime, FEndTime: TDateTime;
    FConstThread, FUnConstThread: array of TTestThread;
  protected
    procedure WMComplete(var Msg: TMessage); message WM_Complete;
  public
    {* 修改const函数变量 *}
    procedure EditConstParameter(const ARecordTest: TRecordTest);
    {* 线程测试函数 *}
    function ConstTestA(const ARecordTest: TRecordTest): Integer;
    function ConstTestB(const ARecordTest: TRecordTest): Integer;
    function ConstTestC(const ARecordTest: TRecordTest): Integer;
    function ConstTestD(const ARecordTest: TRecordTest): Integer;
    function ConstTestE(const ARecordTest: TRecordTest): Integer;
    function ConstTestF(const ARecordTest: TRecordTest): Integer;
    function UnConstTestA(ARecordTest: TRecordTest): Integer;
    function UnConstTestB(ARecordTest: TRecordTest): Integer;
    function UnConstTestC(ARecordTest: TRecordTest): Integer;
    function UnConstTestD(ARecordTest: TRecordTest): Integer;
    function UnConstTestE(ARecordTest: TRecordTest): Integer;
    function UnConstTestF(ARecordTest: TRecordTest): Integer;
  end;

  TTestThread = class(TThread)
  private
    FConst: Boolean;
  protected
    procedure Execute; override;
  end;

var
  FmMain: TFmMain;

implementation

{$R *.dfm}

{ TFmMain }

procedure TFmMain.EditConstParameter(const ARecordTest: TRecordTest);
var
  pPoint: PRecordTest;
begin
  pPoint := @ARecordTest;
  pPoint.A := 1;
  ShowMessage(IntToStr(ARecordTest.A));
end;

procedure TFmMain.btnEditConstClick(Sender: TObject);
var
  ARecordTest: TRecordTest;
begin
  ARecordTest.A := 0;
  EditConstParameter(ARecordTest);
  Inc(ARecordTest.A);
  ShowMessage(IntToStr(ARecordTest.A));
end;

function TFmMain.ConstTestA(const ARecordTest: TRecordTest): Integer;
var
  i, j: Integer;
begin
  j := ARecordTest.A;
  for i := 0 to 5 do
  begin
    j := j + 1;
  end;
  Result := j;
  ConstTestB(ARecordTest);
end;

function TFmMain.ConstTestB(const ARecordTest: TRecordTest): Integer;
var
  i, j: Integer;
begin
  j := ARecordTest.A;
  for i := 0 to 5 do
  begin
    j := j + 1;
  end;
  Result := j;
  ConstTestC(ARecordTest);
end;

function TFmMain.ConstTestC(const ARecordTest: TRecordTest): Integer;
var
  i, j: Integer;
begin
  j := ARecordTest.A;
  for i := 0 to 5 do
  begin
    j := j + 1;
  end;
  Result := j;
  ConstTestD(ARecordTest);
end;

function TFmMain.ConstTestD(const ARecordTest: TRecordTest): Integer;
var
  i, j: Integer;
begin
  j := ARecordTest.A;
  for i := 0 to 5 do
  begin
    j := j + 1;
  end;
  Result := j;
  ConstTestE(ARecordTest);
end;

function TFmMain.ConstTestE(const ARecordTest: TRecordTest): Integer;
var
  i, j: Integer;
begin
  j := ARecordTest.A;
  for i := 0 to 5 do
  begin
    j := j + 1;
  end;
  Result := j;
  ConstTestF(ARecordTest);
end;

function TFmMain.ConstTestF(const ARecordTest: TRecordTest): Integer;
var
  i, j: Integer;
begin
  j := ARecordTest.A;
  for i := 0 to 5 do
  begin
    j := j + 1;
  end;
  Result := j;
end;

function TFmMain.UnConstTestA(ARecordTest: TRecordTest): Integer;
var
  i, j: Integer;
begin
  j := ARecordTest.A;
  for i := 0 to 5 do
  begin
    j := j + 1;
  end;
  Result := j;
  UnConstTestB(ARecordTest);
end;

function TFmMain.UnConstTestB(ARecordTest: TRecordTest): Integer;
var
  i, j: Integer;
begin
  j := ARecordTest.A;
  for i := 0 to 5 do
  begin
    j := j + 1;
  end;
  Result := j;
  UnConstTestC(ARecordTest);
end;

function TFmMain.UnConstTestC(ARecordTest: TRecordTest): Integer;
var
  i, j: Integer;
begin
  j := ARecordTest.A;
  for i := 0 to 5 do
  begin
    j := j + 1;
  end;
  Result := j;
  UnConstTestD(ARecordTest);
end;

function TFmMain.UnConstTestD(ARecordTest: TRecordTest): Integer;
var
  i, j: Integer;
begin
  j := ARecordTest.A;
  for i := 0 to 5 do
  begin
    j := j + 1;
  end;
  Result := j;
  UnConstTestE(ARecordTest);
end;

function TFmMain.UnConstTestE(ARecordTest: TRecordTest): Integer;
var
  i, j: Integer;
begin
  j := ARecordTest.A;
  for i := 0 to 5 do
  begin
    j := j + 1;
  end;
  Result := j;
  UnConstTestF(ARecordTest);
end;

function TFmMain.UnConstTestF(ARecordTest: TRecordTest): Integer;
var
  i, j: Integer;
begin
  j := ARecordTest.A;
  for i := 0 to 5 do
  begin
    j := j + 1;
  end;
  Result := j;
end;

procedure TFmMain.WMComplete(var Msg: TMessage);
begin
  FEndTime := Now;
  mmoText.Lines.Add('Spend Time: ' + IntToStr(MilliSecondsBetween(FStartTime, FEndTime)));
end;

{ TTestThread }

procedure TTestThread.Execute;
var
  ARecordTest: TRecordTest;
begin
  inherited;
  ARecordTest.A := 0;
  while ARecordTest.A < 1000000 do
  begin
    if FConst then
    begin
      Inc(ARecordTest.A);
      FmMain.ConstTestA(ARecordTest);
    end
    else
    begin
      Inc(ARecordTest.A);
      FmMain.UnConstTestA(ARecordTest);
    end;
  end;
  SendMessage(FmMain.Handle, WM_Complete, 0, 0);
end;

procedure TFmMain.btnConstStartClick(Sender: TObject);
var
  i: Integer;
begin
  FStartTime := Now;
  SetLength(FConstThread, StrToInt(cbbConstThreadNum.Text));
  for i := Low(FConstThread) to High(FConstThread) do
  begin
    FConstThread[i] := TTestThread.Create(True);
    FConstThread[i].FreeOnTerminate := True;
    FConstThread[i].FConst := True;
  end;
  for i := Low(FConstThread) to High(FConstThread) do
  begin
    FConstThread[i].Resume;
  end;
  btnConstStart.Enabled := False;
  btnConstStop.Enabled := True;
end;

procedure TFmMain.btnConstStopClick(Sender: TObject);
var
  i: Integer;
begin
  if Length(FConstThread) = 0 then Exit;
  for i := Low(FConstThread) to High(FConstThread) do
  begin
    FConstThread[i].Terminate;
  end;
  SetLength(FConstThread, 0);
  btnConstStart.Enabled := True;
  btnConstStop.Enabled := False;
end;

procedure TFmMain.btnUnConstStartClick(Sender: TObject);
var
  i: Integer;
begin
  FStartTime := Now;
  SetLength(FUnConstThread, StrToInt(cbbUnConstThreadNum.Text));
  for i := Low(FUnConstThread) to High(FUnConstThread) do
  begin
    FUnConstThread[i] := TTestThread.Create(True);
    FUnConstThread[i].FreeOnTerminate := True;
    FUnConstThread[i].FConst := False;
  end;
  for i := Low(FUnConstThread) to High(FUnConstThread) do
  begin
    FUnConstThread[i].Resume;
  end;
  btnUnConstStart.Enabled := False;
  btnUnConstStop.Enabled := True;
end;

procedure TFmMain.btnUnConstStopClick(Sender: TObject);
var
  i: Integer;
begin
  if Length(FUnConstThread) = 0 then Exit;
  for i := Low(FUnConstThread) to High(FUnConstThread) do
  begin
    FUnConstThread[i].Terminate;
  end;
  SetLength(FUnConstThread, 0);
  btnUnConstStart.Enabled := True;
  btnUnConstStop.Enabled := False;
end;

end.