# Avoiding implicit try finally section

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## Overview

Sometimes it's useful to know that compiler can wrap some code in implicit try ... finally block. Essentialy this is needed when you use variable of any type that must be initialized / finalized (i.e. standard procedures Initialize() and Finalize() do something meaningful with it), like AnsiStrings or Variants or dynamic arrays.

E.g. a procedure like

procedure P;
var
S: AnsiString;
begin
... do something with S ...
end;

is actually compiled like

procedure P;
var
S: AnsiString;
begin
Initialize(S);
try
... do something with S ...
finally Finalize(S) end;
end;

This is needed to be sure that the reference count of S will be properly decremented when procedure P exits with exceptions. But in some cases this can significantly affect the speed of given code.

Here's a link to archived discussion on fpc-devel list regarding this issue, with subject "TList slowness in classes" : http://www.mail-archive.com/fpc-devel@lists.freepascal.org/msg01367.html

Note that ansistring temp variables can be created implicitly. The only real way to be sure what's going on is to read the assembler output.

• use {$implicitexceptions off} Make sure however that you use this only on release versions of your program. Debugging can become a problem with that switch especially locating memory leaks and corruption. • split off rarely used code that causes an implicit try finally into separate procedures. (You can use procedures in procedures) • use const parameters rather than value parameters. This avoids the need to change refcount but temporary variables could still be an issue. • use global variables. You have to be careful with reentrancy issues here though and temporary variables could still be an issue. • use non refcounted types like shortstrings. ## Risks and when to apply Warning: These exception frames are generated for a reason. If you leave them out any exception in that code will leave a memory leak In 2007$implicitexceptions was added to the strutils unit. Meanwhile, sysutils has probably followed (to do: verify). For this, the following approach was followed:

• A routine that calls a routine that raises exceptions is unsafe - e.g. strtoint, but not strtointdef.
• A routine that raises exceptions itself is unsafe.
• Very large routines are not worth the trouble, because of the risk and low gains - e.g. date formatting routines.
• Floating point use can raise exceptions that are converted into catchable exceptions by sysutils. I'm not sure if this really is sufficient reason, but I skipped floating point using routines initially for this reason.

## Demo program

Below is a small demo program that

• When run, clearly shows that avoiding an implicit try ... finally block can make code a lot faster. When I run this program on my system, I get
Time of Foo_Normal: 141
Time of Foo_Faster: 17

• Shows a trick how to avoid implicit try ... finally block (without changing the meaning or safety of the code) in some cases (when you don't need to actually use that AnsiString/Variant/something every time procedure is called but e.g. only if some parameter has some particular value).
{$mode objfpc}{$H+}

uses
{BaseUnix, Unix needed only to implement Clock} BaseUnix, Unix,
SysUtils;

function Clock: Int64;
var Dummy: tms;
begin
Clock := FpTimes(Dummy);
end;

procedure Foo_Normal(i: Integer);
var S: string;
begin
if i = -1 then
begin
S := 'Some operation with AnsiString';
raise Exception.Create(S);
end;
end;

procedure Foo_Faster(i: Integer);

procedure RaiseError;
var S: string;
begin
S := 'Some operation with AnsiString';
raise Exception.Create(S);
end;

begin
if i = -1 then RaiseError;
end;

{ Note that when I call Foo_Normal and Foo_ResourceString
i is always >= 0 so Exception is never actually raised.
So string constants SNormal and SResString are not really used. }

const
TestCount = 10000000;
var
i: Integer;
Start: Int64;
begin
Start := Clock;
for i := 0 to TestCount do Foo_Normal(i);
Writeln('Time of Foo_Normal: ', Clock - Start);

Start := Clock;
for i := 0 to TestCount do Foo_Faster(i);
Writeln('Time of Foo_Faster: ', Clock - Start);
end.