String optimization

[pyscripter 579]

I was trying to improve the performance of SynEdit in handling files with super long lines (e.g. 50K characters).  In doing so I came across an unexpected optimization that had a spectacular impact on performance.

 

See this innocent looking function probably written two decades ago (fCasedLine is a PChar):

function TSynCustomHighlighter.GetToken: string;
var
  Len: Integer;
begin
  Len := Run - fTokenPos;
  SetLength(Result, Len);
  if Len > 0 then
    StrLCopy(@Result[1], fCasedLine + fTokenPos, Len);
end;

By measuring I found that this function (called zillions of times) was a bottleneck and replaced it with:

function TSynCustomHighlighter.GetToken: string;
var
  Len: Integer;
begin
  Len := Run - fTokenPos;
  SetString(Result, fCasedLine + fTokenPos, Len);
end;

The impact was massive.

Both SetString and  StrLCopy in x64 are implemented in pascal (not assembly) using Move.   So initially I was puzzled.

 

However here is the reason.  Look at StrLCopy:

function StrLCopy(Dest: PWideChar; const Source: PWideChar; MaxLen: Cardinal): PWideChar;
var
  Len: Cardinal;
begin
  Result := Dest;
  Len := StrLen(Source);
  if Len > MaxLen then
    Len := MaxLen;
  Move(Source^, Dest^, Len * SizeOf(WideChar));
  Dest[Len] := #0;
end;

Can you see the problem?  It is the call to StrLen(Source)!  In my case the Source was very very long strings, the length of which was unnecessarily calculated the hard way.    The lesson here is that you need to identify the bottlenecks by measuring.  You may be in for surprises, sometimes pleasant ones,
 

 

Edited February 20, 2022 by pyscripter

[MarkShark 25]

Wow!  I've actually been bitten by strlen in the past.  Well done sir!!

[Edwin Yip 154]

Well done sir!

BTW, what tool(s) do you use to measure the performance? Thanks.

[pyscripter 579]

27 minutes ago, Edwin Yip said:

BTW, what tool(s) do you use to measure the performance?

TStopwatch and occasionally VTune.

[Guest]

SetString can raise a "EOutOfMemory" or not initialize target string... 🧐