Question

Is this

try
  DoSomethingThatMightThrowAnException;
except
  on E : ESyntaxError do
    begin
    if (E.ErrorCode = errMissingBracket) then
      HandleError
    else
      raise;
    end;
end;

slower than this?

try
  DoSomethingThatMightThrowAnException;
except
  on E : EMissingBracketSyntaxError do
    begin
    HandleError;
    end;
end;

What’s the difference to be expected? Does it matter? Note that this could happen several times through the call stack.

Answer 1

I’ve had a quick look at the assembler the compiler spews out for above code snippets. Turns out that the bytes right after jmp @HandleOnExeption contain data such as the exception class pointers you use in the on clauses (if any).

I’m not that well versed in assembler to know exactly what’s going on, but enough to understand what is roughly going on and come to this conclusion:

I suspect System.pas’ HandleOnException does a call @IsClass already, and passes the exception on if no suitable handler is found, so if you use on e:Exception and re-raise, this will add a little code and make two calls extra:

• one back to your exception handling section (in all cases)
• one call @RaiseAgain (in cases the exception gets re-raised)

So, there’s a difference. A minor one, but still, it’s there.