I'm perplexed. At CodeRage today, Marco Cantu said that CharInSet was slow and I should try a Case statement instead. I did so in my parser and then checked with AQTime what the speedup was. I found the Case statement to be much slower.
4,894,539 executions of:
while not CharInSet (P^, [' ', #10,#13, #0]) do inc(P);
was timed at 0.25 seconds.
But the same number of executions of:
while True do
case P^ of
' ', #10, #13, #0: break;
else inc(P);
end;
takes .16 seconds for the "while True", .80 seconds for the first case, and .13 seconds for the else case, totaling 1.09 seconds, or over 4 times as long.
The assembler code for the CharInSet statement is:
add edi,$02
mov edx,$0064b290
movzx eax,[edi]
call CharInSet
test a1,a1
jz $00649f18 (back to the add statement)
whereas the case logic is simply this:
movzx eax,[edi]
sub ax,$01
jb $00649ef0
sub ax,$09
jz $00649ef0
sub ax,$03
jz $00649ef0
add edi,$02
jmp $00649ed6 (back to the movzx statement)
The case logic looks to me to be using very efficient assembler, whereas the CharInSet statement actually has to make a call to the CharInSet function, which is in SysUtils and is also simple, being:
function CharInSet(C: AnsiChar; const CharSet: TSysCharSet): Boolean;
begin
Result := C in CharSet;
end;
I think the only reason why this is done is because P^ in [' ', #10, #13, #0] is no longer allowed in Delphi 2009 so the call does the conversion of types to allow it.
None-the-less I am very surprised by this and still don't trust my result.
Is AQTime measuring something wrong, am I missing something in this comparison, or is CharInSet truly an efficient function worth using?
Conclusion:
I think you got it, Barry. Thank you for taking the time and doing the detailed example. I tested your code on my machine and got .171, .066 and .052 seconds (I guess my desktop is a bit faster than your laptop).
Testing that code in AQTime, it gives: 0.79, 1.57 and 1.46 seconds for the three tests. There you can see the large overhead from the instrumentation. But what really surprises me is that this overhead changes the apparent "best" result to be the CharInSet function which is actually the worst.
So Marcu is correct and CharInSet is slower. But you've inadvertently (or maybe on purpose) given me a better way by pulling out what CharInSet is doing with the AnsiChar(P^) in Set method. Other than the minor speed advantage over the case method, it is also less code and more understandable than using the cases.
You've also made me aware of the possibility of incorrect optimization using AQTime (and other instrumenting profilers). Knowing this will help my decision re Profiler and Memory Analysis Tools for Delphi and it also is another answer to my question How Does AQTime Do It?. Of course, AQTime doesn't change the code when it instruments, so it must use some other magic to do it.
So the answer is that AQTime is showing results that lead to the incorrect conclusion.