DaVinci Unlimited Software » disassembly http://www.davinciunltd.com Jim McKeeth's blog on creative and innovative Delphi programming. Wed, 09 Nov 2011 10:39:45 +0000 http://wordpress.org/?v=2.8.4 en hourly 1 Order of Enum in Case Statement http://www.davinciunltd.com/2008/07/order-of-enum-in-case-statement/ http://www.davinciunltd.com/2008/07/order-of-enum-in-case-statement/#comments Sun, 13 Jul 2008 00:16:58 +0000 Jim McKeeth http://www.davinciunltd.com/2008/07/order-of-enum-in-case-statement/ A while back my manager asked me if the order of the enums in a Delphi case statement changed performance: i.e. Enums in order being faster then those not.  I was pretty sure it didn’t, but thought it was worth checking out.  Time for a test application and some disassembly . . .

Here are my types and variables, which are the same for both examples.

type
  TMyEnum = (my1, my2, my3, my4);
var
  myEnum: TMyEnum;
  val: Char;

Here are the two examples, side by side, with assembly code to follow.  BTW, compiler optimization was turned on.

Case enum in order Case enum out of order 
  myEnum := my1;
  case myEnum of
    my1: val := '1';
    my2: val := '2';
    my3: val := '3';
    my4: val := '4';
  else
    val := '?';
  end;
 
  myEnum := my1;
  case myEnum of
    my3: val := '3';
    my1: val := '1';
    my4: val := '4';
    my2: val := '2';
  else
    val := '?';
  end;
—– Disassembly —–
case myEnum of case myEnum of
BF9E sub al,$01 C00A sub al,$01
BFA0 jb $bfae C00C jb $c01e
BFA2 jz $bfb2 C00E jz $c026
BFA4 dec al C010 dec al
BFA6 jz $bfb6 C012 jz $c01a
BFA8 dec al C014 dec al
BFAA jz $bfba C016 jz $c022
BFAC jmp $bfbe C018 jmp $c02a
my1: val := ‘1′; my3: val := ‘3′;
BFAE mov bl,$31 C01A mov bl,$33
BFB0 jmp $bfc0 C01C jmp $c02c
my2: val := ‘2′; my1: val := ‘1′;
BFB2 mov bl,$32 C01E mov bl,$31
BFB4 jmp $bfc0 C020 jmp $c02c
my3: val := ‘3′; my4: val := ‘4′;
BFB6 mov bl,$33 C022 mov bl,$34
BFB8 jmp $bfc0 C024 jmp $c02c
my4: val := ‘4′; my2: val := ‘2′;
BFBA mov bl,$34 C026 mov bl,$32
BFBC jmp $bfc0 C028 jmp $c02c
val := ‘?’; val := ‘?’;
BFBE mov bl,$20 C02A mov bl,$20

Same number of lines of assembly code. There may be some internal CPU optimization, but I don’t expect there is any.

Conclusion: Order of the enum in the case statement does not change performance.  Anyone else have any details I missed, or evidence to the contrary?  I guess the next step is to test it in .NET and IL. . . .

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