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March 25th, 2012, 07:37 PM
#5
Re: Switch case or something better!
 Originally Posted by Skizmo
His idea is making it much faster. Each case in the switch costs time. If you put it in an array, only one lookup has to be done instead of testing a lot of cases inside the switch.
Thanks, but now this statement raised my curiosity... 
So I turned the OP's code into something compilable like that:
Code:
// Test6.cpp : Definiert den Einstiegspunkt für die Konsolenanwendung.
//
#include "stdafx.h"
class column {};
column c_01, c_02, c_19;
inline column* ptr2col(unsigned int intin)// Returns a pointer to a column class
{
column* ret;
switch (intin)
{
//c_01 a q
case 1:
case 65: case 97: case 192: case 193: case 194: case 195: case 196: case 197: case 198:
case 224: case 225: case 226: case 227: case 228: case 229: case 230: case 81: case 113:
ret=&c_01;
break;
//c_02 e w
case 2:
case 69: case 101: case 200: case 201: case 202: case 203: case 232:
case 233: case 234: case 235: case 240: case 87: case 119:
ret=&c_02;
break;
// and so on etc. ............................
// till
//c_19 Rest
case 19:
default:
ret=&c_19;
}
return ret;
}
int _tmain(int argc, _TCHAR* argv[])
{
column *p = ptr2col(0);
return 0;
}
And here's what the compiler made out of it (VC++ 2010, release build, default settings except for turning on the assembly listing output):
Code:
; Listing generated by Microsoft (R) Optimizing Compiler Version 16.00.30319.01
TITLE c:\dokumente und einstellungen\administrator\eigene dateien\visual studio 2010\Projects\Test6\Test6\Test6.cpp
.686P
.XMM
include listing.inc
.model flat
INCLUDELIB OLDNAMES
PUBLIC ?ptr2col@@YAPAVcolumn@@I@Z ; ptr2col
PUBLIC ?c_01@@3Vcolumn@@A ; c_01
PUBLIC ?c_02@@3Vcolumn@@A ; c_02
PUBLIC ?c_19@@3Vcolumn@@A ; c_19
EXTRN @__security_check_cookie@4:PROC
?c_01@@3Vcolumn@@A DB 01H DUP (?) ; c_01
ALIGN 4
?c_02@@3Vcolumn@@A DB 01H DUP (?) ; c_02
ALIGN 4
?c_19@@3Vcolumn@@A DB 01H DUP (?) ; c_19
; Function compile flags: /Ogtp
; File c:\dokumente und einstellungen\administrator\eigene dateien\visual studio 2010\projects\test6\test6\test6.cpp
; COMDAT ?ptr2col@@YAPAVcolumn@@I@Z
_TEXT SEGMENT
?ptr2col@@YAPAVcolumn@@I@Z PROC ; ptr2col, COMDAT
; $T5300 = eax
; 12 : column* ret;
; 13 : switch (intin)
00000 0f b6 88 00 00
00 00 movzx ecx, BYTE PTR $LN13@ptr2col[eax]
00007 ff 24 8d 00 00
00 00 jmp DWORD PTR $LN14@ptr2col[ecx*4]
$LN3@ptr2col:
; 14 : {
; 15 : //c_01 a q
; 16 : case 1:
; 17 : case 65: case 97: case 192: case 193: case 194: case 195: case 196: case 197: case 198:
; 18 : case 224: case 225: case 226: case 227: case 228: case 229: case 230: case 81: case 113:
; 19 : ret=&c_01;
0000e b8 00 00 00 00 mov eax, OFFSET ?c_01@@3Vcolumn@@A ; c_01
; 34 : }
; 35 : return ret;
; 36 : }
00013 c3 ret 0
$LN2@ptr2col:
; 20 : break;
; 21 : //c_02 e w
; 22 : case 2:
; 23 : case 69: case 101: case 200: case 201: case 202: case 203: case 232:
; 24 : case 233: case 234: case 235: case 240: case 87: case 119:
; 25 : ret=&c_02;
00014 b8 00 00 00 00 mov eax, OFFSET ?c_02@@3Vcolumn@@A ; c_02
; 34 : }
; 35 : return ret;
; 36 : }
00019 c3 ret 0
$LN8@ptr2col:
; 26 : break;
; 27 :
; 28 : // and so on etc. ............................
; 29 : // till
; 30 : //c_19 Rest
; 31 : case 19:
; 32 : default:
; 33 : ret=&c_19;
0001a b8 00 00 00 00 mov eax, OFFSET ?c_19@@3Vcolumn@@A ; c_19
; 34 : }
; 35 : return ret;
; 36 : }
0001f c3 ret 0
$LN14@ptr2col:
00020 00 00 00 00 DD $LN3@ptr2col
00024 00 00 00 00 DD $LN2@ptr2col
00028 00 00 00 00 DD $LN8@ptr2col
$LN13@ptr2col:
0002c 00 DB 0
0002d 01 DB 1
0002e 02 DB 2
0002f 02 DB 2
00030 02 DB 2
00031 02 DB 2
00032 02 DB 2
00033 02 DB 2
00034 02 DB 2
00035 02 DB 2
00036 02 DB 2
00037 02 DB 2
00038 02 DB 2
00039 02 DB 2
0003a 02 DB 2
0003b 02 DB 2
0003c 02 DB 2
0003d 02 DB 2
0003e 02 DB 2
0003f 02 DB 2
00040 02 DB 2
00041 02 DB 2
00042 02 DB 2
00043 02 DB 2
00044 02 DB 2
00045 02 DB 2
00046 02 DB 2
00047 02 DB 2
00048 02 DB 2
00049 02 DB 2
0004a 02 DB 2
0004b 02 DB 2
0004c 02 DB 2
0004d 02 DB 2
0004e 02 DB 2
0004f 02 DB 2
00050 02 DB 2
00051 02 DB 2
00052 02 DB 2
00053 02 DB 2
00054 02 DB 2
00055 02 DB 2
00056 02 DB 2
00057 02 DB 2
00058 02 DB 2
00059 02 DB 2
0005a 02 DB 2
0005b 02 DB 2
0005c 02 DB 2
0005d 02 DB 2
0005e 02 DB 2
0005f 02 DB 2
00060 02 DB 2
00061 02 DB 2
00062 02 DB 2
00063 02 DB 2
00064 02 DB 2
00065 02 DB 2
00066 02 DB 2
00067 02 DB 2
00068 02 DB 2
00069 02 DB 2
0006a 02 DB 2
0006b 02 DB 2
0006c 00 DB 0
0006d 02 DB 2
0006e 02 DB 2
0006f 02 DB 2
00070 01 DB 1
00071 02 DB 2
00072 02 DB 2
00073 02 DB 2
00074 02 DB 2
00075 02 DB 2
00076 02 DB 2
00077 02 DB 2
00078 02 DB 2
00079 02 DB 2
0007a 02 DB 2
0007b 02 DB 2
0007c 00 DB 0
0007d 02 DB 2
0007e 02 DB 2
0007f 02 DB 2
00080 02 DB 2
00081 02 DB 2
00082 01 DB 1
00083 02 DB 2
00084 02 DB 2
00085 02 DB 2
00086 02 DB 2
00087 02 DB 2
00088 02 DB 2
00089 02 DB 2
0008a 02 DB 2
0008b 02 DB 2
0008c 00 DB 0
0008d 02 DB 2
0008e 02 DB 2
0008f 02 DB 2
00090 01 DB 1
00091 02 DB 2
00092 02 DB 2
00093 02 DB 2
00094 02 DB 2
00095 02 DB 2
00096 02 DB 2
00097 02 DB 2
00098 02 DB 2
00099 02 DB 2
0009a 02 DB 2
0009b 02 DB 2
0009c 00 DB 0
0009d 02 DB 2
0009e 02 DB 2
0009f 02 DB 2
000a0 02 DB 2
000a1 02 DB 2
000a2 01 DB 1
000a3 02 DB 2
000a4 02 DB 2
000a5 02 DB 2
000a6 02 DB 2
000a7 02 DB 2
000a8 02 DB 2
000a9 02 DB 2
000aa 02 DB 2
000ab 02 DB 2
000ac 02 DB 2
000ad 02 DB 2
000ae 02 DB 2
000af 02 DB 2
000b0 02 DB 2
000b1 02 DB 2
000b2 02 DB 2
000b3 02 DB 2
000b4 02 DB 2
000b5 02 DB 2
000b6 02 DB 2
000b7 02 DB 2
000b8 02 DB 2
000b9 02 DB 2
000ba 02 DB 2
000bb 02 DB 2
000bc 02 DB 2
000bd 02 DB 2
000be 02 DB 2
000bf 02 DB 2
000c0 02 DB 2
000c1 02 DB 2
000c2 02 DB 2
000c3 02 DB 2
000c4 02 DB 2
000c5 02 DB 2
000c6 02 DB 2
000c7 02 DB 2
000c8 02 DB 2
000c9 02 DB 2
000ca 02 DB 2
000cb 02 DB 2
000cc 02 DB 2
000cd 02 DB 2
000ce 02 DB 2
000cf 02 DB 2
000d0 02 DB 2
000d1 02 DB 2
000d2 02 DB 2
000d3 02 DB 2
000d4 02 DB 2
000d5 02 DB 2
000d6 02 DB 2
000d7 02 DB 2
000d8 02 DB 2
000d9 02 DB 2
000da 02 DB 2
000db 02 DB 2
000dc 02 DB 2
000dd 02 DB 2
000de 02 DB 2
000df 02 DB 2
000e0 02 DB 2
000e1 02 DB 2
000e2 02 DB 2
000e3 02 DB 2
000e4 02 DB 2
000e5 02 DB 2
000e6 02 DB 2
000e7 02 DB 2
000e8 02 DB 2
000e9 02 DB 2
000ea 02 DB 2
000eb 00 DB 0
000ec 00 DB 0
000ed 00 DB 0
000ee 00 DB 0
000ef 00 DB 0
000f0 00 DB 0
000f1 00 DB 0
000f2 02 DB 2
000f3 01 DB 1
000f4 01 DB 1
000f5 01 DB 1
000f6 01 DB 1
000f7 02 DB 2
000f8 02 DB 2
000f9 02 DB 2
000fa 02 DB 2
000fb 02 DB 2
000fc 02 DB 2
000fd 02 DB 2
000fe 02 DB 2
000ff 02 DB 2
00100 02 DB 2
00101 02 DB 2
00102 02 DB 2
00103 02 DB 2
00104 02 DB 2
00105 02 DB 2
00106 02 DB 2
00107 02 DB 2
00108 02 DB 2
00109 02 DB 2
0010a 02 DB 2
0010b 00 DB 0
0010c 00 DB 0
0010d 00 DB 0
0010e 00 DB 0
0010f 00 DB 0
00110 00 DB 0
00111 00 DB 0
00112 02 DB 2
00113 01 DB 1
00114 01 DB 1
00115 01 DB 1
00116 01 DB 1
00117 02 DB 2
00118 02 DB 2
00119 02 DB 2
0011a 02 DB 2
0011b 01 DB 1
?ptr2col@@YAPAVcolumn@@I@Z ENDP ; ptr2col
_TEXT ENDS
PUBLIC _wmain
; Function compile flags: /Ogtp
; COMDAT _wmain
_TEXT SEGMENT
_argc$ = 8 ; size = 4
_argv$ = 12 ; size = 4
_wmain PROC ; COMDAT
; 40 : column *p = ptr2col(0);
; 41 : return 0;
00000 33 c0 xor eax, eax
; 42 : }
00002 c3 ret 0
_wmain ENDP
_TEXT ENDS
END
You see that the compiler actually uses a two-stage lookup (while I would have expected an ordinary direct lookup), the first lookup fetching an index (from $LN13@ptr2col) into a short table of jump target addresses ($LN14@ptr2col) from where it then gets the actual address to do an indirect jump to. That way it sacrifices a tiny bit of runtime performance for an almost four-fold reduction in lookup table size. It's most likely easy to tweak the optimization settings so that the compiler favors performance over table size reduction, but already the above confirms my point from the last post, so i did no further testing.
I was thrown out of college for cheating on the metaphysics exam; I looked into the soul of the boy sitting next to me.
This is a snakeskin jacket! And for me it's a symbol of my individuality, and my belief... in personal freedom.
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