CHAPTER 13. SWITCH()/CASE/DEFAULT CHAPTER 13. SWITCH()/CASE/DEFAULT
Ajumptableis just array of pointers, like the one described later:18.5 on page 271.13.3 When there are severalcasestatements in one block
Here is a very widespread construction: severalcasestatements for a single block:#include <stdio.h>void f(int a)
{
switch (a)
{
case 1:
case 2:
case 7:
case 10:
printf ("1, 2, 7, 10\n");
break;
case 3:
case 4:
case 5:
case 6:
printf ("3, 4, 5\n");
break;
case 8:
case 9:
case 20:
case 21:
printf ("8, 9, 21\n");
break;
case 22:
printf ("22\n");
break;
default:
printf ("default\n");
break;
};
};int main()
{
f(4);
};It’s too wasteful to generate a block for each possible case, so what is usually done is to generate each block plus some kind
of dispatcher.13.3.1 MSVC
Listing 13.10: Optimizing MSVC 2010
1 $SG2798 DB '1, 2, 7, 10', 0aH, 00H
2 $SG2800 DB '3, 4, 5', 0aH, 00H
3 $SG2802 DB '8, 9, 21', 0aH, 00H
4 $SG2804 DB '22', 0aH, 00H
5 $SG2806 DB 'default', 0aH, 00H
6
7 _a$ = 8
8 _f PROC
9 mov eax, DWORD PTR _a$[esp-4]
10 dec eax
11 cmp eax, 21
12 ja SHORT $LN1@f
13 movzx eax, BYTE PTR $LN10@f[eax]
14 jmp DWORD PTR $LN11@f[eax*4]
15 $LN5@f:
16 mov DWORD PTR _a$[esp-4], OFFSET $SG2798 ; '1, 2, 7, 10'