CHAPTER 13. SWITCH()/CASE/DEFAULT CHAPTER 13. SWITCH()/CASE/DEFAULT
call _printf
add esp, 4
jmp SHORT $LN7@f
$LN2@f:
push OFFSET $SG743 ; 'two', 0aH, 00H
call _printf
add esp, 4
jmp SHORT $LN7@f
$LN1@f:
push OFFSET $SG745 ; 'something unknown', 0aH, 00H
call _printf
add esp, 4
$LN7@f:
mov esp, ebp
pop ebp
ret 0
_f ENDP
Our function with a few cases in switch() is in fact analogous to this construction:
void f (int a)
{
if (a==0)
printf ("zero\n");
else if (a==1)
printf ("one\n");
else if (a==2)
printf ("two\n");
else
printf ("something unknown\n");
};
If we work with switch() with a few cases it is impossible to be sure if it was a real switch() in the source code, or just a pack
of if() statements. This implies that switch() is like syntactic sugar for a large number of nested if()s.
There is nothing especially new to us in the generated code, with the exception of the compiler moving input variableato
a temporary local variabletv64^1.
If we compile this in GCC 4.4.1, we’ll get almost the same result, even with maximal optimization turned on (-O3option).
Optimizing MSVC
Now let’s turn on optimization in MSVC (/Ox):cl 1.c /Fa1.asm /Ox
Listing 13.2: MSVC_a$ = 8 ; size = 4
_f PROC
mov eax, DWORD PTR _a$[esp-4]
sub eax, 0
je SHORT $LN4@f
sub eax, 1
je SHORT $LN3@f
sub eax, 1
je SHORT $LN2@f
mov DWORD PTR _a$[esp-4], OFFSET $SG791 ; 'something unknown', 0aH, 00H
jmp _printf
$LN2@f:
mov DWORD PTR _a$[esp-4], OFFSET $SG789 ; 'two', 0aH, 00H
jmp _printf
$LN3@f:
mov DWORD PTR _a$[esp-4], OFFSET $SG787 ; 'one', 0aH, 00H
jmp _printf
$LN4@f:
mov DWORD PTR _a$[esp-4], OFFSET $SG785 ; 'zero', 0aH, 00H
jmp _printf
_f ENDP
(^1) Local variables in stack are prefixed withtv—that’s how MSVC names internal variables for its needs