A plan for a loop. For, while, do-while all are follow similar implementation.
init expressions
t = test expression
if(!t)
goto done
loop:
body
update expression
if(t)
goto loop
done:
to check a variable against multiple cases, which are constants themselves. switch doesn’t check against each statement sequentially. Maintaining an array of labels where to jump and looking up that array to jmp to location without actually checking other conditions. This works efficiently if the cases are close, but we need a hash table if cases are far apart. [request clarification]
Example:
switch(n){
case 100:
result = 100;
break;
case 102:
result = 102;
case 103:
result = 103;
break;
default:
result = 0;
}
To get an idea of how it is implemented in assembly, consider this C snippet:
static void* jumptable[4] = {&&label_A, &&label_default, &&label_B, &&label_C};
//&& is GCC-sepecific sytanx to get the address of labels. unary operator
unsigned index = n - 100;
int result;
if(index > 3){goto label_default;}
goto *(jumptable[index]);
//`*` is necessary here because the array stores the address of labels.
//goto doesn't need the address of the label, but the label itself.
//hence the dereferencing
label_A:
result = 100;
goto done; //notice the `done` label here
label_B:
result = 102; //notice the absence of `goto done` here
// and the fact the subsequent lines will be executed
label_C:
result = 103;
goto done;
label_default:
result = 0;
done:
return result;
Assembly snippet on how to set up a jump table, assume n is already mov‘d into %eax
subl $100, %eax # n = n-100
compl %6, %eax # compare 6 with n
jge label_default
jmp *jumptable( , %eax, 4)
where jumptable is an array defined in .rodata section
.section .rodata
.align 4
.jumptable
.long label_A
.long label_B
.long label_C