Walter Bright wrote:
>> void bar(bool foo) {
>> int a = void;
>> if (foo) {
>> a = 1;
>> } else {
>> a = 2; // Reuse variable.
>> }
>> int c = 3;
>> }
>>
>> You now only have two variables, but both of them coexist at the end of
>> the function. Unless the compiler applies a clever optimization, the
>> compiler is now forced to allocate space for two variables on the stack.
>
> Not necessarily. The optimizer uses a technique called "live range
> analysis" to determine if two variables have non-overlapping ranges. It
> uses this for register assignment, but it could just as well be used for
> minimizing stack usage.
That's the optimization I was referring to. It works for ints, but not
for RAII types. It also doesn't (necessarily) work if you reorder the
function:
void bar(bool foo) {
int a = void;
int c = 3;
if (foo) {
a = 1;
} else {
a = 2; // Reuse variable.
}
}
Of course, a good optimizer can still reorder the declarations in this
case, or even eliminate the whole function body (since it doesn't do
anything).
--
Rainer Deyke - [email protected]
