a question about code optimization

[龙海涛 <longhaitao at otsc dot com dot cn>]

---------code-----------
vector<int> a;
/*do somthing*/
for(int i = 0; i<a.size(); i++) {
	/* do something*/
}
--------code end--------

my question is:
is it possible that the compiler will transform the code to this:

int __tmp = a.size();
for(int i = 0; i < __tmp; i++) {
	/*do something*/
}

i think it is impoossible,because in size() function, the programmer
can change some global variables, so the compiler could not do that.

if gcc can, could you tell me how?

i am sorry in advance because i am not familiar with the code optimization.
so i do not know how to search the gcc manual and gcc internal manual to find
answer.could anyone tell me how to find this in the manual？or at least give
some keywords?