C variable allocation time and space_问答_开发者

If i have a test.c file with the following

#include ...
int global = 0;

 int main() {
 int local1 = 0;

  while(1) {
  int local2开发者_如何学JAVA = 0;
  // Do some operation with one of them
 }
 return 0;
}

So if I had to use one of this variables in the while loop, which one would be preferred?

Maybe I'm being a little vague here, but I want to know if the difference in time/space allocation is actually relevant.

If you are wondering whether declaring a variable inside a for loop causes it to be created/destroyed at every iteration, there is nothing really to worry about. These variables are not dynamically allocated at runtime, nothing is being malloced here - just some memory is being set aside for use inside the loop. So having the variable inside is just the same as having it outside the loop in terms of performance.

The real difference here is scope not performance. Whether you use a global or local variable only affects where you want this variable to be visible.

In case you're wondering about performance differences: most likely there aren't any. If there are theoretical performance differences, you'll find it hard to actually devise a test to measure them.

A decision like this should not be based on performance but semantics. Unless the semantic behavior of a global variable is required, you should always use automatic (local non-static) variables.

As others have said and surely will say, there are unlikely to be any differences in performance. If there are, the automatic variable will be faster.

The C compiler will have an easier time making optimizations on the variables declared local to the function. The global variable would require an optimizer to perform "Inter-Procedural Data Flow Analysis", which isn't that commonly done.

As an example of the difference, consider that all your declarations initialize the variable to zero. However, in the case of the global variable, the compiler cannot use that information unless it verifies that no flow of control in your program can change the global prior to using it in your example function. In the case of the locally declared ("automatic") variables, there is no way the initial value can be changed by another function (in particular, the compiler verifies that their address is never passed to a sub-function) and the compiler can perform "killed definitions" and "value liveness" analysis to determine whether the zero value can be assumed in some code paths.

Of the two local variables, as a guideline, the optimizer will always have an easier time optimizing access to the variable with the smaller (more limited) scope.

Having stated the above, I would suggest that other answers concerning a bias toward semantics over optimizer-meta-optimization is correct. Use the variable which causes the code to read best, and you will be rewarded with more time returned to you than assisting the def-use optimization calculation.

In general, avoid using a global variable, or any variable which can be accessed more broadly than absolutely necessary. Limited scoping of variables helps prevent bugs from being introduced during later program maintenance.

There are three broad classes of variables: static (global), stack (auto), and register.

Register variables are stored in CPU registers. Registers are very fast word-sized memories, which are integrated in the CPU pipeline. They are free to access, but there are a very limited number of them (typically between 8 and 32 depending on your processor and what operations you're doing).

Stack variables are stored in an area of RAM called the stack. The stack is almost always going to be in the cache, so stack variables typically take 1-4 cycles to access.

Generally, local variables can be either in registers or on the stack. It doesn't matter whether they are allocated at the top of a function or in a loop; they will only be allocated once per function call, and allocation is basically free. The compiler will put variables in registers if at all possible, but if you have more active variables than registers, they won't all fit. Also, if you take the address of a variable, it must be stored on the stack since registers don't have addresses.

Global and static variables are a different beast. Since they are not usually accessed frequently, they may not be in cache, so it could take hundreds of cycles to access them. Also, since the compiler may not know the address of a global variable ahead of time, it may need to be looked up, which is also expensive.

As others have said, don't worry too much about this stuff. It's definitely good to know, but it shouldn't affect the way you write your programs. Write code that makes sense, and let the compiler worry about optimization. If you get into compiler development, then you can start worrying about it. :)

Edit: more details on allocation:

Register variables are allocated by the compiler, so there is no runtime cost. The code will just put a value in a register as soon as the value is produced.

Stack variables are allocated by your program at runtime. Typically, when a function is called, the first thing it will do is reserve enough stack space for all of its local variables. So there is no per-variable cost.