C++ performance of global variables

Question

For clarification: I know how evil globals are and when not to use them :)

• Is there any performance penalty when accessing/setting a global variable vs. a loca

Answer 1

In addition to other answers, I would just point out that accessing a global variable in a multithreaded environment is likely to be more expensive because you need to ensure it is locked properly and the threads may wait in line to access it. With local variables it is not an issue.

Answer 2

It depends but usually yes although it is a micro issue. Global variables should be reference-able from many contexts, which means that putting them into a register is not possible. While in the case of local variables, that is possible and preferable. In fact, the more narrower the scope the more the compiler has the opportunity to optimize access/modifying that variable.

Answer 3

Local variables are probably "faster" in many cases, but I don't think the performance gain would be noticeable or outweigh the additional maintenance cost of having many global variables. Everything I list below either has a negligible cost or can easily be dwarfed by just about any other inefficiency in your program. I would consider these to be a perfect example of a micro-optimization.

Local variables are on the stack, which is more likely to be in the cache. This point is moot if your global variable is frequently used, since it will therefore also be in the cache.

Local variables are scoped to the function - therefore, the compiler can presume that they won't be changed by any other function calls. With a global, the compiler may be forced to reload the global value.

On some 64-bit machines, getting the address of a global variable is a two-step process - you must also add the 32-bit offset of the global to a 64-bit base address. Local variables can always be directly accessed off of the stack pointer.