Do global variables mean faster code?

Question

I read recently, in an article on game programming written in 1996, that using global variables is faster than passing parameters.

Was this ever true, and if so, is

Answer 1

When you switch from parameters to global variables, one of three things can happen:

• it runs faster
• it runs the same
• it runs slower

You will have to measure performance to see what's faster in a non-trivial concrete case. This was true in 1996, is true today and is true tomorrow.

Leaving the performance aside for a moment, global variables in a large project introduce dependencies which almost always make maintenance and testing much harder.

When trying to find legitimate uses of globals variables for performance reasons today I very much agree with the examples in Preet's answer: very often needed variables in microcontroller programs or device drivers. The extreme case is a processor register which is exclusively dedicated to the global variable.

When reasoning about the performance of global variables versus parameter passing, the way the compiler implements them is relevant. Global variables typically are stored at fixed locations. Sometimes the compiler generates direct addressing to access the globals. Sometimes however, the compiler uses one more indirection and uses a kind of symbol table for globals. IIRC gcc for AIX did this 15 years ago. In this environment, globals of small types were always slower than locals and parameter passing.

On the other hand, a compiler can pass parameters by pushing them on the stack, by passing them in registers or a mixture of both.