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

Perhaps a micro optimisation, and would probably be wiped out by optimisations your compiler could generate without resort to such practices. In fact the use of globals may even inhibit some compiler optimisations. Reliable and maintainable code would generally be of greater value, and globals are not conducive to that.

Using globals to replace function parameters renders all such functions non-reentrant, which may be a problem if multi-threading is used - not a common practice in game development in 1996, but more common with the advent of multi-core processors. It also precludes recursion, although that is probably less of an issue since recursion has its own issues.

In any significant body of code, there is likely to be more mileage in higher-level optimisation of algorithms and data structures. Moreover there are options open to you other than global variables that avoid parameter passing, most especially C++ class-member variables.

If the habitual use of global variables in your code makes a measurable or useful difference to its performance, I would question the design first.

For a discussion of the problems inherent in global variables and some ways to avoid them see A Pox on Globals by Jack Gannsle. The article relates to embedded systems development, but is generally applicable; its just that some embedded systems developers think they have good reason to use globals, probably for all the same misguided reasons used to justify it in game development.