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    <p>Two specific issues that I've encountered in the past:</p> <p><strong>First:</strong> If you're attempting to separate e.g. render phase (const access to most game state) from logic phase (non-const access to most game state) for whatever reason (decoupling render rate from logic rate, synchronizing game state across a network, recording and playback of gameplay at a fixed point in the frame, etc), globals make it very hard to enforce that.</p> <p>Problems tend to creep in and become hard to debug and eradicate. This also has implications for threaded renderers separate from game logic, or the like (the other answers cover this topic thoroughly).</p> <p><strong>Second:</strong> The presence of many globals tends to bloat the <a href="http://en.wikipedia.org/wiki/Literal_pool" rel="nofollow noreferrer">literal pool</a>, which the compiler typically places after each function.</p> <p>If you get to your state through either a single "struct GlobalTable" which holds globals or a collection of methods on an object or the like, your literal pool tends to be a lot smaller, decreasing the size of the <code>.text</code> section in your executable.</p> <p>This is mostly a concern for instruction set architectures that can't embed load targets directly into instructions (see e.g. fixed-width ARM or Thumb version 1 instruction encoding on ARM processors). Even on modern processors I'd wager you'll get slightly smaller codegen.</p> <p>It also hurts doubly when your instruction and data caches are separate (again, as on some ARM processors); you'll tend to get two cachelines loaded where you might only need one with a unified cache. Since the literal pool may count as data, and won't always start on a cacheline boundary, the same cacheline might have to be loaded both into the i-cache and d-cache.</p> <p>This may count as a "microoptimization", but it's a transform that's relatively easily applied to an entire codebase (e.g. <code>extern struct GlobalTable { /* ... */ } the;</code> and then replace all <code>g_foo</code> with <code>the.foo</code>)... And we've seen code size decrease between 5 and 10 percent in some cases, with a corresponding boost to performance due to keeping the caches cleaner.</p>
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    1. POIn game programming are global variables bad?
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    1. COI had no idea about any of that, thank you very much for sharing it! I'm always interested in learning how to optimize code, and this sounds like a great trick :D
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    2. COMy pleasure. Remember, though, as with anything that looks like a "trick" you should always make sure you need it before applying it -- use a profiler, or check disasm before and after. It's worthwhile to try things like this as experiments (if you're not on a deadline), to gain better understanding, but not always worth keeping the results. =) Stuff like this tends to vary so much from compiler to compiler and from CPU to cpu that the "show me the profile" mantra is always worth keeping in mind.
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