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POShould I store C array values in local variables if I'm going to reference them repeatedly?
 

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  1. POShould I store C array values in local variables if I'm going to reference them repeatedly?
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    <p>I'm writing some optimized C code that basically runs through an array and does something to each element. What it does depends on the current value of the element so something like:</p> <pre><code>for (i=0; i &lt; a_len; i++) { if (a[i] == 0) { a[i] = f1(a[i]); } else if (a[i] % 2 == 0) { a[i] = f2(a[i]); } else { a[i] = 0; } </code></pre> <p>I'm returning to C after many years working in dynamic languages, where my practice has been to try to write straightforward code and not create lots of local variables for things that I can just refer to directly, like a[i] above. <em>I am very much aware that best practices are to write readable code and trust that the compiler is smarter than you and will do good optimizations.</em></p> <p>If I were writing the code above in assembler, I would load a[i] into a register once and then just use that value each time because I know that a[] is private memory and won't change between references. However, even a smart compiler might do a load every time because it can't be sure that the memory hasn't changed. (Or do I have to explicitly declare "a" volatile for the compiler to not make this optimization?).</p> <p>So, my question is: should I expect better performance by rewriting with a local variable like so:</p> <pre><code>for (i=0; i &lt; a_len; i++) { val = a[i]; if (val == 0) { a[i] = f1(val); } else if (val % 2 == 0) { a[i] = f2(val); } else { a[i] = 0; } </code></pre> <p>Or does stuff like -O3 take care of this automatically for me? The code I'm optimizing takes days to run, so even modest improvements will make a difference.</p>
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    1. COdepends heavily on what "a" is (from where it comes. if it is a function parameter, things are more complicated than if it is a local array). in your case, all clauses are mutually exclusive and i think a halfway decent compiler will produce the same assembler code.
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    2. COa is a function parameter
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    3. COUse register T val = a[i]; Then it is stored in a register for latter access. Basically what you expect the compiler to do/optimize for you, but so you are sure.
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