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POEfficiency of data structures in C99 (possibly affected by endianness)
 

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  1. POEfficiency of data structures in C99 (possibly affected by endianness)
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    <p>I have a couple of questions that are all inter-related. Basically, in the algorithm I am implementing a word <code>w</code> is defined as four bytes, so it can be contained whole in a <code>uint32_t</code>. </p> <p>However, during the operation of the algorithm I often need to access the various parts of the word. Now, I can do this in two ways:</p> <pre><code>uint32_t w = 0x11223344; uint8_t a = (w &amp; 0xff000000) &gt;&gt; 24; uint8_t b = (w &amp; 0x00ff0000) &gt;&gt; 16; uint8_t b = (w &amp; 0x0000ff00) &gt;&gt; 8; uint8_t d = (w &amp; 0x000000ff); </code></pre> <p>However, part of me thinks that isn't particularly efficient. I thought a better way would be to use union representation like so:</p> <pre><code>typedef union { struct { uint8_t d; uint8_t c; uint8_t b; uint8_t a; }; uint32_t n; } word32; </code></pre> <p>Using this method I can assign <code>word32 w = 0x11223344;</code> then I can access the various parts as I require (<code>w.a=11</code> in little endian).</p> <p>However, at this stage I come up against endianness issues, namely, in big endian systems my struct is defined incorrectly so I need to re-order the word prior to it being passed in.</p> <p>This I can do without too much difficulty. My question is, then, is the first part (various bitwise ands and shifts) efficient compared to the implementation using a union? Is there any difference between the two generally? Which way should I go on a modern, x86_64 processor? Is endianness just a red herring here?</p> <p>I could inspect the assembly output of course, but my knowledge of compilers is not brilliant. I would have thought a union would be more efficient as it would essentially convert to memory offsets, like so:</p> <pre><code>mov eax, [r9+8] </code></pre> <p>Would a compiler realise that is what happening in the bit-shift case above?</p> <p>If it matters, I'm using C99, specifically my compiler is clang (llvm).</p> <p>Thanks in advance.</p>
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    1. COHave you measured the performance? Is the code which access this data structure critical in terms of performance? How different are the two approaches in terms of performance?
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      1. USDavid Heffernan
    2. CONo not yet. I was wondering which way to implement, you see. Basically it's an implementation of AES I'm building, so yes it is critical. I can either invert the endianness of the entire data array and revert it at the end or I can do lots of bit shifting operations. Which is more efficient depends on the compiler, which is what I don't know. I may well simply have to measure both ways, we shall see.
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    3. COwhy don't you read some open source code that does this and see how they do it. Even better would be to use an existing implementation.
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      1. USDavid Heffernan
 

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