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POAlignment, total size and SSE
 

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    <p>I'm trying to <a href="http://pointclouds.org/documentation/tutorials/adding_custom_ptype.php" rel="nofollow">define a custom point type</a> for the <code>PCL</code> library. In that tutorial, they're talking about memory alignment, so I started off by trying to understand how it works.</p> <p>In <a href="http://www.geeksforgeeks.org/structure-member-alignment-padding-and-data-packing/" rel="nofollow">this page</a>, they present a rather simple way of calculating the total alignment of a structure. For example, this structure</p> <pre><code>// Alignment requirements // (typical 32 bit machine) // char 1 byte // short int 2 bytes // int 4 bytes // double 8 bytes // structure C typedef struct structc_tag { char c; double d; int s; } structc_t; </code></pre> <p>will have a size of 24: </p> <pre><code>1 byte for the char + 7 bytes of padding + 8 bytes for the double + 4 bytes for the int + 4 bytes of padding </code></pre> <p>and for <code>g++ 4.8.1</code>, <em>sizeof</em> returns 24. So far, so good.</p> <p>Now, in <code>PCL</code> they're defining the point types with this scheme (here's the most simple point, that holds the position in each axis) for <code>SSE</code> alignment.</p> <pre><code>union { float data[4]; struct { float x; float y; float z; }; }; </code></pre> <p><em>sizeof</em> returns 16. With the union it is made sure that the <code>point type</code> is SSE aligned (I read <a href="http://www.codeproject.com/Articles/4522/Introduction-to-SSE-Programming" rel="nofollow">here</a> that is 16 byte alignment) and with the struct the axis values are accessible.</p> <p>Quoting from the <code>PCL</code> docs:</p> <blockquote> <p>The user can either access points[i].data[0] or points[i].x for accessing say, the x coordinate.</p> </blockquote> <p>Is my reasoning valid until here?</p> <hr> <p>In my case, I want to change the floats for doubles in order to have more precision in the <code>X</code> and <code>Y</code> axis.</p> <p>So, is it enough to declare the point type as:</p> <pre><code>union { float data[4]; struct { double x; double y; float z; }; }; </code></pre> <p>? <em>sizeof</em> returns 24, which is not a multiple of 16 (so I understand it's not SSE aligned) but it is "double aligned".</p> <p>My question is, how can I define my point type to be able to store the <code>X</code> and <code>Y</code> coordinates as double and still be <code>SSE aligned</code>?</p> <p><strong>PS:</strong> Also, if any of you know of a good resource for this, please tell me. I want to understand better this topic.</p> <p><strong>PS 2:</strong> I forgot to tell, the platform I'm trying all of this is a 64 bit one.</p> <p><strong>PS 3:</strong> If possible, I'm interested in <code>pre-C++11</code> solutions. A compiler as old as <code>g++ 4.4</code> (and its <code>MinGW</code> counterpart) must be able to build the new point type.</p>
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    1. USAdri C.S.
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      1. VTUpMod
    1. COcould you use structure packing pragmas?
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      1. USelSnape
    2. COHave you considered the [`alignas`](http://en.cppreference.com/w/cpp/language/alignas) specifier?
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      1. USBrett Hale
    3. CO@BrettHale, I dindn't and I forgot (again) to mention that I can't use `C++11` solutions. I need to be able to compile the point type with old compilers (the oldest one being g++ 4.4). I'll update the info in the post right away.
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      1. USAdri C.S.
 

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