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POQWORD shuffle sequential 7-bits to byte-alignment with SIMD SSE...AVX
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  1. POQWORD shuffle sequential 7-bits to byte-alignment with SIMD SSE...AVX
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    copied!<p>I would like to know if the following is possible in any of the SIMD families of instructions.</p> <p>I have a qword input with 63 significant bits (never negative). Each sequential 7 bits starting from the LSB is shuffle-aligned to a byte, with a left-padding of 1 (except for the most significant non-zero byte). To illustrate, I'll use letters for clarity's sake.</p> <p>The result is only the significant bytes, thus 0 - 9 in size, which is converted to a byte array.</p> <pre><code>In: 0|kjihgfe|dcbaZYX|WVUTSRQ|PONMLKJ|IHGFEDC|BAzyxwv|utsrqpo|nmlkjih|gfedcba Out: 0kjihgfe|1dcbaZYX|1WVUTSRQ|1PONMLKJ|1IHGFEDC|1BAzyxwv|1utsrqpo|1nmlkjih|1gfedcba </code></pre> <p>Size = 9</p> <pre><code>In: 00|nmlkjih|gfedcba Out: |0nmlkjih|1gfedcba </code></pre> <p>Size = 2</p> <p>I do understand the padding is separate. The shuffle-aligning is my question. Is this possible?</p> <p><strong>EDIT 2</strong></p> <p>Here is my updated code. Gets a sustained 46 M / sec for random-length input on single thread Core 2 Duo 2 GHz, 64 bit.</p> <pre><code>private static int DecodeIS8(long j, ref byte[] result) { if (j &lt;= 0) { return 0; } int size; // neater code: gives something to break out of while (true) { result[0] = (byte)((j &amp; 0x7F) | 0x80); size = 0; j &gt;&gt;= 7; if (j == 0) break; result[1] = (byte)((j &amp; 0x7F) | 0x80); size++; j &gt;&gt;= 7; if (j == 0) break; result[2] = (byte)((j &amp; 0x7F) | 0x80); size++; j &gt;&gt;= 7; if (j == 0) break; result[3] = (byte)((j &amp; 0x7F) | 0x80); size++; j &gt;&gt;= 7; if (j == 0) break; result[4] = (byte)((j &amp; 0x7F) | 0x80); size++; j &gt;&gt;= 7; if (j == 0) break; result[5] = (byte)((j &amp; 0x7F) | 0x80); size++; j &gt;&gt;= 7; if (j == 0) break; result[6] = (byte)((j &amp; 0x7F) | 0x80); size++; j &gt;&gt;= 7; if (j == 0) break; result[7] = (byte)((j &amp; 0x7F) | 0x80); size++; j &gt;&gt;= 7; if (j == 0) break; result[8] = (byte)j; return 9; } result[size] ^= 0x80; return size + 1; } </code></pre>
 

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