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POSimulating packusdw functionality with SSE2
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copied!<p>I'm implementing a fast x888 -> 565 pixel conversion function in <a href="http://cgit.freedesktop.org/pixman/" rel="noreferrer">pixman</a> according to the algorithm described <a href="http://software.intel.com/sites/landingpage/legacy/mmx/MMX_App_24-16_Bit_Conversion.pdf" rel="noreferrer">by Intel [pdf]</a>. Their code converts x888 -> 555 while I want to convert to 565. Unfortunately, converting to 565 means that the high bit is set, which means I can't use signed-saturation pack instructions. The unsigned pack instruction, packusdw wasn't added until SSE4.1. I'd like to implement its functionality with SSE2 or find another way of doing this.</p> <p>This function takes two XMM registers containing 4 32-bit pixels each and outputs a single XMM register containing the 8 converted RGB565 pixels.</p> <pre><code>static force_inline __m128i pack_565_2packedx128_128 (__m128i lo, __m128i hi) { __m128i rb0 = _mm_and_si128 (lo, mask_565_rb); __m128i rb1 = _mm_and_si128 (hi, mask_565_rb); __m128i t0 = _mm_madd_epi16 (rb0, mask_565_pack_multiplier); __m128i t1 = _mm_madd_epi16 (rb1, mask_565_pack_multiplier); __m128i g0 = _mm_and_si128 (lo, mask_green); __m128i g1 = _mm_and_si128 (hi, mask_green); t0 = _mm_or_si128 (t0, g0); t1 = _mm_or_si128 (t1, g1); t0 = _mm_srli_epi32 (t0, 5); t1 = _mm_srli_epi32 (t1, 5); /* XXX: maybe there's a way to do this relatively efficiently with SSE2? */ return _mm_packus_epi32 (t0, t1); } </code></pre> <p>Ideas I've thought of:</p> <ul> <li><p>Subtracting 0x8000, _mm_packs_epi32, re-adding 0x8000 to each 565 pixel. I've tried this, but I can't make this work.</p> <pre><code>t0 = _mm_sub_epi16 (t0, mask_8000); t1 = _mm_sub_epi16 (t1, mask_8000); t0 = _mm_packs_epi32 (t0, t1); return _mm_add_epi16 (t0, mask_8000); </code></pre></li> <li><p>Shuffle data instead of packing it. Works for MMX, but since SSE 16-bit shuffles work on only the high or low 64-bits, it would get messy.</p></li> <li><p>Save high bits, set them to zero, do the pack, restore them afterwards. Seems quite messy.</p></li> </ul> <p>Is there some other (hopefully more efficient) way I could do this?</p>

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