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    <p>Firstly you can speed up the original code a little by factoring out the multiply and getting rid of the branch:</p> <pre><code> int whiteCount = 0; for (int q = i; q &lt; i + windowHeight; q++) { const bool * const row = &amp;imageData[q * W]; for (int w = j; w &lt; j + windowWidth; w++) { whiteCount += row[w]; } } </code></pre> <p>(This assumes that <code>imageData[]</code> is truly binary, i.e. each element can only ever be 0 or 1.)</p> <p>Here is a simple NEON implementation:</p> <pre><code>#include &lt;arm_neon.h&gt; // ... int i, w; int whiteCount = 0; uint32x4_t v_count = { 0 }; for (q = i; q &lt; i + windowHeight; q++) { const bool * const row = &amp;imageData[q * W]; uint16x8_t vrow_count = { 0 }; for (w = j; w &lt;= j + windowWidth - 16; w += 16) // SIMD loop { uint8x16_t v = vld1q_u8(&amp;row[j]); // load 16 x 8 bit pixels vrow_count = vpadalq_u8(vrow_count, v); // accumulate 16 bit row counts } for ( ; w &lt; j + windowWidth; ++w) // scalar clean up loop { whiteCount += row[j]; } v_count = vpadalq_u16(v_count, vrow_count); // update 32 bit image counts } // from 16 bit row counts // add 4 x 32 bit partial counts from SIMD loop to scalar total whiteCount += vgetq_lane_s32(v_count, 0); whiteCount += vgetq_lane_s32(v_count, 1); whiteCount += vgetq_lane_s32(v_count, 2); whiteCount += vgetq_lane_s32(v_count, 3); // total is now in whiteCount </code></pre> <p>(This assumes that <code>imageData[]</code> is truly binary, <code>imageWidth &lt;= 2^19</code>, and <code>sizeof(bool) == 1</code>.)</p> <hr> <p>Updated version for <code>unsigned char</code> and values of 255 for white, 0 for black:</p> <pre><code>#include &lt;arm_neon.h&gt; // ... int i, w; int whiteCount = 0; const uint8x16_t v_mask = { 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1 }; uint32x4_t v_count = { 0 }; for (q = i; q &lt; i + windowHeight; q++) { const uint8_t * const row = &amp;imageData[q * W]; uint16x8_t vrow_count = { 0 }; for (w = j; w &lt;= j + windowWidth - 16; w += 16) // SIMD loop { uint8x16_t v = vld1q_u8(&amp;row[j]); // load 16 x 8 bit pixels v = vandq_u8(v, v_mask); // mask out all but LS bit vrow_count = vpadalq_u8(vrow_count, v); // accumulate 16 bit row counts } for ( ; w &lt; j + windowWidth; ++w) // scalar clean up loop { whiteCount += (row[j] == 255); } v_count = vpadalq_u16(v_count, vrow_count); // update 32 bit image counts } // from 16 bit row counts // add 4 x 32 bit partial counts from SIMD loop to scalar total whiteCount += vgetq_lane_s32(v_count, 0); whiteCount += vgetq_lane_s32(v_count, 1); whiteCount += vgetq_lane_s32(v_count, 2); whiteCount += vgetq_lane_s32(v_count, 3); // total is now in whiteCount </code></pre> <p>(This assumes that <code>imageData[]</code> is has values of 255 for white and 0 for black, and <code>imageWidth &lt;= 2^19</code>.)</p> <hr> <p><em>Note that all the above code is untested and may need some further work.</em></p>
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    1. COSorry - a couple of typos in there (now fixed) - I should have mentioned that this is untested code so it may need some further work - I was just trying to present the general idea.
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    2. COSorry to be bothering again.. but I'm getting an error at the line uint8x16_t v = vld1q_u8(&row[j]); saying that – Cannot initialize a variable of const unit8_t * (aka unsigned char *) with value of type const bool *- Any idea what the problem might be ??
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    3. COAnd I'm using a bool* for image data as I assumed it will be faster as it takes only 1 bit of memory per value
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