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copied!<p>There are three possibilities I see here.</p> <p>First, your data might not suit wide comparisons. If there's a high chance that <code>(*tptr & *xptr) != *tptr</code> within the first few blocks, the plain C++ version will almost certainly always be faster. In that instance, your SSE will run through more code & data to accomplish the same thing.</p> <p>Second, your SSE code may be incorrect. It's not totally clear here. If <code>no_blocks_</code> is identical between the two samples, then <code>start + i</code> is probably having the unwanted behavior of indexing into 128-bit elements, not 32-bit as the first sample.</p> <p>Third, SSE <em>really</em> likes it when instructions can be pipelined, and this is such a short loop that you might not be getting that. You can reduce branching significantly here by processing more than one SSE block at once.</p> <p>Here's a quick untested shot at processing 2 SSE blocks at once. Note I've removed the <code>block != xblock</code> branch entirely by keeping the state outside of the loop and only testing at the end. In total, this moves things from 1.3 branches per <code>int</code> to 0.25.</p> <pre><code>bool equal(unsigned const *a, unsigned const *b, unsigned count) { __m128i eq1 = _mm_setzero_si128(); __m128i eq2 = _mm_setzero_si128(); for (unsigned i = 0; i != count; i += 8) { __m128i xa1 = _mm_load_si128((__m128i const*)(a + i)); __m128i xb1 = _mm_load_si128((__m128i const*)(b + i)); eq1 = _mm_or_si128(eq1, _mm_xor_si128(xa1, xb1)); xa1 = _mm_cmpeq_epi32(xa1, _mm_and_si128(xa1, xb1)); __m128i xa2 = _mm_load_si128((__m128i const*)(a + i + 4)); __m128i xb2 = _mm_load_si128((__m128i const*)(b + i + 4)); eq2 = _mm_or_si128(eq2, _mm_xor_si128(xa2, xb2)); xa2 = _mm_cmpeq_epi32(xa2, _mm_and_si128(xa2, xb2)); if (_mm_movemask_epi8(_mm_packs_epi32(xa1, xa2)) != 0xFFFF) return false; } return _mm_movemask_epi8(_mm_or_si128(eq1, eq2)) != 0; } </code></pre> <p>If you've got enough data and a low probability of failure within the first few SSE blocks, something like this should be at least somewhat faster than your SSE.</p>

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