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copied!<p>The __popcnt*() builtins are for AMD's Advanced Bit Manipulation (ABM) instructions. See <a href="http://blogs.amd.com/developer/2007/09/26/barcelona-processor-feature-advanced-bit-manipulation-abm/">http://blogs.amd.com/developer/2007/09/26/barcelona-processor-feature-advanced-bit-manipulation-abm/</a></p> <p>The _mm_popcnt_u*() intrinsics are for Intel's implementation, which aren't part of SSE4.2 per se, but were implemented around the same time. See <a href="http://en.wikipedia.org/wiki/SSE4#POPCNT_and_LZCNT">http://en.wikipedia.org/wiki/SSE4#POPCNT_and_LZCNT</a></p> <p>According to <a href="http://chessprogramming.wikispaces.com/Population+Count">http://chessprogramming.wikispaces.com/Population+Count</a> , both implementations are binary compatible, in spite of their different intrinsic names.</p> <p>Intel's <a href="http://download.intel.com/products/processor/manual/325462.pdf">architecture manual</a> states that:</p> <blockquote> <p>Before an application attempts to use the POPCNT instruction, it must check that the processor supports SSE4.2 (if CPUID.01H:ECX.SSE4_2[bit 20] = 1) and POPCNT (if CPUID.01H:ECX.POPCNT[bit 23] = 1).</p> </blockquote> <p>AMD's <a href="http://support.amd.com/us/Processor_TechDocs/24594_APM_v3.pdf">AMD64 Architecture Programmer's Manual Volume 3: General Purpose and System Instructions</a> says</p> <blockquote> <p>Support for the POPCNT instruction is indicated by ECX bit 23 (POPCNT) as returned by CPUID function 0000_0001h. Software MUST check the CPUID bit once per program or library initialization before using the POPCNT instruction, or inconsistent behavior may result.</p> </blockquote> <p>I can't see any reason why popcnt would require the presence of SSE4.2, so I think that checking bit 23 of ECX is sufficient to determine popcnt's presence.</p> <p>AMD's Barcelona, the first AMD CPU to have popcnt, didn't fully implement SSE4, so it's possible that Intel's architecture manual suggests a method for determine presence which will work on Intel CPUs and fail on even qualified AMD CPUs.</p>

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