Doubts about _mm_popcnt_u64()

[Cardoso]

Hi,
In my code I have:

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#define myPopCnt(bb) _mm_popcnt_u64(bb)
#define PopCnt(v) __popcnt64(v)

I have two doubts I would like to clear up.
1 - Both _mm_popcnt_u64() and __popcnt64() return a 64bit value but In my code I use 32bit ints to hold that value, is that safe or should I cast to an int to store that index?

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int index = myPopCnt(bb); 

or

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int index = (int)myPopCnt(bb); 

2 - Which is the fastest? _mm_popcnt_u64() or __popcnt64() ?

[Bo Persson]

On a modern x86-64 system those intrinsics are the same, as the CPU has a hardware POPCNT instruction. Use whatever your compiler supports.

You can add a typecast if the compiler protests, as we all know that the result can never be above 64. So a cast would be safe.

As an alternative, in C++ you could use std::popcount that already returns an int.

https://en.cppreference.com/w/cpp/numeric/popcount