CODEWITHSUNDEEP
gccx86clangbit-manipulationinstructions
Steven Lu
Steven Lu

Reputation: 43527

I want both implementations of __builtin_popcountll

I have some code that I'd like to run the fast built-in CPU instruction popcnt (when __builtin_popcountll is compiled using proper flags such as with g++ -mpopcnt or clang++ -march=corei7, this happens), but also be able to fall-back to code when cpuid reveals a CPU not supporting the HW instruction.

Of course, to get the fall-back code that I trust the compiler folks have implemented right (so I don't have to bring in C or asm code to do my popcount) I need a separate compilation unit that is compiled without the -mpopcnt or -march=corei7 flags.

Is linking together separately compiled code the only way? Are there no compiler intrinsics or other types of hints or other built-ins I don't know about that I can use to have it generate the fallback popcount code?

Upvotes: 5

Views: 3410

Answers (3)

Anon E. Mouse
Anon E. Mouse

Reputation: 21

gcc has a feature called "multiversioning" specifically for this.

Upvotes: 2

Max
Max

Reputation: 2859

You could call the "fall back code" directly. I believe it's accessible of the standard libraries as:

int __popcountsi2 (int a)
int __popcountdi2 (long a)
int __popcountti2 (long long a)

Upvotes: 0

leecbaker
leecbaker

Reputation: 3741

I don't know for sure, but the cost of putting in code necessary to select between the popcnt instruction and a fallback implementation might have a larger performance hit than simply going with a non-popcnt implementation all of the time.

To switch in an alternate implementation (doing the switching at the site of the popcnt), you will need at least the following:

  • Load and check the cpuid bit (CPUID.01H:ECX.POPCNT[Bit 23])
  • Branch to select popcnt or alternate implementation
  • Potentially save/restore registers needed for the alternate implementation
  • Potentially move data to SIMD registers, if the alternate implementation is SSE or AVX based

I suspect that the cost prohibits an effective implementation of the intrinsic that you describe.

Upvotes: 2

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