Re: [boost] [dynamic_bitset] Using of GCC built-in functions may increase performance of some methods

Subject: Re: [boost] [dynamic_bitset] Using of GCC built-in functions may increase performance of some methods
From: Brian Kuhl (kuhlenough_at_[hidden])
Date: 2018-08-24 21:49:59

But not on ARM or PowerPC says the embedded guy. Though both Gcc and clang support __builtin_popcount() across all processors, which is what we are using these days. I did notice one library does a call out openCl for popcount ()

I honestly was thinking about the same sort approach, though all compile time based. But I wasn’t sure what you would do with a signed value? As the builtin treats everything as unsigned.  
Do you mask out the sign bit or just cast it, or consider it an error to take a bit count of a signed value?

Sent from my iPhone

> On Aug 22, 2018, at 10:08 PM, Gavin Lambert via Boost <boost_at_[hidden]> wrote:
> 
>> On 23/08/2018 09:16, Andrey Semashev wrote:
>> I think such an optimization would be useful. Note that MSVC also has
>> intrinsics for popcount[1], although I don't think those are supported
>> when the target CPU doesn't implement the corresponding instructions.
>> You would have to check at compile time whether the target CPU
>> supports it (e.g. by checking if __AVX__ is defined).
> 
> While compile-time detection is better, if you can do it (because it lets it be completely inlined); if the compile-time detection fails, you can still do runtime detection, eg. by defining something like:
> 
> // header file
> extern int (*popcnt64)(uint64_t);
> 
> // source file
> static bool is_popcnt_supported()
> {
>    int info[4] = { 0 };
>    __cpuid(info, 1);
>    return (info[2] & 0x00800000) != 0;
> }
> 
> static int popcnt64_intrinsic(uint64_t v)
> {
>    return /* _mm_popcnt_64(v) or __builtin_popcountll(v) */;
> }
> 
> static int popcnt64_emulation(uint64_t v)
> {
>   // code that calculates it with bit twiddling
> }
> 
> static int popcnt64_auto(uint64_t v)
> {
>    popcnt64 = is_popcnt_supported()
>        ? &popcnt64_intrinsic
>        : &popcnt64_emulation;
>    return popcnt64(v);
> }
> 
> int (*popcnt64)(uint64_t) = &popcnt64_auto;
> 
> Repeat for other argument sizes as needed.  You could probably do something fancier with C++11 guaranteed static initialisation, but this will work on all compilers.
> 
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