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0 pointstoth6mo ago0 comments

> An example you'll see in say a modern C compiler is that if you write the obvious loop to calculate how many bits are set in an int, the actual machine code on a brand new CPU should be a single population count instruction, C provides neither intrinsics (like Rust) not a dedicated "popcount" feature, so you can't write that but it's obviously what you want here and yup an optimising C compiler will do that.

C compilers definitely have intrinsics for this, for GCC for instance it is `__builtin_popcount`.

And apparently it has even standard language support for it since C23, it's `stdc_count_ones` [1] and in C++ you have `std::popcount` [2]

[1] https://en.cppreference.com/w/c/numeric/bit_manip.html [2] https://en.cppreference.com/w/cpp/numeric/popcount.html

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tialaramex6mo ago

The existence of platform specific hacks is not interesting. In reality what happens is that software which has at any point cared about being portable doesn't use them.

But yes stdc_count_ones is indeed the intrinsic you'd want here, and only a few years after I stopped writing C, so thanks for mentioning that.

std::popcount is C++ but it's also kinda miserable that it took until C++ 20 and yet they still only landed the unsigned integer types, even though C++ 20 also insists the signed integers have two's complement representation, so the signed integers do have these desirable properties in fact but you can't use that.

articulatepang6mo ago

> In reality what happens is that software which has at any point cared about being portable doesn't use them.

I don't think this generalization is actually true. Fast portable software compiles conditionally based on the target platform, picking the fast platform-specific intrinsic, and falls back to a slow but guaranteed portable software implementation. This pattern is widespread in numerical linear algebra, media codecs, data compressors, encryption, graphics, etc.

tothOP6mo ago

Maybe we are just quibbling over semantics but the compiler intrinsic here is '__builtin_popcount'. 'stdc_count_ones' is a standard library element that presumably will be implemented using the intrinsic.

And FWIW all major C/C++ have for a long time have had a an intrinsic for this. In clang it even has the same name, Visual Studio it's something like just '_popcount'. So it has long been easy to roll your own macro that works everywhere.

tialaramex6mo ago

Yes, just semantics. But I don't think I can agree that because you could have ensured this works portably people actually did. That's not been my experience.

Yesterday I watched that "Sea of Thieves" C++ 14 to C++ 20 upgrade story on Youtube, that feels much more like what I've seen - code that shouldn't have worked but it did, kept alive by people whose priority is a working game.

gpderetta6mo ago

__builtin_popcount is not platform specific.

tialaramex6mo ago

OK, sure, vendor specific then. C23 does not promise this incantation, it's presumably a GCCism.

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0 pointstoth6mo ago0 comments

C compilers definitely have intrinsics for this, for GCC for instance it is `__builtin_popcount`.

And apparently it has even standard language support for it since C23, it's `stdc_count_ones` [1] and in C++ you have `std::popcount` [2]

[1] https://en.cppreference.com/w/c/numeric/bit_manip.html [2] https://en.cppreference.com/w/cpp/numeric/popcount.html

0 comments

tialaramex6mo ago

The existence of platform specific hacks is not interesting. In reality what happens is that software which has at any point cared about being portable doesn't use them.

But yes stdc_count_ones is indeed the intrinsic you'd want here, and only a few years after I stopped writing C, so thanks for mentioning that.

articulatepang6mo ago

> In reality what happens is that software which has at any point cared about being portable doesn't use them.

tothOP6mo ago

tialaramex6mo ago

Yes, just semantics. But I don't think I can agree that because you could have ensured this works portably people actually did. That's not been my experience.

gpderetta6mo ago

__builtin_popcount is not platform specific.

tialaramex6mo ago

OK, sure, vendor specific then. C23 does not promise this incantation, it's presumably a GCCism.

j / k navigate · click thread line to collapse