Re: bug - boost::math::ccmath::copysign not work

From: Proton (matt_at_[hidden])
Date: 2022-08-21 21:25:20

> On Aug 21, 2022, at 1:26 PM, Gero Peterhoff via Boost <boost_at_[hidden]> wrote:
>
> Am 19.08.22 um 03:53 schrieb Proton via Boost:
>> Using __builtin_bit_cast should be fine for conditional C++17 support since we already rely on __builtin_is_constant_evaluated. I’ll play with it this weekend and see where it get us. Worst case is we static_assert unsupported cases like John suggested.
>
> Hi Matt,
> I see that you have started work on ccmath::signbit/copysign/etc. I also. Our basic problem is that std::bit_cast is only available with C++20. You are already trying to provide this for C++17 (BOOST_MATH_BIT_CAST). My questions:
> - can you *guarantee* that BOOST_MATH_BIT_CAST works on/with all platforms/compilers ?
> - if not: does it make sense to upgrade ccmath to C++20 so that std::bit_cast is available ?
>
> Otherwise I can only oracle and provide code to the best of my knowledge - without guarantee that it works on/with all platforms/compilers :-(
>
> regards
> Gero
>
>
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Gero,

I can make no such guarantees. I do not think it makes sense to upgrade to C++20 because BOOST_MATH_BIT_CAST macro enables C++17 support. If you look at the comments even with a move to C++20 Clang does not yet support constexpr std::bit_cast/__builtin_bit_cast with bit-fields. I have provided layouts for 64, 80, and 128 bit long doubles so most platforms should be covered if the toolchain supports it.

Matt