[PATCH v2] libstdc++: implement constexpr memory algorithms

[Giuseppe D'Angelo]

Hello,

On 25/02/2025 23:46, Jonathan Wakely wrote:
>> Maybe we can get away with unconditionally declaring this
>> _GLIBCXX26_CONSTEXPR?  If the compiler doesn't support constexpr
>> placement new then the 'constexpr' would be silently dropped at
>> instantiation time.  This would be in line with C++23 P2448R2 which
>> made it no longer IFNDR to declare a constexpr function template
>> for which no specialization is actually constexpr.
> 
> Yeah, for internal functions that aren't ever compiled as C++98, we
> can often just make them constexpr. It will never be called during
> constant evaluation in C++20 or older, but that's usually fine.
> 
> In this case though, would the placement new make it ill-formed in
> Clang 18, which didn't support P2448R2?

Yes, I had the same question. Cppreference says that Clang 17/18 have a 
partial implementation of P2448, not sure what that means:

https://en.cppreference.com/w/cpp/23

A quick test shows that it seems happy with a placement new in a 
constexpr function in C++17 mode, but I'm not positive it won't complain 
in some other case.

So I'm going to leave these macros alone if you don't mind.


>> We typically call __is_constant_evaluated fully qualified (though I
>> don't remember why since it's not eligible for ADL?)
> 
> I don't think we're consistent, and it's not necessary.
> 
> But we could simplify things a little by doing:
> 
> #if __glibcxx_raw_memory_algorithms >= 202411L // >= C++26
>    if consteval {
>      return std::__do_uninit_copy(__first, __last, __result);
>    }
> #endif
> 
> We don't need to use the __is_constant_evaluated() wrapper, or even
> the std::is_constant_evaluated() function, because this is C++26 code
> so we know 'if consteval' works. Clang supports it since version 14,
> which is too old to support any C++26 mode, so every Clang that
> supports -std=c++2c also supports 'if consteval'.

I'm not sure if the extra FTM check makes it more readable, but I guess 
`if consteval` is strictly better than the wrappers, so I'll amend this way.

New patch is attached.

Thank you,

-- 
Giuseppe D'Angelo
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