Implement C++20 constexpr <algorithm>, , and <array>

[Ed Smith-Rowland via libstdc++]

On 3/29/19 9:23 AM, Jakub Jelinek wrote:
> On Fri, Mar 29, 2019 at 09:10:26AM -0400, Ed Smith-Rowland via gcc-patches wrote:
>> Greetings,
>>
>> This patch implements C++20 constexpr for <algorithm>, <uility>, <array>.
>>
>> It's a large patch but only affects C++20 and the volume is mostly test
>> cases.
>>
>> This differs from the previous patch in actually testing constexpr :-\ and
>> in the addition of wrappers for __builtin_memmove and __builtin_memcmp that
>> supply constexpr branches if C++20 and is_constant_evaluated().
> +    void*
> +    __memmove(_Tp* __dst, const _Tp* __src, ptrdiff_t __num)
> +    {
> +#if __cplusplus > 201703L
> +      if (is_constant_evaluated())
> +       {
> +         for(; __num > 0; --__num)
> +           {
> +             *__dst = *__src;
> +             ++__src;
> +             ++__dst;
> +           }
> +         return __dst;
> +       }
> +      else if (__num)
> +#endif
> +       return __builtin_memmove(__dst, __src, sizeof(_Tp) * abs(__num));
> +      return __dst;
> ..
> 	  const ptrdiff_t _Num = __last - __first;
>            if (_Num)
> -           __builtin_memmove(__result, __first, sizeof(_Tp) * _Num);
> +           __memmove(__result, __first, _Num);
> ..
> 	  const ptrdiff_t _Num = __last - __first;
>            if (_Num)
> -           __builtin_memmove(__result - _Num, __first, sizeof(_Tp) * _Num);
> +           __memmove(__result - _Num, __first, _Num);
>
> Why the abs in there, that is something that wasn't previously there and
> if the compiler doesn't figure out that __last >= __first, it would mean
> larger emitted code for the non-constexpr case.  As memmove argument is
> size_t, wouldn't it be better to make __num just size_t and remove this abs?
> Also, wouldn't it be better to have on the other side the __num == 0
> handling inside of __memmove, you already have it there for C++2a, but not
> for older.  You could then drop the if (_Num) guards around __memmove.

memmove needs to be able to work with __last < __first also.

I was getting negative __num and when passed to __builtin_memmove which 
takes size_t got blowups.

I'm not sure why it worked before. __builtin_memmove will work with 
__last < __first and sensible positive __num.

When I tried to do what __builtin_memmove or ::memmove must do with 
unsigned num I would need to branch on __last < __first

and copy backwards.  But pointer comparisons were getting caught as 
non-constexpr.

I'll look at the __num==0 (noop) testing.

>
> Also, shouldn't the is_constant_evaluated() calls be guarded with
> _GLIBCXX_HAVE_BUILTIN_IS_CONSTANT_EVALUATED ?  Without that it won't be
> defined...

I am trying for a C++20-only patch (hoping to get it in for 9) so I used 
the library function and tested __cplusplus > 201703L.

We could do _GLIBCXX_HAVE_BUILTIN_IS_CONSTANT_EVALUATED and then we 
could use these for lower version. Maybe stage 1?

>
> 	Jakub
>