[llvm-dev] RFC: Constructing StringRefs at compile time

[Mehdi Amini via llvm-dev]

> On Nov 25, 2016, at 4:34 AM, Malcolm Parsons via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> On 24 November 2016 at 15:04, Hal Finkel <hfinkel at anl.gov> wrote:
>>> Creating constexpr StringRefs isn't trivial as strlen isn't portably
>>> constexpr and std::char_traits<char>::length is only constexpr in
>>> C++17.
>> 
>> Why don't we just create our own traits class that has a constexpr length, and then we can switch over to the standard one when we switch to C++17?
> 
> GCC and Clang treat __builtin_strlen as constexpr.
> MSVC 2015 doesn't support C++14 extended constexpr. I don't know how
> well it optimises a recursive strlen.
> 
> This works as an optimisation for GCC and Clang, and doesn't make
> things worse for MSVC:
> 
>     /// Construct a string ref from a cstring.
>     LLVM_ATTRIBUTE_ALWAYS_INLINE
> +#if __has_builtin(__builtin_strlen)
> +    /*implicit*/ constexpr StringRef(const char *Str)
> +        : Data(Str), Length(Str ? __builtin_strlen(Str) : 0) {}
> +#else
>     /*implicit*/ StringRef(const char *Str)
>         : Data(Str), Length(Str ? ::strlen(Str) : 0) {}
> +#endif

The only reason I didn’t go with this solution was that an MSVC built clang would take a long time to startup if StringRef are present in global tables.

— 
Mehdi