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

[Mueller-Roemer, Johannes Sebastian via llvm-dev]

What about going for

template<unsigned N>
constexpr StringRef(const char (&Str)[N])

and avoiding strlen entirely for string literals?

-----Original Message-----
From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of Malcolm Parsons via llvm-dev
Sent: Friday, November 25, 2016 13:34
To: Hal Finkel <hfinkel at anl.gov>
Cc: llvm-dev at lists.llvm.org
Subject: Re: [llvm-dev] RFC: Constructing StringRefs at compile time

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

--
Malcolm Parsons
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