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

[Mehdi Amini via llvm-dev]

> On Nov 28, 2016, at 9:47 AM, David Blaikie via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> OK - good to know. (not sure we're talking about pessimizing it - just not adding a new/possible optimization, to be clear)

This does not seem that clear to me. The motivation seems to be able to create global table of StringRef, which we don’t do because the lack fo constexpr of static initializers right now.
Moving forward it would mean making clang a lot slower when built with MSVC if we were going this route.

— 
Mehdi


> 
> Just out of curiosity - are there particular reasons you prefer or need to ship an MSVC built version, rather than a bootstrapped Clang?
> 
> On Mon, Nov 28, 2016 at 9:24 AM Robinson, Paul <paul.robinson at sony.com <mailto:paul.robinson at sony.com>> wrote:
> So I wouldn't personally worry too much about performance degredation when built with MSVC - if, when building a stage 2 on Windows (building Clang with MSVC build Clang) you do end up with a compiler with the desired performance characteristics - then that's probably sufficient.
> 
>   <>
> Hold on there—we deliver an MSVC-built Clang to our licensees, and I would really rather not pessimize it.
> 
> --paulr
> 
>  
> 
> From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org <mailto:llvm-dev-bounces at lists.llvm.org>] On Behalf Of David Blaikie via llvm-dev
> Sent: Friday, November 25, 2016 8:52 AM
> To: Mueller-Roemer, Johannes Sebastian; Malcolm Parsons; Hal Finkel; llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>
> 
> Subject: Re: [llvm-dev] RFC: Constructing StringRefs at compile time
> 
>  
> 
>  
> 
> On Fri, Nov 25, 2016 at 6:10 AM Mueller-Roemer, Johannes Sebastian via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
> 
> What about going for
> 
> template<unsigned N>
> constexpr StringRef(const char (&Str)[N])
> 
> and avoiding strlen entirely for string literals?
> 
>  
> 
> You'd at least want an assert in there (that N - 1 == strlen(Str)) in case a StringRef is ever constructed from a non-const char buffer that's only partially filled.
> 
> But if we can write this in such a way that it performs well on good implementations - that seems sufficient. If getting good performance out of the compiler means bootstrapping - that's pretty much the status quo already, as I understand it.
> 
> So I wouldn't personally worry too much about performance degredation when built with MSVC - if, when building a stage 2 on Windows (building Clang with MSVC build Clang) you do end up with a compiler with the desired performance characteristics - then that's probably sufficient.
> 
>  
> 
> 
> -----Original Message-----
> From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org <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 <mailto:hfinkel at anl.gov>>
> Cc: llvm-dev at lists.llvm.org <mailto: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 <mailto: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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