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

Mon Nov 28 11:38:09 PST 2016

The fact that the templatized constructor falls down because of the
possibility of initializing StringRef with a stack-allocated char array
kills that idea in my mind.

I feel like the only two reasonable solutions are

1) allow UDL for this case, document that this is an exception and that
UDLs are still not permitted anywhere else, and require (by policy, since I
don't know of a way to have the compiler force it) that this UDL be used
only in global constructors.  One idea to help "enforce" this policy would
be to give the UDL a ridiculously convoluted name, like
`string_ref_literal`, so that one would have to write
"foo"_string_ref_literal, and then provide a macro like `#define LITERAL(x)
x_string_ref_literal`, so that the user writes `StringRef s[] = {
LITERAL("a"), LITERAL("b") };  I'm not sure if that's better or worse than
`StringRef s[] = { "a"_sr, "b"_sr };`, but at least it's greppable this way.

2) Don't allow global tables of StringRefs.

On Mon, Nov 28, 2016 at 11:30 AM Mehdi Amini via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> On Nov 28, 2016, at 11:27 AM, David Blaikie <dblaikie at gmail.com> wrote:
>
>
>
> On Mon, Nov 28, 2016 at 11:01 AM Mehdi Amini <mehdi.amini at apple.com>
> wrote:
>
> 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.
>
>
> Ah, fair - perhaps I misunderstood/misrepresented, apologies. Figured this
> was just an attempt to reduce global initializers in arrays we already
> have. Any pointers on where the motivation is described/discussed?
>
>
> This thread started with: "There is a desire to be able to create
> constexpr StringRefs to avoid static initializers for global tables
> of/containing StringRefs.”
>
> I don’t have more information, but maybe Malcolm can elaborate?
>
> —
> 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>
> 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] *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
>
>
> *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> 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] 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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