[llvm-dev] Use of C++17 in the LLVM codebase

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Simon Pilgrim via llvm-dev

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Nov 23, 2021, 7:57:26 AM11/23/21
to llvm...@lists.llvm.org
Making better use of C++17 came up several times in devmtg chats this year.

I notice we have a number of cases in LLVM source which allows
(optional) use of C++17, but it is never mandatory or assumed:

e.g  StringRef.h

#if __cplusplus > 201402L
#include <string_view>
#endif

...

    // Constexpr version of std::strlen.
    static constexpr size_t strLen(const char *Str) {
#if __cplusplus > 201402L
      return std::char_traits<char>::length(Str);
#elif __has_builtin(__builtin_strlen) || defined(__GNUC__) || \
    (defined(_MSC_VER) && _MSC_VER >= 1916)
      return __builtin_strlen(Str);
#else
      const char *Begin = Str;
      while (*Str != '\0')
        ++Str;
      return Str - Begin;
#endif
    }

According to https://llvm.org/docs/CodingStandards.html - we only accept
up to standard C++14 code that is supported by our minimum
gcc/clang/msvc versions.

Does anyone know what in particular is preventing us from bumping the
minimum compiler versions to make 'compiler supported' C++17 a similar
requirement to build LLVM at this time?

https://lists.llvm.org/pipermail/llvm-dev/2018-October/127045.html
https://reviews.llvm.org/D47073

AFAICT, the last time this was discussed (back in 2018/9), some
developers still had a dependency on Ubuntu 1604 LTS (and other old
distros), whose default gcc didn't have adequate C++17 support. Support
for 1604 has been extended to April 2026 - I'm guessing we're not going
to wait until then?

Cheers, Simon.

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Tobias Hieta via llvm-dev

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Nov 23, 2021, 10:23:25 AM11/23/21
to Simon Pilgrim, llvm...@lists.llvm.org
On Tue, Nov 23, 2021 at 1:57 PM Simon Pilgrim via llvm-dev
<llvm...@lists.llvm.org> wrote:
> According to https://llvm.org/docs/CodingStandards.html - we only accept
> up to standard C++14 code that is supported by our minimum
> gcc/clang/msvc versions.
>
> Does anyone know what in particular is preventing us from bumping the
> minimum compiler versions to make 'compiler supported' C++17 a similar
> requirement to build LLVM at this time?

There is a process for this that's documented here:
https://llvm.org/docs/DeveloperPolicy.html#id23

An RFC like this should be written:
https://lists.llvm.org/pipermail/llvm-dev/2019-January/129452.html

I don't think anything is blocking it except that someone needs to do
the work to investigate the different toolchains and summarize it in a
RFC email to the list.

-- Tobias

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