[all-commits] [llvm/llvm-project] e1f59a: Mark some std::string functions noinline. (#72869)

[James Y Knight via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: e1f59ad9f6b4df592925e39462ba95bb299eca26
      https://github.com/llvm/llvm-project/commit/e1f59ad9f6b4df592925e39462ba95bb299eca26
  Author: James Y Knight <jyknight at google.com>
  Date:   2023-11-28 (Tue, 28 Nov 2023)

  Changed paths:
    M libcxx/include/__config
    M libcxx/include/string

  Log Message:
  -----------
  Mark some std::string functions noinline. (#72869)

The intent of these particular functions, since their introduction, was
to NOT be inlinable.

However, the mechanism by which this was accomplished was non-obvious,
and stopped working when string is compiled for C++20.

A longstanding behavior specified by the C++ standard is that
instantiation of the body of a template function is suppressed by an
extern template declaration -- unless the function is explicitly marked
either constexpr or inline. Of course, if the body is not instantiated,
then it cannot possibly be inlined, and thus all the functions listed in
libcxx/include/__string/extern_template_lists.h were uninlineable.

But, in C++20 mode, string functions were annotated constexpr, which
means they _are_ instantiated, and do become inlineable. And, in fact,
they do get inlined, which has caused noticeable binary-size growth for
users.

For example, in C++17,
`std::string f(std::string *in) { return *in; }`
does not inline the copy-constructor call, and instead generates a call
to the exported function defined in the libc++ shared library.

I think we probably don't want to mark all functions that are currently
in the extern template list as noinline, as many of them really are
reasonable inlining candidates. Thus, I've restricted this change to
only the few functions that were clearly intended to be outlined.

See commits like b019c5c0372eb08800327efb5e7955ce918b75d1 (and some
others like it) for background, in which functions were removed from the
extern template list in the unstable ABI in order to allow the
short-string case to be inlined, while moving the long-string case to a
separate function, added to the extern template list.