Skip to content

[libc++] Mark __{emplace,push}_back_slow_path as noinline #94379

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Merged
merged 1 commit into from
Sep 12, 2025
Merged

[libc++] Mark __{emplace,push}_back_slow_path as noinline #94379

Changes from all commits
Commits
Show all changes
1 commit
Select commit
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
32 changes: 26 additions & 6 deletions libcxx/include/__vector/vector.h
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -1142,6 +1142,24 @@ vector<_Tp, _Allocator>::__emplace_back_slow_path(_Args&&... __args) {
return this->__end_;
}

// This makes the compiler inline `__else()` if `__cond` is known to be false. Currently LLVM doesn't do that without
// the `__builtin_constant_p`, since it considers `__else` unlikely even through it's known to be run.
// See https://llvm.org/PR154292
Comment on lines +1145 to +1147
Copy link
Member

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Suggested change
// This makes the compiler inline `__else()` if `__cond` is known to be false. Currently LLVM doesn't do that without
// the `__builtin_constant_p`, since it considers `__else` unlikely even through it's known to be run.
// See https://llvm.org/PR154292
// This function performs an if-else on the given condition, calling either `__if()` or `__else()`. The `__if()` branch is annotated as being likely.
//
// However, it increases the likelihood that the `__else()` branch will be inlined if `__cond` is known to be a constant by
// the optimizer, which LLVM currently doesn't do when naively applying the [[likely]] attribute.
// See https://llvm.org/PR154292

template <class _If, class _Else>
_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 void __if_likely_else(bool __cond, _If __if, _Else __else) {
if (__builtin_constant_p(__cond)) {
if (__cond)
__if();
else
__else();
} else {
if (__cond) [[__likely__]]
__if();
else
__else();
}
}

template <class _Tp, class _Allocator>
template <class... _Args>
_LIBCPP_CONSTEXPR_SINCE_CXX20 inline
Expand All @@ -1152,12 +1170,14 @@ _LIBCPP_CONSTEXPR_SINCE_CXX20 inline
#endif
vector<_Tp, _Allocator>::emplace_back(_Args&&... __args) {
pointer __end = this->__end_;
if (__end < this->__cap_) {
__emplace_back_assume_capacity(std::forward<_Args>(__args)...);
++__end;
} else {
__end = __emplace_back_slow_path(std::forward<_Args>(__args)...);
}
std::__if_likely_else(
__end < this->__cap_,
[&] {
__emplace_back_assume_capacity(std::forward<_Args>(__args)...);
++__end;
},
[&] { __end = __emplace_back_slow_path(std::forward<_Args>(__args)...); });

this->__end_ = __end;
#if _LIBCPP_STD_VER >= 17
return *(__end - 1);
Expand Down
Loading