[all-commits] [llvm/llvm-project] ab3a9e: [libc] clean up futex usage (#91163)

[Schrodinger ZHU Yifan via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: ab3a9e724d87a4272782f76b90fb0872a6a86939
      https://github.com/llvm/llvm-project/commit/ab3a9e724d87a4272782f76b90fb0872a6a86939
  Author: Schrodinger ZHU Yifan <yifanzhu at rochester.edu>
  Date:   2024-05-07 (Tue, 07 May 2024)

  Changed paths:
    M libc/src/__support/threads/linux/CMakeLists.txt
    M libc/src/__support/threads/linux/callonce.cpp
    A libc/src/__support/threads/linux/futex_utils.h
    M libc/src/__support/threads/linux/futex_word.h
    M libc/src/__support/threads/linux/mutex.h
    M libc/src/__support/threads/linux/thread.cpp
    M libc/src/__support/threads/mutex.h
    M libc/src/__support/threads/thread.cpp
    M libc/src/threads/linux/CMakeLists.txt
    M libc/src/threads/linux/CndVar.h

  Log Message:
  -----------
  [libc] clean up futex usage (#91163)

# Motivation

Futex syscalls are widely used in our codebase as synchronization
mechanism. Hence, it may be worthy to abstract out the most common
routines (wait and wake). On the other hand, C++20 also provides
`std::atomic_notify_one/std::atomic_wait/std::atomic_notify_all` which
align with such functionalities. This PR introduces `Futex` as a subtype
of `cpp::Atomic<FutexWordType>` with additional
`notify_one/notify_all/wait` operations.

Providing such wrappers also make future porting easier. For example,
FreeBSD's `_umtx_op` and Darwin's `ulock` can be wrapped in a similar
manner.

### Similar Examples

1. [bionic
futex](https://android.googlesource.com/platform/bionic/+/refs/heads/main/libc/bionic/bionic_futex.cpp)
2. [futex in Rust's
std](https://github.com/rust-lang/rust/blob/8cef37dbb67e9c80702925f19cf298c4203991e4/library/std/src/sys/pal/unix/futex.rs#L21)



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