libc/src/__support/threads/linux/callonce.h

   1 //===-- Linux callonce fastpath -------------------------------------------===//
   2 //
   3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   4 // See https://llvm.org/LICENSE.txt for license information.
   5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8 #ifndef LLVM_LIBC_SRC___SUPPORT_THREADS_LINUX_CALLONCE_H
   9 #define LLVM_LIBC_SRC___SUPPORT_THREADS_LINUX_CALLONCE_H
  10
  11 #include "src/__support/macros/config.h"
  12 #include "src/__support/threads/linux/futex_utils.h"
  13
  14 namespace LIBC_NAMESPACE_DECL {
  15 using CallOnceFlag = Futex;
  16
  17 namespace callonce_impl {
  18 static constexpr FutexWordType NOT_CALLED = 0x0;
  19 static constexpr FutexWordType START = 0x11;
  20 static constexpr FutexWordType WAITING = 0x22;
  21 static constexpr FutexWordType FINISH = 0x33;
  22
  23 // Avoid cmpxchg operation if the function has already been called.
  24 // The destination operand of cmpxchg may receive a write cycle without
  25 // regard to the result of the comparison.
  26 LIBC_INLINE bool callonce_fastpath(CallOnceFlag *flag) {
  27   return flag->load(cpp::MemoryOrder::RELAXED) == FINISH;
  28 }
  29 } // namespace callonce_impl
  30
  31 } // namespace LIBC_NAMESPACE_DECL
  32 #endif // LLVM_LIBC_SRC___SUPPORT_THREADS_LINUX_CALLONCE_H